Ozonation of Organic and Polymer Compounds

Ozonation of Organic & Polymer Compounds Authors: Gennady E. Zaikov and Slavcho K. Rakovsky Ozonation of Organic and ...

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Ozonation of Organic & Polymer Compounds

Authors: Gennady E. Zaikov and Slavcho K. Rakovsky

Ozonation of Organic and Polymer Compounds

Gennady E. Zaikov and Slavcho K. Rakovsky

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2009 by

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C

ontents

Introduction....................................................................................................1 1

Kinetics and Mechanism of Ozone Reactions with Organic and Polymeric Compounds in the Liquid Phase........................................9

1.1

Reaction mechanisms .........................................................9 1.1.1

1.2

2

Thermochemical analysis ..............................................11

Structural-kinetic investigations ........................................12 1.2.1

Normal parafﬁns and isoparafﬁns ................................12

1.2.2

Cycloparafﬁns ..............................................................20

1.2.3

Method for estimation of reaction mechanisms ...........26

1.2.4

Application of ERM .....................................................29

1.3

Cyclohexane .....................................................................36

1.4

Cumene ............................................................................47 1.4.1

Ozonation in the presence of transition metal compounds ...................................................................51

1.4.2

Ozonolysis in the presence of NiO ...............................57

1.4.3

Ozonolysis in the presence of Mo and V oxides ...........60

1.4.4

Cumenehydroperoxide .................................................64

1.5

Polyethylene and polypropylene .......................................68

1.6

Polystyren .........................................................................74

Ozonolysis of oxygen-containing organic compounds ..................93

2.1

Alcohols ...... 932.1.1 Application of Estimation of Reaction Mechanism (ERM) .........................................................100

2.2

Ketones ...........................................................................102 iii

Ozonation of Organic and Polymer Compounds

2.3

Hydrotrioxides ...............................................................110

2.4

Synthesis of oxolanes ......................................................124

2.5

2.4.1

Derivatives of 4-hydroxymethyl-1,3-dioxolane ...........124

2.4.2

Phenyl ethers of 4-hydroxymethyl-1,3-dioxolane .......124

2.4.3

Alkyl ethers of 4-hydroxymethyl-1,3-dioxolane .........125

2.4.4

5-Nonylene-1,2,4-trioxolane ......................................127

Ethers .............................................................................130 2.5.1

2.6

Hydroxybenzenes ...........................................................140 2.6.1

2.7

2.8 3

Application of ERM ..................................................146

Carbohydrates and model compounds ............................149 2.7.1

2,3-Butanediol ...........................................................153

2.7.2

1,2-Cyclohexanediol ...................................................155

2.7.3

Mannitol and its derivatives .......................................156

2.7.4

A-D-Glucose ...............................................................158

2.7.5

A-D-Methyl-glucose....................................................160

2.7.6

B-Cyclodextrine and starch .........................................161

Catalytic ozonolysis and oxidation .................................162

Ozonolysis of alkenes in liquid phase............................................179

3.1

3.2

iv

Application of the ERM ............................................135

Oleﬁns ............................................................................179 3.1.1

Mechanisms ...............................................................179

3.1.2

Kinetics ......................................................................189 3.1.2.1

Gas phase ...................................................189

3.1.2.2

Liquid phase ...............................................191 3.1.2.2.1

Effect of solvent .......................203

3.1.2.2.2

Effect of conﬁguration ..............205

3.1.2.2.3

Effect of structure .....................209

Polydienes .......................................................................216

Contents

4

3.2.1

Polybutadiene .............................................................219

3.2.2

Cis-1,4-polyisoprene ...................................................227

3.2.3

Polychloroprene .........................................................228

3.2.4

Butadienenitrile rubbers .............................................230

3.2.5

Ethylenepropylene rubbers .........................................235

Degradation and Stabilisation of Rubber ......................................251

4.1

Ozonolysis of Elastomers in Elastic State ........................252

4.2

Antiozonants ..................................................................261 4.2.1

Mechanism of Action .................................................263

4.2.2

Synthesis .....................................................................267 4.2.2.1

Paraphenylenediamine ................................267 4.2.2.1.1

Alkylation in the Presence of Hydrogen ......................................267

4.2.2.1.2

Alkylation in the Absence of Hydrogen ......................................273

4.2.2.2

Hydroquinolines.........................................275

4.2.2.3

N,N´-Disubstituted Hexahydropyrimidines 276

4.2.2.4

N-Substituted Dimethylpyrols ....................276

4.2.2.5

Enamines ....................................................276

4.2.2.6

Nitrone Compounds ...................................277

4.2.2.7

Derivatives of 3(5)-Methylpyrazone ...........278

4.2.2.8

Enolethers ..................................................279

4.2.2.9

Ethers .........................................................279

4.2.2.10 Cyclic and Acyclic Acetals and Ketals .........280 4.2.2.11 Other Classes of Compounds ....................280 4.2.2.11.1 Bis-Alkylaminophenoxy Alkanes .... 280 4.2.2.11.2 Derivatives of 2,2,7,7,-Tetramethyl-1,4-Diazocyclopentane ......281 4.2.2.11.3 Bis-Cyclopentadienyl Compounds ..

v

Ozonation of Organic and Polymer Compounds

281 4.2.2.11.4 Alkylnaphthenes .......................281 4.2.2.11.5 Aminomethylene Derivatives of Furane ......................................282 4.2.2.11.6 Lactams ....................................282 4.2.2.12 Sulfur-Containing Compounds ...................282 4.2.2.13 Si-Containing Compounds .........................285 4.2.2.14 P-Containing Compounds ..........................285 4.2.3

Application .................................................................286

4.2.4

High Molecular Antiozonants ....................................296 4.2.4.1

4.3

Apparatus for Ozone Resistance Determination ............301

4.4

Evaluation of Industrial Stabilisers .................................305

4.5

5

vi

Ethylene-Propylene Rubber With Diene Monomer (EPDM) ...............................................299

4.4.1

Antioxidant Action .....................................................306

4.4.2

Atmospheric Ageing ...................................................308

4.4.3

Antiozonant Action ....................................................308

Prediction .......................................................................312 4.5.1

Oxidation ...................................................................312

4.5.2

Ozonolysis ..................................................................312

4.6

Efﬁciency of Antiozonants Under the Conditions of Various Deformations ..................................................................319

4.7

Effect of Vulcanisate’s Structure ......................................327

Quantum Chemical Calculations of Ozonolysis of Organic Compounds ......................................................................................359

5.1

Alkanes ...........................................................................359

5.2

Oxygen-containing Compounds ....................................364 5.2.1

Water..........................................................................364

5.2.2

Methanol ....................................................................364

Contents

5.3

5.4

5.5

5.2.3

Ethylene Glycol ..........................................................368

5.2.4

Formaldehyde and Acetone ........................................368

5.2.5

Dimethylether .............................................................371

Sulfur-containing Compounds .......................................373 5.3.1

Hydrogen Sulﬁde ........................................................373

5.3.2

Methylsulﬁde .............................................................374

5.3.3

Dimethylsulﬁde ..........................................................375

Nitrogen-containing Compounds ...................................377 5.4.1

Ammonia ...................................................................377

5.4.2

Methylamine ..............................................................380

5.4.3

Dimethylamine ...........................................................381

5.4.4

Trimethylamine ..........................................................384

Phosphorus-containing Compounds ...............................384 5.5.1

Phosphine ...................................................................384

5.5.2

Methylphosphine ........................................................385

5.5.3

Dimethylphosphine ....................................................387

5.5.4

Trimethylphosphine ....................................................388

Abbreviations .............................................................................................395 Index .........................................................................................................399

vii

Ozonation of Organic and Polymer Compounds

viii

I

ntroduction

Introduction The development of ozone chemistry started in 1840 when Schönbein discovered the gas which had a distinctive smell, and which formed near to the electrodes of electrical machines [1, 2]. He called it ‘ozone’, which in Greek means ‘smell’. It is a highly reactive gas - an allotropic modiﬁcation of oxygen containing three atoms of oxygen [3, 4]. Upon studying the reaction of ozone with ethylene, which appears to be the ﬁrst ozone reaction ever performed, it was shown that the ozone reacts with the double bonds, and products of their cleavage are formed. At the beginning of the last century Harries showed ﬁrstly that natural rubber reacts rapidly with ozone. He applied ozone as ‘chemical scissors’ to degrade the natural rubber and on the basis of the cut-off products he determined the elastomer structure and proposed the mechanism of the ozone reaction with the elastomers [5-12]. At present the mechanism of ozone reaction with double bonds, as proposed by Criegee [13-16], is widely accepted. The development of analytical methods opens possibilities for intensive and expanded research on the reactions of ozone with various polymers, elastomers and chemical compounds [17-25]. Since 1958 to the present day, a number of reviews and monographs describing and discussing the various aspects of ozone chemistry have been published [26-37]. The intensive research on this topic is illustrated convincingly by the huge number of publications, more than 10,000 titles in the patent and scientiﬁc literature, covering all aspects of ozone chemistry and physics, ozone preparation, application, storage, decomposition, etc. The study of the kinetics and mechanism of ozone reactions is an important ﬁeld in modern science. It is closely related to the solution of the problem of ‘ozone holes’, the development of physical, organic, inorganic and polymer chemistry and biochemistry in connection with ozone, chemical kinetics, the theory and utilisation of the reactivity of chemical compounds with ozone, the development of new highly efﬁcient technologies for the chemical industry, electronics, ﬁne organic synthesis, the solution of ecological and medical problems by employing ozone, and the degradation

1

Ozonation of Organic and Polymer Compounds of organic, polymer, elastomer and biological materials, etc., and their stabilisation against ozone’s harmful action [39, 40]. The oxidative, antibacterial and antiviral properties of ozone make it very attractive for the puriﬁcation of drinking water, the treatment of natural, waste and process water, artiﬁcial pools, waste gases and contaminated soils, in human and veterinary medicine, sterilisation, etc. Usually ozone can be found in the atmosphere at an altitude of 25-30 km. Here it is generated photochemically and its concentration reaches values of 10-20 ppm. Ozone absorbs ultraviolet (UV) light in the region of 200-300 nm wavelength and protects life on the earth from its harmful effects. The natural concentration of ozone at the ground level is in the range of 0.005-0.01 ppm, but in some cases it may approach values up to 1 ppm. At ground level, ozone is produced by photochemical reactions in the ‘smog phenomenon’, during the decay of some seaweed, during thunder and lightning storms, near high-voltage conduits, and in the vicinity of radiation sources, UV-applying units, lasers and radar units, electrolysis, and galvanic and welding moulding apparatus. The control of all these sources of ozone generation is rather difﬁcult and in many cases they cause ecological pollution of dwellings and work places with harmful ozone levels. Ozone is highly toxic in concentrations greater than 0.1 mg/m3. The presence of ozone in the lower atmospheric layer leads to the appearance of undesirable phenomena related to the earth’s ﬂora and fauna, and degradation of valuable organic, inorganic, polymeric and biological objects, materials and articles. In this connection the harmful effect of ozone on human health, rubber goods, polymeric materials should be outlined [31, 41-77]. The purposeful application of ozone promotes invention and the development of novel and improvement of well-known methods for its generation and analysis, and the means and methods for its more effective application, etc. [31, 61, 69-79]. A number of laboratory and industrial methods for its synthesis have been proposed. The modern ozonation stations have a capacity of 102-103 kg O3/day with ozone concentrations not higher than 5-7% vol. In some particular cases, solid or liquid ozone, or ozone deposited on inorganic supports such as silica gel, or absorbed in freons, etc. [80-89], is used. The joint research work on ozone chemistry has been intensively developing since 1972 [90-92]. We should mention that several hundred publications, reviews, plenary lectures, oral and poster presentations and patents have been published during this time interval. Serious attention has been paid to applied research work leading to the solving of the stabilisation problem of pneumatic tyres and rubber technical goods.

2

Introduction

References 1. C.F. Schonbein, Poggendorff’s Annalen der Physik und Chemie, 1840, 49, 616. 2. C.F. Schonbein, Comtes Rendus Hebdomadaires des Séances de l’Académie des Sciences, 1840, 10, 706. 3. C. Nebel in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, John Wiley, New York, NY, USA, 1981, 16, 650. 4. M. Horwath, L. Vitzky and G. Huttner, Ozone, Academy Kiado, Budapest, Hungary, 1985. 5. H. Staudinger, Chemische Berichte, 1925, 58, 1088. 6. C.D. Harries, Chemische Berichte, 1904, 37, 2708. 7. C.D. Harries, Chemische Berichte, 1905, 38, 1195. 8. C.D. Harries, Chemische Berichte, 1912, 45, 936; 9. C.D. Harries, Untersuchungen über die Naturlichen und Kunstlichen Kautschukarten, Springer Verlag GmbH, Berlin, Germany, 1919. 10. C.D. Harries, Chemische Berichte, 1907, 40, 4905. 11. C.D. Harries, Chemische Berichte, 1908, 41, 1227. 12. C.D. Harries and V. Weiss, Liebig’s Annalen der Chemie, 1905, 343, 369. 13. R. Criegee, Annalen, 1949, 564, 9. 14. R. Criegee, Annalen, 1953, 583, 1. 15. R. Criegee, Record of Chemical Progress, 1957, 18, 111. 16. R. Criegee, Angewandte Chemie, 1975, 21, 765. 17. S. Hatakeyama, H. Lai, S. Gao and K. Murano, Chemistry Letters, 1993, 8, 1287. 18. W.S. Schutt, M.E. Sigman and Y.Z. Li, Analytica Chimica Acta, 1996, 319, 3, 369. 19. M.J. Lee, H. Arai and T. Miyata, Chemistry Letters, 1994, 6, 1069.

3

Ozonation of Organic and Polymer Compounds 20. Y.S. Hon and J.L. Yan. Tetrahedron Letters, 1994, 35, 11, 1743. 21. Y.S. Hon and L. Lu. Tetrahedron Letters, 1993, 34, 33, 5309. 22. R. Atkinson, E.C. Tuazon, J. Arey and S.M. Aschmann, Atmospheric Environment, 1995, 29, 3423. 23. T.L. Rakitskaya, A.Y. Bandurko, A.A. Ennan and V.V. Litvinskaya, Kinetics and Catalysis, 1994, 35, 5, 705. 24. S.W. Mcelvany, J.H. Callahan, M.M. Ross, L.D. Lamb and D.R. Huffman, Science, 1993, 260, 5114, 1632. 25. R. Keshavaraj and R.W. Tock, Advances in Polymer Technology, 1994, 13, 2, 149. 26. S.D. Razumovskii, S.K. Rakovski, D.M. Shopov, and G.E. Zaikov, Ozone and Its Reactions with Organic Compounds, Publishing House of the Bulgarian Academy of Sciences, Soﬁa, Bulgaria, 1983. [in Russian] 27. Ozonation in Organic Chemistry, Volumes I and II, Ed., P.S. Bailey, Organic Chemistry Monographs, Volume 39, Academic Press, New York, NY, USA, 1978 and 1982. 28. S.D. Razumovskii and G.E. Zaikov, Ozone and Its Reactions with Organic Compounds, Elsevier, Amsterdam, The Netherlands, 1984. 29. P.S. Bailey, Chemical Reviews, 1958, 58, 925. 30. Ozone Chemistry and Technology, Advances in Chemistry Series, Volume No.21, ACS, Washington, DC, USA, 1959. 31. Ozone Reactions with Organic Compounds, Ed., P.S. Bailey, Advances in Chemistry Series Volume No.112, ACS, Washington, DC, USA, 1972. 32. G.Y. Ishmuratov, R.Y. Kharisov, V.N. Odinokov and G.A. Tolstikov, Uspekhi Khimii, 1995, 64, 6, 580. 33. B. Dhandapani and S.T. Oyama, Applied Catalysis B - Environmental, 1997, 11, 2, 129. 34. S.K. Rakovsky in Polymer Materials Encyclopedia, Ed., J.C. Solomone, CRC Press, Boca Ratan, FL, USA, 1996, p.1878. 35. ACS Division of the Petroleum Chemistry - Preprints, 1972, 16, 2.

4

Introduction 36. Handbook of Ozone Technology and Applications, Volume 2, Eds., R.G. Rice and A. Netzer, Butterworth Publishers, Boston, MA, USA, 1983. 37. Ozone: Science & Engineering, 1978, 1, 1-4; 38. Ozone: Science & Engineering, 1998, 20, 1-4. 39. D.P. Chock, G. Yarwood, A.M. Dunker, R.E. Morris, A.K. Pollack and C.H. Schleyer, Atmospheric Environment, 1995, 29, 21, 3067. 40. R.D. Bojkov and V.E. Fioletov, Journal of Geophysical Research – Atmosphere, 1995, 100, D8, 16537. 41. J.M. Giovannoni, F. Muller, A. Clappier and A.G. Russell, Tropospheric Modelling and Emission Estimation, 1997, 7, 111. 42. S.M. George and F.E. Livingston, Surface Review and Letters, 1997, 4, 4, 771. 43. M.T. Benjamin and A.M. Winer, Atmospheric Environment, 1998, 32, 1, 53. 44. B. Frakes and B. Yarnal, International Journal of Climatology, 1997, 17, 13, 1381. 45. L. Coy and R. Swinbank, Journal of Geophysical Research - Atmosphere, 1997, 102, D22, 25763. 46. W.R. Stockwell, F. Kirchner, M. Kuhn and S. Seefeld, Journal of Geophysical Research - Atmosphere, 1997, 102, D22, 25847. 47. B. Ramacher, J. Rudolph and R. Koppmann, Tellus B – Chemical and Physical Meteorology, 1997, 49, 5, 466. 48. R. Sander, R. Vogt, G.W. Harris and P.J. Crutzen, Tellus B – Chemical and Physical Meteorology, 1997, 49, 5, 522. 49. P.A. Ariya, V. Catoire, R. Sander, H. Niki and G.W. Harris, Tellus B – Chemical and Physical Meteorology, 1997, 49, 5, 583. 50. J. Rudolph, B. Ramacher, C. Plassdulmer, K.P. Muller and R. Koppmann, Tellus B – Chemical and Physical Meteorology, 1997, 49, 5, 592. 51. Bulletin of the American Meteorological Society, 1997, 78, 11, 2681. 52. P.A. Newman, J.F. Gleason, R.D. McPeters and R.S. Stolarski, Geophysical Research Letters, 1997, 24, 22, 2689.

5

Ozonation of Organic and Polymer Compounds 53. R.B. Pierce, T.D. Fairlie, E.E. Remsberg, J.M. Russell and W.L. Grose, Geophysical Research Letters, 1997, 24, 22, 2701. 54. J.P. McCormack, L.L. Hood, R. Nagatani, A.J. Miller, W.G. Planet and R.D. McPeters, Geophysical Research Letters, 1997, 24, 22, 2729. 55. M.K. Dubey, G.P. Smith, W.S. Hartley, D.E. Kinnison and P.S. Connell, Geophysical Research Letters, 1997, 24, 22, 2737. 56. S. Solberg, T. Krognes, F. Stordal, O. Hov, H.J. Beine, D.A. Jaffe, K.C. Clemitshaw and S.A. Penkett, Journal of Atmospheric Chemistry, 1997, 28, 1-3, 209. 57. R. Schmitt and A. Volzthomas, Journal of Atmospheric Chemistry, 1997, 28, 1-3, 245. 58. C. Nevison and E. Holland, Journal of Geophysical Research - Atmospheres, 1997, 102, D21, 25519. 59. Proceedings of the World Environmental Conference, Rio de Janeiro, Brazil, 1992. 60. Proceedings of the World Environmental Conference, Kyoto, Japan, 1998. 61. M.K.W. Ko, N.D. Sze and M.J. Prather, Nature, 1994, 367, 6463, 505. 62. V.A. Basiuk, Uspekhi Khimii, 1995, 64, 11, 1073. 63. V. Cataldo and O. Ori, Polymer Degradation and Stability, 1995, 48, 2, 291. 64. R.W. Murray in Techniques and Methods of Organic and Organometallic Chemistry, Ed., D.B. Denney, Marcel Dekker, New York, NY, USA, 1969, p.1-32. 65. Proceedings of the Second International Symposium on Ozone Technology, Eds., R.G. Rice, P. Pichet, M-A. Vincent, Montreal, Quebec, Canada, 1975. 66. M.J.S. Dewar, J.C. Hwang and D.R. Kuhn, Journal of the American Chemical Society, 1991, 113, 3, 735. 67. J.S. Belew in Oxidation: Techniques and Applications in Organic Synthesis, Volume 1, Ed., R.L. Augustine, Marcel Dekker, New York, NY, USA, 1969. 68. R.L. Kuczkowski, Chemical Society Reviews, 1992, 21, 79. 69. J. Prousek and A. Klcova, Chemicke Listy, 1997, 91, 8, 575. 6

Introduction 70. C.A. Zaror, Journal of Chemical Technology and Biotechnology, 1997, 70, 1, 21. 71. B.I. Tarunin, V.N. Tarunina, M.N. Klimova and M.Y. Ratkova, Zhurnal Obshchei Khimii, 1997, 67, 5, 806. 72. E. Otal, D. Mantzavinos, M.V. Delgado, R. Hellenbrand, J. Lebrato, I.S. Metcalfe and A.G Livingston, Journal of Chemical Technology and Biotechnology, 1997, 70, 2, 147. 73. F.J. Benitez, J. Beltranheredia, J.L. Acero and M.L. Pinilla, Journal of Chemical Technology and Biotechnology, 1997, 70, 3, 253. 74. B.J. Finlaysonpitts and J.N. Pitts, Science, 1997, 276, 5315, 1045. 75. H.K Roscoe and K.C. Clemitshaw, Science, 1997, 276, 5315, 1065. 76. F.H. Tiefenbrunner, H.G. Moll, A. Grohmann, D. Eihelsdorfer, K. Seidel and G. Golderer, Ozone: Science Engineering, 1990, 12, 393. 77. F. Hegeler, and H. Akiyama, Japanese Journal of Applied Physics, Part 1, 1997, 36, 8, 5335. 78. S. Rakovski, V. Podmasterev, S. Razumovskii and G. Zaikov, International Journal of Polymeric Materials, 1991, 15, 2, 123. 79. R.A. Kerr, Science, 1996, 271, 5245, 32. 80. D.P. Uﬁmkin, V.V. Lunin, L.V. Sabitova, L.N. Burenkova, V.A. Voblikova and S.N. Tkachenko, Zhurnal Fizicheskoi Khimii, 1995, 69, 11, 1964. 81. B Dhandapani and S.T. Oyama, Chemistry Letters, 1995, 6, 413. 82. A. Naydenov, R. Stoyanova and D. Mehandjiev, Journal of Molecular Catalysis A - Chemical, 1995, 98, 1, 9. 83. I.V. Martynov, V.I. Demidyuk, S.N. Tkachenko and M.P. Popovich, Zhurnal Fizicheskoi Khimii, 1994, 68, 11, 1972. 84. E. Merz and F. Gaia, Ozone: Science and Engineering, 1990, 12, 401. 85. J. Kitayama and M. Kuzumoto, Journal of Physics D - Applied Physics, 1997, 30, 17, 2453. 86. V.A. Basiuk, Uspekhi Khimii, 1995, 64, 11, 1073.

7

Ozonation of Organic and Polymer Compounds 87. S. Rimmer, J.R. Ebdon and M.J. Shepherd, Reactive & Functional Polymers, 1995, 26, 1-3, 145. 88. L.V. Ruban, S.K. Rakovsky and A.A. Popov, Izvestiya AN SSSR Seriya Khimicheskaya, 1976, 40, 9, 1950. 89. D.P. Uﬁmkin, V.V. Lunin, L.V. Sabitova, L.N. Burenkova, V.A. Voblikova and S.N. Tkachenko, Zhurnal Fizicheskoi Khimii, 1995, 69, 11, 1964. 90. S.K. Rakovsky, Kinetics and Mechanism of the Ozone Reactions with Parafﬁns in Liquid Phase, Institue of Chemical Physics, Moscow, 1975. [PhD Thesis] 91. M.P. Anachkov, Kinetics and Mechanism of the Ozone Reaction with Diene Rubbers in Solution, Institute Neftekhimii, Moscow, 1982. [PhD Thesis] 92. Proceedings of the 1st International Micro Symposium on Ozone Degradation of Polymers, Soﬁa, Bulgaria, 1984.

8

1

Kinetics and Mechanism of Ozone Reactions with Organic and Polymeric Compounds in the Liquid Phase

The oxidation of low molecular weight hydrocarbons by ozone is one of their few reactions that takes place both at low and ambient temperatures and transforms them into oxygen-containing products [1-12]. This reaction has been investigated by a number of authors [13-15]. The action of ozone as an initiator of oxidation processes [16-18], a modifying polymer agent [19-21] and an oxidiser in the preparation of alcohols, ketones and some other compounds is of special interest [22-24]. The kinetics and mechanism of this reaction have been intensively discussed [25-46]. Our investigations [47-64] are also dedicated to these problems.

1.1 Reaction mechanisms The ﬁrst studies of ozone reactions with parafﬁns were carried out in the last century using the examples of methane, ethane, propane and butane in the gas phase [2-8]. It was established that upon the absorption of 1 mole of ozone 1 mole of ﬁnal product is formed. These authors proposed two different mechanisms of ozone action: ozone is decomposed into atomic oxygen, which initiates the oxidation process [2] (M.1.1) and ozone interacts directly with the alkane [3-5] (M.1.2). O 3 = O2 + O RH + O m Rv + vOH

(M.1.1)

RH + O3 m ROv + vO2H

(M.1.2)

Later it was shown that mechanism M.1.1 can be realised only in the gas phase at temperatures >>50-60 oC [28, 29]. Thus this reaction is of no importance in the liquid phase whereby the reactions are usually carried out at ≤50-60 oC. On the other hand mechanism M.1.2 assume the formation of H2O2 in quantities proportional to the absorbed amount of ozone, but really in the liquid phase only a small amount of hydroperoxide is formed [1] which means that the role of the mechanism in M.1.2 in ozonation of alkanes is negligible.

9

Ozonation of Organic and Polymer Compounds The ozonolysis of alkanes in the gas phase leads to the formation of excited species, which determines the speciﬁc character of these reactions [65]. The ﬁrst studies in the liquid phase were carried out by Azinger and co-workers [16] on ozonation of octadecane. They came to the conclusion that ozone directly attacks the CH2 groups. Upon ozonolysis of decalin and adamantane the tert-CH groups are attacked, leading to the formation of the respective tert-alcohols. In cyclohexane, ozone reacts with the CH2 groups giving rise to cyclohexanol and cyclohexanone formation [16]. The kinetics of intermediate and end-product formation has been studied and the mechanism of ozone reaction with tetradecane is suggested to be [66]: RH + O3 m RO2v + vOH

(M.1.3)

Although the basic reaction in M.1.3 is exothermal its occurrence in two steps appears to be more entropy favourable than in one as assumed by this mechanism in regard to entropy. Hamilton and co-workers [6, 67] consider the most probable mechanism to be the ozone attack on the CH bonds via a 1,3-transition state (M.1.4): RH + O3 TS m [Rv + HO3v Rv + HO3 ] m ROOOH (1) (M.1.4) n ROH + O2 (s) (2);

cage

n

Rvl+ lvOH + O2 (t) (3)

n RO2v + O2, which is followed by H-atom abstraction and hydrotrioxide (1), alcohol, singlet oxygen (2), peroxy radical and triplet oxygen (3) leaving the kinetic cage in the bulk liquid phase. They postulate that the products in the cage have a radical character, while according to Myurei [68] they are more probably ionic pairs. Nangiya and Benson [28] assume the mechanism M.1.4 to be more probable, but the interaction between ozone and CH bonds is not synchronised.

10

Ozonolysis of low and high molecular weight saturated hydrocarbons Denissov and co-workers [26] considered the reaction of ozone with C-H bonds within the framework of the parabolic model of the potential surface and they also adhere to mechanism M.1.4. Another view of the mechanism of alkane ozonolysis is that Rv, HOv radicals are kinetically responsible for the reaction proceeding and that ROH, Rv, HOv and O2 leave the kinetic cage into the solution (M.1.5) [51]: RH + O3 m [Rv + HOv + O2]

(M.1.5)

However, one cannot exclude the possibility of RO2v radical formation from ROOOH as a precursor via the mechanism of ozone insertion into the CH bonds (M.1.6): RH + O3 m ROOOH

(M.1.6)

ROOOH could act as a precursor in all the mechanisms discussed in the literature [9, 67]. This reaction is probably entropically disadvantageous, because it goes through a cyclic transition state and it is necessary to assume a ﬁve-fold coordinated C atom.

1.1.1 Thermochemical analysis The heat of ozone reactions with parafﬁns via different mechanisms are calculated using the example of methane ozonation. Experimental and calculated (?) values of the bond energies and heat of formation [69, 70] have been used: CH3-H = 104 kcal/ mol; OO-O = 24; CH3O-H = 102; CH3OO-H = 77?; CH3OOO-H = 70?; CH3OOOH = 51?; CH3O-OOH = 47?; CH3-OH = 91; CH3-OOH = 69?; CH3-OOOH = 61?; H-Ov = 102; H-OOv = 77; H-OOOv = 62; C-Ov = 78; C-OOv = 59; and C-OOOv = 48 kcal/mol and the heat of the reactions (Q) in the various mechanisms are as shown in Table 1.1.

Table 1.1 Heats of ozone reactions (Q) Mechanism Q (kcal/mol)

M.1.1 26

M.1.2 -27

M.1.3 -33

M.1.4 66

M.1.5 26

M.1.6 -5

For alkanes with secondary and tertiary C-H bonds with lower bond energies, the Q values for each reaction will be higher by 10-15 kcal.

11

Ozonation of Organic and Polymer Compounds

1.2 Structural-kinetic investigations As we have shown above there are two approaches concerning the interaction of ozone with parafﬁns: ozone insertion in the CH bonds and H-atom abstraction. We should note, however, that the effect of parafﬁn composition in this reaction has not been studied systematically, which limits the possibilities for analysis and elaboration of the mechanism of these reactions [25, 59]. The variety of the proposed mechanisms and the insufﬁcient kinetic data means a systematic investigation of the reaction will need to cover a wide spectrum of conditions and theoretical and experimental research methods.

1.2.1 Normal parafﬁns and isoparafﬁns We have studied the kinetics of ozone reactions with a series of parafﬁns of different structure [47, 59]. The rates of the reactions (W) have been determined at stationary and dynamic conditions. By following the change of [O3] with time (t) under static conditions we have determined the rate constant of the pseudomonomolecular reaction ka = k × [RH] and the bimolecular constant – k: W = d [O3]/dt = k [RH]0.[O3] = ka [O3]

(1.1)

The constant ka has been determined during the half-decomposition time interval: ka= ln2/T1/2

(1.2)

or based on the slope of the curve in coordinates ln([O3]o/[O3])/t. The reaction in an open system has been carried out by bubbling ozone through the parafﬁn phase and following spectrophotometrically its concentration in the ultraviolet (UV) region (254-300 nm). The course of the concentration change has the following pattern: ﬁrst it drops down abruptly from [O3]o, the concentration at the reactor inlet, to [O3]g, the concentration at the reactor outlet, after which for a long time it remains parallel to the x-axis, at a distance of $[O3] = {[O3]o – [O3]g}. The analysis of the kinetic curves has been done on the basis of its stationary part, when the rate of the chemical reaction becomes equal to the rate of ozone consumption W = WO3, or: k.[RH].[O3]l = W.$[O3]

(1.3)

from which it follows that: k = W.$[O3]/([RH].[O3]l) 12

(1.4)

Ozonolysis of low and high molecular weight saturated hydrocarbons where W = v/V is the relative rate, v is the rate of the gas flow, V is the volume of the liquid phase; in one of the models used [O 3 ] l = A × [O 3 ] 9 the ozone concentration in the liquid, A is Henry’s coefﬁcient [31]. This model is valid in all cases when the rate of ozone absorption is greater than the rate of the chemical reactions. Several criteria could by applied if Henry’s law is observed in the course of absorption and there proceeds an irreversible ﬁrst-order chemical reaction (k1): k1.T << 1, where T = 1/W, or DO3.k1/kL2<<1 where DO3 is the diffusion coefﬁcient of ozone in solution; kL= DO3/D is the coefﬁcient of mass transfer in liquid phase; and D is thickness of the boundary layer in the hydrodynamic model renovation surface; or kL=(DO3.s)1/2, where s is the time interval of renovation; in the bubbling method with small bubbles with diameters up to 2.5-3 mm kL = 0.31 × (gN)1/3 × (DO3/N)2/3, where N = H/R is the kinematic viscosity of the solvent, H is the viscosity of the solvent, R is the solvent density, g is the earth acceleration. Usually the kL value is 0.1-0.05 cm/s. In all other cases [O3]l is a complicated function of [O3]g and [O3]0 and it requires special investigation [54]. Conventionally, v = 0.1 l/min, V = 10 ml and W = 0.167/s, [O3]o z 10-6-10-3 M, the solvent is CCl4, for which A z 1.8-3, [RH] z 10-3-101 M depending on the temperature and for the parafﬁns A z 1.8-2.5. The bubbling reactors are made of glass with a grating G2, G3 or G4 built in their lower end, with a cylindrical form and the inner diameter F = 1.7-3.7 cm and height h = 7-15 cm. The accuracy of thermal control was p 0.1 oC. By applying the kinetic methods specified above we studied the influence of concentration, composition, solvent nature and temperature on the kinetics of a deﬁnite reaction. The rates of the studied reaction were linearly dependent on [O3] and, with some exceptions, linearly dependent on [RH] (e.g., adamantane in the ﬁrst case and 3-methylpentane in the second case) (Figure 1.1). A linear dependence of 1/kobs on [RH] for 3-methylpentane has been observed while the 1/kobs values for adamantane do not depend on [RH] at [RH] < 0.3 M and on the molar ratio of [RH]/[O3] <1 × 104. During, ozonation of n-octane, n-pentane, 3-methylpentane and iso-pentane, Williamson and Cvetanovich [17] observed an increase of reaction rate (W) at low [RH] and ratios [RH]/[O3] <1 × 104. They concluded that ozone interacts with some microimpurities that initiate the chain reactions.

13

Ozonation of Organic and Polymer Compounds

9 3-methylpentane

kobs-1, M.s

8

7

6

adamantane

5 0

10

20

30

40

[RH], mM

Figure 1.1 The dependence of 1/kobs on [RH] Our explanation of the dependence of 1/kobs on [RH] is that the reaction passes through a complex multistep pathway. To clarify the possible pathways of the reaction several kinetic schemes are applied. For example Scheme 1.1 provides a good agreement between the applied kinetic and chemical criteria and the experimental data which includes the formation of a complex [R...H...O3]. RH + O3

k1

R...H...O3

k2

products

k -1

R...H...O3

+ RH

k3

2RH + O3

Scheme 1.1 We have studied the case when the equilibrium concentration of the complex is not affected by the occurrence of reactions 2 and 3. The running concentration of the complex can be speciﬁed by the ozone concentration as follows: [R...H...O3] = [O3]0 – [O3] , Under quasistationary conditions in relation to [R...H...O 3] and taking into consideration the material balance, we obtain for kobs: kobs = k1k2/{k-1 + k2 + (k1 + k3) [RH]},

14

(1.5)

Ozonolysis of low and high molecular weight saturated hydrocarbons and in reciprocal form: 1/kobs = (1/k1 + k-1/k1k2 ) + {(k1 +k3)/k1k2}.[RH]

(1.6)

It follows from Equation 1.6 that 1/kobs will increase linearly with the increase of [RH]. From the ordinate intercept one can determine (1/k1 + Kd/k2 ), where Kd = k-1/ k1 is the dissociation constant of the complex and from the slope angle the value of the ratio (k1 + k3)/k1k2. In our calculations we have assumed that k2<< k-1 according to Scheme 1.1. Hence the intercept gives Kd/k2 and at k1>>k3 the 1/k2 value can be found from the slope. The last assumption transforms Scheme 1.1 into a mechanism of the Michaelis-Menten type (Scheme 1.2). RH + O3

k1

R...H...O3

k2

products

k -1

Scheme 1.2 Then Equation 1.5 is transformed into: kobs = k1k2/(k-1 + k2 + k1[RH]),

(1.7)

and Equation 1.6 becomes: 1/kobs = (1/k1 + k-1/k1.k2 ) + (1/k2 )[RH]

(1.8)

In the case when the equilibrium constant of [R...H...O3] varies as a result of the proceeding of reactions 2, 1 and 3, 1/kobs will be equal to: 1/kobs = (1/k1 + k-1/k1k2 ) + k3/k1k2.[RH]

(1.6a)

and 1/k 1 will be determined from the intercept as K d << k 2 , and k 3 /k 1 k 2 from the slope, respectively. In order to assess the validity of the kinetic schemes we have determined experimentally the rate constants of ozone interactions with 24 parafﬁns of various structures. The rate constants, depending on [RH], are given in Table 1.2 whereby the constants calculated from the ordinate intercepts are shown in column 3 and those obtained from the slope according to Equations 1.6a and Equation 1.8 are shown in column 4.

15

Ozonation of Organic and Polymer Compounds

Table 1.2 Rate constants of ozone reactions with parafﬁns in CCl4 at 20 oC [47] No.

Hydrocarbon

k p 10% (M-1. s-1)

(k3/k2)/k2 (M-1)

1.

n-Pentane

0.015

0.001/15

2.

n-Hexane

0.019

0.0013/15

3.

n-Heptane

0.021

0.0012/17

4.

n-Octane

0.023

-

5.

n-Nonane

0.026

-

6.

n-Decane

0.029

<0.001<29

7.

n-Tetradecane

0.036

-

8.

n-Octadecane

0.048

<0.001/<48

9.

3-Me-pentane

0.15

0.75/0.2

10.

2,3-Di-Me-pentane

0.29

11/0.026

11.

2,4-Di-Me-pentane

0.08

-

12.

2,3,4-Tri-Me-pentane

0.59

40/0/015

13.

2-Methyl-hexane

0.13

5.2/0.025

14.

3-Me-hexane

0.20

7.4/0.027

15.

2,2-Dimethyl-hexane

0.015

<<0.001/<<15

16.

2,4-Dimethyl-hexane

0.13

-

17.

2,2,5-Trimethyl-hexane

0.19

9.0/0.021

18.

3-Methyl-heptane

0.20

9.0/0.022

19.

Hexa-methyl-ethane

0.0002

-

20.

Ethyl-n-propane

0.05

-

21.

Cyclohexane

0.01

<<0.001/<<14

22.

Norbornane

0.014

<<0.001/<<14

23.

Adamantane

0.22

<<0.1/<<2.2

24.

Isooctane

0.14

-

The analysis of the data in Table 1.2 shows that the kinetic models, most probably do not conform to the experimental results although they describe formally some of the experimental observations in Figure 1.1 and Table 1.2 For some of the representatives

16

Ozonolysis of low and high molecular weight saturated hydrocarbons of n-, iso- and cyclo-parafﬁns no dependence of 1/kobs on [RH] has been established (e.g., compounds 4, 5, 7, 11, 16, 19, 20, 24) or this dependence is within the limits of the graphic presentation, as for compounds 1, 2, 3, 6, 8, 15, 21, 22 and 23. A good correlation has been found for compounds 9, 10, 12, 13, 14, 17, 18 as all these are iso-parafﬁns with 1, 2 or 3 CH3 groups in the side chains. These isomers always contain microimpurities that are hardly separable, which have higher reactivity than the parent parafﬁns and they can affect the processes of di-, tri-merisation of the hydrocarbon molecules. It is seen that the values of k1 depend on the structure and type of the C-H bonds in the alkane molecule - for the primary bonds (I) it is 0.0002 (no.19), for the secondary bonds (II), 0.048 (no.8) and for the tertiary (III), 0.20 (no.18). These bonds differ in their energies which have a ratio of 1:0.98:0.96 [69, 70]. The effect of the bond energies assumes that the C-H bond in the transition state is highly elongated [75]. The enthalpy is within the limits of 0-10 kcal. According to Hammond’s rule under these conditions the transition state would be more similar to the ﬁnal product than to the starting compound which means that the reaction coordinate has a relatively high value. This justiﬁes to a great extent the assumption that the dependence of 1/kobs on [RH] is not so much related to the chemistry of the reaction but rather to the experimental conditions because it is observed only in some cases and it is concentration limited. In addition, W has a constant value in solvents of various natures (CCl4, CHCl3, CH3CN, decane). In the ozonation of parafﬁns the activation energies approach values of 14-15 kcal/mol, while reactions through a complex formation should have Ea <10 kcal/mol. The UV spectrum of ozone in n-hexane solution is similar to that in the gas phase. The insigniﬁcant bathochromic shift of 6 nm does not lead to a broadening of the absorption band. The hypsochromic effect leads to a change in the molecular absorbance (E) by 10% from 1820 to 2002 M-1.cm-1. The blue colour of ozone is preserved in the n-hexane solution. The infrared (IR) spectra of the gaseous ozone and of its solution in n-hexane are practically identical and new bands are not identiﬁed. In the nuclear magnetic resonance spectra of the n-hexane solutions of ozone at a temperature of 80 oC and after a long time no changes in the spectrum of n-hexane are observed. Only upon ozonolysis of oleﬁns at 100 oC in the IR cell and the analysis of the IR spectra, is the formation of P-complexes presumed [76, 77]. Thus the reaction of ozone with parafﬁns is more likely to be a bimolecular process, excluding preliminary complex formation, which leads directly to the formation of the reaction products: RH + O3 m products (k)

17

Ozonation of Organic and Polymer Compounds and then the data extrapolated to [RH]m0 in column 3 of Table 1.2 will correspond to a bimolecular constant k. We have assumed that the reaction of ozone with the homologous series of n-parafﬁns is isoentropic and the differences in their reactivities are determined by the energies of the C-H bonds. In order to eliminate random factors we have related the relative rate constants to that of n-hexane: kn/k6 = exp (-0.3$D/RT)

(1.9)

where $D = Dn – D6 is the difference between the energies of C-H bonds in kcal/ mol; n is the carbon number for the corresponding parafﬁn; 0.3 is the empirical coefﬁcient. At 293 K, R = 1.99 cal/(mol.K) and k6 = 0.019 M-1.s-1: kn = 0.019.exp (-0.514.$D)

(1.10)

The energies of C-H bonds for primary (I), secondary (II) and tertiary (III) C atoms are calculated according to the following equations [70]: DI = 93.6 + 830.4 f, g, i, j,...

(1.11)

DII = 85.6 + 830.4 f, g, i, j,...

(1.12)

DIII = 77.6 + 830.4 f, g, i, j,...

(1.13)

where f, g, i, j,... denote the number of C atoms in each linear hydrocarbon chain bonded to the C atom, where at the energy of the C-H bonds is calculated. The k values estimated by Equation 1.10 for parafﬁns comprising only primary C-H bonds (e.g., methane, neopentane) are within the limits 0.00078 w 0.000013; with secondary C-H bonds (n-parafﬁns) - 0.48 w 0.00078 and with tertiary C-H bonds (sec-butane, alkylsubstituted n-parafﬁns) - 2.93 w 0.048. The ratio in between to the maximum and minimum rates is 1:61:3756 and 1:60:3692, respectively. The calculated values of k and D of the C-H bonds for some parafﬁns are presented in Table 1.3 and a comparison with the experimental data has been made. It is seen that the calculated values of k and D are in a very good agreement with the experimental results of Table 1.2 and are also in a complete conformity with the data from the literature [2-15]. A slight difference is noticed only for hydrocarbons containing primary C-H bonds. Because of their lower reactivity insigniﬁcant amounts of microimpurities might increase the experimental values of k. Thus the

18

Ozonolysis of low and high molecular weight saturated hydrocarbons difference between the calculated and the experimental values of k could be used as a measure for checking the purity of parafﬁns.

Table 1.3 Calculated and experimental kinetic and thermodynamic parameters of ozone reactions with some parafﬁns No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

Parafﬁn Methane Ethane n-Propane n-Butane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane n-Undecane n-Dodecane n-Tridecane n-Tetradecane n-Octadecane Polyethylene 3-Me-pentane 2,3-Di-Me-pentane 2,4-Di-Me-pentane 2,3,4-Tri-Me-pentane 2-Me-hexane 3-Me-hexane 2,2-Di-Me-hexane 2,4-Di-Me-hexane 2,2,5-Tri-Me-hexane 3-Me-heptane Ethylcyclopropane Neopentane Hexamethylethane Cyclohexane Norbornane Adamantane 2,2,4-Tri-Me-pentane

kcal/kexp (M-1.s-1) 1.2 × 10-5/6.6 × 10-4 1.52 × 10-4/6.3 × 10-3 1.8 × 10-3/3.4 × 10-3 5 × 10-2/5.8 × 10-3 0.013/0.015 0.019/0.019 0.029/0/021 0.033/0.023 0.039/0.026 0.041/0.029 0.043 0.046 0.046 0.047/0.036 0.048/0.048 0.048 0.15/0.15 0.08/0.29 0.06/0.08 0.04/0.59 0.10/0.13 0.23/0.20 0.01/0.015 0.17/0.13 0.08/0.19 0.26/0.20 0.15/0.05 1.12 × 10-4/3.8 × 10-5 0.00016/0.00002 0.01/0.01 0.014/0.014 0.22/0.22 0.05/0.14

DC-H (Kcal) 101.6 96.8 92 90 88.2 87.4 86.6 86.3 86 85.9 85.8 85.7 85.66 85.66 85.607 85.6 83.36 84.64 85.02 85.92 84.20 82.59 88.42 83.10 84.61 82.28 88.6? 97.4 96.67 88.6? 99? 82.6? 85.54

19

Ozonation of Organic and Polymer Compounds The energies of C-H bonds, given in column 4 of Table 1.3 for linear and branched parafﬁns are calculated based on Equations 1.11-1.13 and for cycloparafﬁns 27, 30 and 32 from the rate constants. The latter approach can be regarded as a very good method for determination of thermodynamic parameters on the basis of kinetic data. The good correlation between the calculated and experimental values of k testiﬁes that the H-atom abstraction mechanism is more adequate for describing the interaction of ozone with alkanes.

1.2.2 Cycloparafﬁns The kinetics and mechanism of ozone reactions with cycloparafﬁns ranging from cyclopentane to cyclododecane have been studied. From a thermodynamic point of view cycloparafﬁns contain only equivalent C-H bonds and their energies, calculated according to Equations 1.11-1.13, using a model of inﬁnite chain, are 88.9-88.5 kcal/ mol, and for cyclopropane and cyclobutane they are 91 and 89.5 kcal/mol, respectively. The literature values for the former compound are within the limits of 94 ± 3 kcal/ mol and for the latter they are 100.4 ± 2 and 95 ± 3 kcal/mol [28]. The deviations of the calculated and literature values could be attributed to the approximations of the chosen model which considers both the small and the big cycles as inﬁnitely large. In both cases, however, it could be stated that after cyclobutane, D does not actually depend on the cycle size. From a stereochemical point of view, however, the cycloparafﬁns differ considerably with respect to number, stability and symmetry of the conformers in which they exist, to the number of equatorial, axial, pseudoequatorial and pseudoaxial, exo- and endo-H atoms, to the deviations from the equilibrium values of the bond lengths, valent and torsion angles and steric hindrance in the cycles. Upon ozonation the energy of the system will either decrease and the reaction will be accelerated or increase and the reaction will be retarded and the reactivity of cycloparafﬁns would be different irrespective of the similar energies of the C-H bonds in them. The rate will also be determined by the product of the numbers of symmetry S: j

S = Ï Cv i=1

where CN is the axis of symmetry; N is the order of identity per one motion; and j is the number of axes of symmetry (e.g., cyclopropane has one axis of symmetry of third order or three-fold C3N and then S = 3, cyclobutane has three axes of symmetry of second order C2N and S = 8, and cyclohexane has one axis of third order C3N and one of second C2N and S = 6) of the initial compounds and of the activated complex (AC), and under other conditions, the increase of symmetry will increase W.

20

Ozonolysis of low and high molecular weight saturated hydrocarbons Using the PCMOD4 program based on molecular mechanics methods [78, 79], we calculated the steric energies (SE) of cycloparafﬁns and their radical forms (c-CnH2n and c-CvnH2n-1). First, we carried out calculations on the various stereoisomers and selected the conformer with the lowest energy (e.g., ‘envelope’ for C5 and ‘chair’ for C6) and SERH. Then we simulated the H-atom abstraction at consecutive moves of the reaction centre at each nonequivalent C-H bond in this conformer and thus we selected the model with minimum SER. Five calculations were made for cyclo-C5, 6 for cyclo-C6, 7 for cyclo-C7, etc. Thus by the difference $SE = SERH–SER we estimated whether or not the transition sp3 m sp2 is favourable. The results from the calculations are given in Table 1.4.

Table 1.4 Conformations, numbers of symmetry (S), calculated SERH and SER, $SE and SERH and the experimental SER of cycloparafﬁns S

SERH (kcal/ mol)

SER (kcal/ mol)

$SE (kcal/ mol)

SERH (exp) (kcal/mol)

Propane

3

26.2

39.9

-13.7

27.6

2.

Butane

4

24.0

21.7

2.3

26.4

3.

Pentane

1

5.5

3.4

2.1

6.5

4.

Hexane

6

0.0

0.0

0.0

0

5.

Heptane

2

7.1

4.1

3.0

6.3

6.

Octane

8

12.5

8.0

4.5

9.6

7.

Nonane

1

15.6

9.8

5.8

12.6

8.

Decane

10

15.3

9.3

6.0

12.0

9.

Undecane

1

14.6

9.9

4.7

-

10.

Dodecane

16

10.2

6.2

4.0

3.6

No.

c-Cn

1.

Conformation

Note: The designations of the conformations of cycloparafﬁns from top to bottom are: triangle, plane square, envelope, chair, twist chair, twist crown, twist crown, twist crown, twist crown and square with 4 butane segments in chair conformation. For the cycles bigger than C7, the conformations shown are among those which differ by not more than 1 kcal/mol but have the highest symmetry. Under real conditions the intrinsic (real) number of symmetry is between 1 and its maximum value and it will be determined by the share of each conformer: we have looked for a coincidence between the theoretical and the experimental values of SE [80]; we have calculated SERH.(exp) from the thermochemical values of the heats of combustion of cycloparafﬁns in the gaseous phase according to the expression (Qn/n-Q6/6), where n is the number of methylene groups in the cycle, and Qn and Q6 are the heats of combustion of the cycloparafﬁns and cyclohexane, respectively.

21

Ozonation of Organic and Polymer Compounds The reaction with cyclopropane and cyclohexane appears to be the slowest due to the increase in energy cycle upon the sp3 m sp2 transition. In the other cases the energy decreases and this leads to a rise of the reaction rates. The parameters of cycloalkane ozonolysis [48, 51] obtained experimentally are depicted in Table 1.5.

Table 1.5 Kinetic parameters of ozone reaction with cycloparafﬁns (20 oC, CCl4) Parameters 3

k × 10 p 10% (M-1.s-1) A × 10-7 (M-1.s-1) Ea p 0.5 (kcal/mol)

Pentane

Hexane

Heptane

Octane

Nonane

Decane

Dodecane

24.9

10.0

229.0

480.0

890.0

408.0

62.0

2.7

38.2

2.2

1.7

24.3

170.0

168.0

12.2

14.3

10.8

10.2

11.4

13.0

14.1

For the elimination of some factors which are common for the reactivity of cycloparafﬁns we have used the values of the relative constants: kn/k6 = (An/A6).exp(-$E/RT)

(1.14)

where n is the number of carbon atoms in the cycle; $E = En-E6 is the difference between the activation energies of the n-th cycloparafﬁn and that of cyclohexane. Taking into consideration the experimental data in Table 1.2 the values of the parameters in the right term of Equation 1.14 are shown in Table 1.6: The decrease of $E to the left and to the right from cyclohexane indicates the sp3 m sp2 transition in cyclohexane is energetically most unfavourable. The variations in An/A6 and $E show the occurrence of the compensation effect (CEF), which in this particular case could be ascribed to the different SE of the cycles and to their symmetry.

Table 1.6 Values for parameters in Equation 1.14 C5

C6

C7

C8

C9

C10

C12

$E (kcal/mol)

–2.1

0

–3.5

–4.1

–2.9

–1.3

–0.2

Exp(-$E/RT)

35.9

1

383.7

1064.2

138.4

9.1

1.4

An/A6

0.071

1

0.058

0.045

0.636

4.45

4.34

22

Ozonolysis of low and high molecular weight saturated hydrocarbons On the basis of the expression for the rate constant in the theory of the AC including the entropy of the activation complex [81, 82]: k = e2.(RT/Nah).exp($S/R).exp(-$E/RT)

(1.15)

where e is the base of natural logarithm; Na is Avogadro’s number; h is Planck’s constant. The values of $S have been calculated in Table 1.7.

Table 1.7 Values of $S $S [cal/ (mol.K)]

C5

C6

C7

C8

C9

C10

C12

–19.46

–14.46

–19.87

–20.39

–15.06

–11.17

–11.20

The values of entropy increase from small, C5, to medium, C7-C9 cycles, and are lower for the large C10 and C12 but even in the latter case the rate constants are higher than that of cyclohexane. The combined solution of Equations 1.14 and 1.15 leads to: kn/k6 = exp($$S/R).exp(-$E/RT)

(1.16)

where $$S = $Sn - $S6. The change of $$S and $E is additional evidence for the occurrence of CEF. We have found an empirical relationship between the reactivity of cycloparafﬁns and their steric energy (Equation 1.17) [48]: log(kn/k6) = X.(SE)1/2

(1.17)

where X is an empiric coefﬁcient and SE is the steric energy. Thus by applying Equation 1.17 (the data for the steric energies are available) it is possible to evaluate k for new compounds. The experimental dependence of the reactivity of cycloalkanes on their steric energy is shown in Figure 1.2.

23

Ozonation of Organic and Polymer Compounds

1

C9

2.0

C8

lg(kn/k6)

1.5

C10

C7

1.0

C4 C3

C12

2

C5

0.5

C6 0.0 0

1

2

3

4

5

6

1/2

(SE) , kcal

Figure 1.2 The dependence of the cycloalkanes reactivity on their steric energy. 1 for cycles > C6; 2 - for cycles < C6

It follows from Figure 1.2 that a = 0.54 and 0.16 for curves 1 and 2, respectively. The experimental point for C10, which deviates from the straight line by 16% (the experimental value of log(kn/k6) is 1.6 and the extrapolated value is 1.9, as denoted by a circle on curve 1). The same notation is used on curve 2 for the extrapolated points for log(kn/k6) of C4 and C3 with values of 0.8 and 0.83, respectively. The calculated rate constants for the ozone reaction with these two cycloalkanes will amount to 63 and 67 M-1.s-1, respectively. The difference in the slopes of the two curves is most probably associated with the more hindered sp3 m sp2 transition in smaller cycles. This fact is also supported by the data on the values of SE in Table 1.4. The theoretical and kinetic studies reveal that ozone reacts with cycloalkanes via H-atom abstraction. The reactivities of cycloalkanes depend on the direction of the steric energy change during the sp3 m sp2 transition in AC. The dependence of the rate constant of cyclooctane ozonolysis on its concentration is presented in Figure 1.3. The variation of [RH] and [O3] up to three orders of magnitude does not affect k and the rate of the reaction is described by the following equation: W = k.[RH]1.[O3]1

24

(1.18)

Ozonolysis of low and high molecular weight saturated hydrocarbons

0.5

0.4

cyclooctane

k, M-1.s -1

0.3

0.2

0.1

0.0 0

10

20

30

40

[RH], mM

Figure 1.3 Dependence of the rate constant of ozone reaction with cyclooctane at 10 oC The semilogarithmic anamorphosis of the kinetic curves of ozone consumption at [RH]>>[O3] upon ozonolysis of H12 cyclo-6 and D12 cyclo-C6 is shown in Figure 1.4.

0.4

D12

lg[O3]0/[O3]t

0.3

0.2 H12 0.1

0.0 0

1

2

3

4

5

6

Time, min

Figure 1.4 Semilogarithmic anamorphosis of the kinetic curves of ozone consumption upon ozonolysis of H12 cyclo-6 and D12 cyclo-C6 ozone consumption at 28 oC

25

Ozonation of Organic and Polymer Compounds The reaction of ozone decomposition follows ﬁrst-order kinetics and the rate of the reaction with D12 cyclo-6 is found to be higher. The ratio kH/kD = 5.4 is indicative of the occurrence of the primary isotope effect, which indicates that the H-atom abstraction is the rate-limiting step of the reaction.

1.2.3 Method for estimation of reaction mechanisms The method for the estimation of reaction mechanism (ERM), developed by us, is based on the comparison of values, calculated by means of the AC theory and experimental values of pre-exponents. According to the concepts developed in the AC theory [8183] the equation for the rate constant k of a bimolecular reaction is: k = G.C.[RT/(Nah)].[F#/(FaFb)].Ffrexp[-Ea/(RT)]

(1.19)

where F, Fa and Fb are the corresponding total statistical sums of the AC and of the initial compounds A and B. The total statistical sums are a product of the separate statistical sums of the state: F = Ft.Fr.Fv.Fe.Fn

(1.20)

where t, r, v, e and n denote the corresponding translational, rotational, vibration, electronic and nuclear sums of the state. The translational sum of the state at 3 degrees of freedom is described by Equation 1.21: Ft = h-3.(2PmkT)3/2

(1.21)

where m is the mass of the molecule; k is Boltzmann’s constant; and T is the absolute temperature. The values of Ft are within the limits 1024-1025 cm3 per molecule. The rotational sum of the state for nonlinear molecule is: Fr = 8P2(8P3IAIBIC)1/2(kT)3/2h-3

(1.22)

where I A I B I C are the principal moments of inertia of the molecule, when the origin of the coordinate lies at the mass centre of the molecule. The v a l u e s o f t h e s e s u m s d o n o t e x c e e d 1 0 2- 1 0 3 c m 3/ m o l e c u l e . The equation for estimation of the statistical sum of the vibration movement is: Fv = (1- e -hN/kT)-1 26

(1.23)

Ozonolysis of low and high molecular weight saturated hydrocarbons where N is the vibrational frequency. The magnitude of FN for one degree of freedom is of the order of 1-10 cm3 per molecule, as the vibrations in the molecules are within the range of 100-3000 cm-1, i.e., N is in the interval 3 × 1012 – 9 × 1013 cm-1 that is close to the value of kT/h = 6.1 × 1012 cm-1 at -173 oC. The statistical sums of the free rotator are: Ffr = (8P3Ia kT)1/2h-1S-1

(1.24)

where Ia is the reduced moment of inertia in regard to the axis of rotation. The electronic sum of the state for adiabatic processes, also including ozone reactions, is demonstrated by: Fe = 1

(1.25)

The nuclear sum of the state for Frank-Condon’s processes, which is the case with most of the chemical transformations, is given by: Fn = 1

(1.26)

The transmission coefﬁcient (G) takes into account the AC probability to be converted into reaction products. In the case of adiabatic reactions it is equal to 1, and for multiple nonadiabatic reactions it may be much smaller than 1. The coefﬁcient L takes into account the tunnel effects associated with the reaction pathway via the quantum way through percolation ‘below barrier’ of the potential surface. The probability for tunnelling depends on: (1) the mass of the particle m and the smaller it is, the greater is the probability for transition, (2) the barrier width D its decrease leads to increase in the tunnelling probability, (3) its height Ea - with its decrease the probability for tunnelling increases, and (4) the steepness of the potential surface at the depression point, (d2U/dx2) - its rise results in increasing the tunnelling probability. According to Gamov [83] the probability for tunnelling via triangle barrier is:

C = [2m(Ea-U)]1/2.exp[-8PD/(3h)]

(1.27)

where U is the height of tunnelling occurrence. In the case of reactions proceeding via redistribution of atoms at room or higher temperatures the tunnel effect is extremely small and L is equal to 1. This is also true for the reactions, studied by us.

27

Ozonation of Organic and Polymer Compounds For the calculation of the rate constant according to Equation 1.19 it is necessary to estimate the potential surface of the reaction and the statistical sums of AC, RH and O3. This is a typical quantum-chemical task, which has been the subject of intensive study by various researchers and we will not focus our attention on the difficulties and successes in this respect [84-88]. Easier methods for the calculation of potential surfaces are those that are semiempirical, such as Mopac 6 [87]. Its use will be presented later on. The estimation of the statistical sums provides important information about the mechanism of chemical reactions. For performing the calculations it is necessary to know the geometry of the starting molecules and AC and the vibration frequency of the bond on the reaction coordinate. The geometry of the starting molecules is usually well known [70], while the geometry of the AC is a more complicated problem, as the potential surface of the reaction is unknown. It is difﬁcult to determine the reaction coordinate, usually done only by indirect data. In order to ﬁnd an acceptable solution of this problem we have applied Franck-Condon’s principle [89] and Hammond’s empirical rule [75], which connect the thermal effect of the reaction with the reaction coordinate. These two principles allow the determination of the geometrical parameters of the AC with an accuracy up to 10%. The thermodynamic approach of the pre-exponential factor calculation in the Arrhenius equation can be given by the expression: A = G.C.[RT/(Nah)].[F#/(FaFb)].Ffr

(1.28)

These considerations were used in developing a computer program for calculating A and the statistical sums of internal rotation [90]. In addition the program also includes the calculation of A and the steric factor p, according to the collision theory. It also takes into account the number of equivalent H atoms and the proceeding of the reaction in the liquid phase. For calculation of A it is necessary to deﬁne the geometrical parameters of AC. Principally there exist two forms of the AC - linear and cyclic. In the ﬁrst an inner rotation is possible while in the second it is absent. The values of A calculated for the linear AC in each case will be higher than those calculated with the cyclic form. The calculated values of A were compared with the experimental values and on the basis of the degree of coincidence one or another mechanism could be preferred. In this manner the method for ERM gives additional possibilities for clarifying the investigated reaction mechanism.

28

Ozonolysis of low and high molecular weight saturated hydrocarbons

1.2.4 Application of ERM Using ERM we have analysed mechanisms M.1.1-M.1.6. According to these mechanisms, the reactions take place via two forms of the AC - linear (LC) and cyclic (CC) (Scheme 1.3).

O R R'

C

O H

O

O

O O

;

R

C R'

R"

H

.

R" CC

LC

Scheme 1.3 LC is associated with the H-atom bond cleavage mechanism and CC with ozone incorporation inside the C-H bonds. LC is characterised by the ozone interaction with the C-H bond via one of its terminal atoms and thus there is a possibility for free bond rotation around the H-O and O-O bonds, while in the CC it forms a ﬁve-membered cycle with the participation of a ﬁve-fold coordinated carbon. Table 1.8 summarises the calculated values of A for the two AC forms. Next we shall focus our attention on estimating the value of A calculated for each separate reaction [48, 52]. The ratio between the statistical sums of electronic and nuclear movement of the AC and those of the initial compounds is 1 as the change of energy is smooth and the electron-nuclear states are not changed; G and L are equal to 1 under the experimental conditions. The value of A can be presented by two multipliers [63]: Ao is a geometric factor, which is determined by the AC geometry and A1 is the contribution by the internal degrees of freedom. In this case we can write the following expressions: A0 = (kT/h) . (Ft#/FtRHFtO3).(Fr#/FrRHFrO3).Ffr#

(1.29)

A1 = (Fv#/FvC-H).(FvR#/FvR).(FvO3#/FvO3).J

(1.30)

where Fv are the statistical sums of the vibrations of the reaction centre, i.e., C-H bond in the initial alkane; J = Jo × exp(-Eh/RT) is the coefﬁcient considering the highest energy that the rotator must overcome at its twist to 360°.

29

30 13.5 13.5

2.12

1.99 1.99

1.88

1.81

1.69

2

6 1

1

2

16

C7

C8

C9

C10

C12

where J = (kT/h) × (Ft#/FtRHFtO3)

8.3

2.29

6

C6

278

97

34

2.5

0.8

2.54

1

C5

IAIBIC × 10-6 (au.Å2)

S

Cycle

J × 10-7 (M-1.s-1)

13

7.68

4.53

2.88 -

2.25

1.23

0.71

S.Ffr

32

12.2

5.40

2.54 2.54

2.02

0.59

0.44

IAIBIC × 10-8 au.Å2

#

#

4.40

2.72

1.81

1.24 1.24

1.10

0.60

0.52

Fr × 10-6

1.41

1.39

1.37

1.34 1.34

1.32

1.28

1.24

Ffr × 10-3

40.3

5.6

3.2

21.5 3.6

8.56

26.6

6.84

A0 × 10-7

3.55

0.48

0.28

1.85 0.31

0.74

2.29

0.59

A × 10-7 (M-1.s-1)

9.95

1.34

0.77

5.17 0.86

2.06

6.40

1.65

Al × 10-7 (M-1.s-1)

Table 1.8 Pre-exponent (A) of the ozone reaction with cycloalkanes at 27 oC calculated by the method of the activated complex

Ozonation of Organic and Polymer Compounds

Ozonolysis of low and high molecular weight saturated hydrocarbons The sums at 27 oC were calculated by using the following equations: Ft = 3.86.109[(mO3 + mRH)/mO3.mRH]

(1.31)

S.Fr = 78 (IAIBIC )1/2

(1.32)

S.Ffr = 6.24 I’ 1/2

(1.33)

Fv = (1- e -hN/kT)-1

(1.34)

It should be noted that the values of Ao thus calculated are relatively reliable. Literature values are used for ozone [29, 64] for the length of the O-O bond, 1.278 p 0.003 Å, and for the OOO angle, 116° 49a p 30a; the main inertial moments IaIbIc are 7.875 × 10-40, 62.844 × 10-40 and 70.888 × 10-40, respectively; S = 2, Fe and Fn = 1. For a cycloalkane the values of the length of the C-C and C-H bonds are 1.54 Å and 1.09 A, respectively, and a value 109.5 for the CCC=CCH=HCH angles has been used. In Table 1.8 the values of IaIbIc are calculated according to well-known methods [1, 89]. We have selected the most stable conformers of the cycles known [80], conﬁrmed also by the calculations according to the Alinger method (Table 1.4). Thus the preferred conformation for cyclopentane is envelope (Cs), for cyclohexane - chair, for cycloheptane - twisted chair (C2v), for cyclooctane - twisted crown with 3 axes C2v, regular crown with 2 axes C2v or knocked crown without any elements of symmetry. The product IAIBIC is nearly the same for all conformations. However, bearing in mind that the different degrees of symmetry would affect the calculation of A we have used two conformations with numbers of symmetry of 6 and 1. The conformation state of cyclononane is a crown with a symmetry number of 1 but it is insufﬁciently studied and for this reason we have combined the calculations by the Alinger method with the X-ray data for cyclononylamine bromohydrate [80]. The conformation of cyclododecylamine-1,6 dihydrohydrate, which has only one axis of symmetry of second order, is taken as a base for cyclodecane. According to the X-ray data the cyclododecane has a square shape conformation with butane segments at its sides. This conformation has one axis of fourth and two of second order and its number of symmetry is 16. Some of the most stable conformations of cycloalkanes are represented in Figure 1.5.

31

Ozonation of Organic and Polymer Compounds

C5

C6

C8

C7

C10

C9

C11

C12

Figure 1.5 Most stable cycloalkane conformations The structure of LC was elaborated on the basis of the following geometrical parameters: the H...O bond length was estimated according to Pauling’s formulae [91]; dH..O = dH-O-0.6.ln(r), where r is the bond order, dH...O is the bond length in the complex and dH-O is the length of ordinary H-O bond equal to 0.95 Å. Considering that the maximum coordinate on the potential surface can be only approximately determined, the bond order was deﬁned within a wide range from 0.2 to 0.8 with dH...O varying from 1.92 to 1.08 Å. The actual value of dH...O lies most likely in this interval and thus the value of 1.35 Å at r = 0.513 was used for the calculations. The lengths of the C-H and O-O bonds in LC are increased by 10% compared with their normal values; the H...O-O angle is accepted as 100° (in water HOH it is 104.5° and in HOOH the HOO angle is 96.6°). The calculation of Ffr was carried out for two asymmetry rotators: one for rotation around the x-axis coinciding with the H...O bond and the second one with rotation around the y-axis coinciding with the O-O bond: y R R'

C

O H

O

O x

R"

The reduced moments of inertia around the y-axis are found to be equal for all cycles and have the value of kg\m2. However, those around the x-axis increase with the rise of the cycloparafﬁn masses and their values are shown in Table 1.9.

32

Ozonolysis of low and high molecular weight saturated hydrocarbons

Table 1.9 Values for moments of inertia around the x-axis C5

C6

C7

C8

C9

C10

C12

50

53

56

58

60

62

64

The values of Ao are presented in Table 1.8 which depends on the LC geometry only and to a smaller extent on the potential surface. The values of A1 could be calculated with lower accuracy as they depend to a greater extent on the form of the potential surface. For the accurate calculation of Ffr it is necessary to know the values of the rotation barrier Efr. The latter were calculated by us using MOPAC6 and for the rotation around the x- and y-axes they are 1.8 and 7.5 kcal, respectively. These values correlate well with the energy of NO rotation in the nitromethane molecule [91]. The correction coefﬁcient was also determined using Pitzer’s tables [92]. Although the latter are designed for symmetric rotators as Pitzer has shown, they can also be successfully applied for the approximate estimation of the thermodynamic functions of asymmetric rotators. The ratio of the vibration statistical sums (F#RHv × F#O3v)/(FRHv × FO3v), which stands for the variations of the vibration frequencies amounts to 1. At the same time one can expect for the ozone molecule, which is characterised by two stretching vibrational frequencies at 1106 and 1036 cm-1 and a deformational one at 703 cm-1 a decrease in the frequency at 703 cm-1, but not lower than the value within the range of 150-200 cm-1 to become kinetically active at kg\m2. The contribution of the vibrational sums to the thermodynamic functions is small and is accepted to be 1 for calculating the ratio F#rcv/FC-Hv at such moderate temperatures as kg\m2, where F#rcv is the vibrational statistical sum of the reaction centre. At higher temperatures, however, this ratio may be much higher [93, 94]. The accurate determination of the vibrational frequency of the reaction centre requires knowledge of the form of the potential surface near the maximum point. For that reason this value could only be approximately evaluated. The frequency of the deformational vibrations of the reaction centre C...H...C lies in the range of 100-400 cm-1 [94]. For the reaction centre C-H...O this value is probably shifted to the higher frequency kinetically nonactive range. This is due to the fact that NO-H lies in the range of 1000-1600 cm-1 and that of C-H in the 400-1000 cm-1 region. Hence, the contribution of the vibrational sums should not differ from 1. However one cannot rule out the probability of the vibrational sum of the reaction centre for bigger cycles reaching values 5-10% higher than those of the small cycles.

33

Ozonation of Organic and Polymer Compounds Even in this case the trend in the calculations remains the same. Upon increasing the inertial moment of cycloalkanes as shown above, the vibrational frequency should also increase as it is inversely proportional to its square root. For example, if we accept that N = 200 cm-1 for C5, then for C12 it will be 120 cm-1. In this case the vibrational term will be increased 3.5-fold. It should be noted, however, that the calculations have been made for reactions in the gas phase, while for those in a liquid medium the values of A would be different. According to the collision [95] and AC [81-83, 96] theories the number of collisions in the liquid phase is 2-3 times higher than in the gas phase. Both theories predict that at El = Eg the value of A will be 3 times higher than that in the gas phase. In reference [91] it is supposed that the increase of k for a bimolecular reaction proceeding in liquid phase will be about 5 times. The same tendency is observed when comparing the experimental results from the ozonolysis of acyclic hydrocarbons in the gas phase [2-10] and in an inert solvent, CCl4 [47]. The ratio of the pre-exponential factors is in the range of 2-3. The aforementioned considerations give us grounds to introduce a coefﬁcient for the ratio Al/Ag equal to 2.8 (Table 1.8, column 11). When comparing the experimental results with the theoretical data the experimental value of A is usually related to the number of the active centres. It is well known [6] that the H atoms in cycloparafﬁns exist in various conformations in respect to the cycle planes - they can be oriented in various positions from equatorial to axial and from endo- to exo-. In cyclohexane, for example, the three equatorial H atoms are 7 times more reactive that the three axial H atoms [74]. The H atoms in the bigger cycles are even more nonequivalent as, besides the axial and equatorial atoms in them, there are also hydrogen atoms lying within the cycles of which a transannular interaction is characteristic. This can be clearly demonstrated by the models of Dreiding and by the various stereochemical programs. The conformation analysis performed demonstrates that the H-atom distribution in the studied cycles is as shown in Table 1.10.

Table 1.10 Hydrogen atom distribution in the studied cycles C5

C6

C7

C8

C9

C10

C12

ax

10

6

8

8

10

10

16

eq

-

6

4

4

6

6

-

exo

-

-

2

4

2

4

8

ax = pseudoaxial, eq = pseudoequational, exo = internal

34

Ozonolysis of low and high molecular weight saturated hydrocarbons In order to compare the experimental values of A with the calculated values we relate the experimental values of A per the number the equatorial H atoms (Table 1.11).

Table 1.11 The experimental and calculated values of A per one equatorial H atom A, M-1.s-1

C5

C6

C7

C8

C9

C10

C12

Aexp × 10

-7

0.32

3.34

0.36

0.29

2.48

22.8

12.9

Acal × 10-7

1.65

6.40

2.06

0.86

0.77

1.34

9.90

The experimental values of A vary from 0.3 × 107 to 23 × 107 M-1.s-1. It has been found that the values of A calculated for the AC with LC lie within the range (0.810) × 107, while those for the CC are in the range of 105–106 M-1.s-1. The lower (about 130 times) values in the latter case are related to the lack of free rotation and to the more compact structure (2-4 times) of this complex. However in this case the eventual increase of A, on account of the occurrence of low frequency vibrations of the reaction centre which in our opinion can bring about not more than 2-3 times increase, should be taken into account. Thus the values of A calculated with CC should be about 100 times lower than the experimental values. The agreement of A calculated with LC with the experimental values of A becomes even better when comparing them for each cycle alone. In fact the differences in the values of A for all cycles do not exceed 3, which fact will be discussed further. The obtained results show that the probability of the reaction to occur via a cyclic transition state is negligibly small. Consequently, the probability of the ozone reaction with the C-H bonds taking place through a mechanism of incorporation or formation of oxy-radicals in one step is also small. It follows from these two mechanisms that the reaction should take place in at least two steps: H-atom abstraction giving rise to (RU + UOOOH) or to (RU + HOU +O2). Because the endothermicity of the latter is lower by 16 kcal/mol it is thermodynamically more favourable. In our opinion, one of the reasons for the existence of such a variety of schemes describing the reaction RH + O3 is due to the lack of consideration of entropy factors. As a result of taking them into account it can be seen that the abstraction of the H atom is the limiting step in the ozonolysis of parafﬁns. In view of the possibilities, the ERM is a universal method because it can be applied to every type of chemical reaction.

35

Ozonation of Organic and Polymer Compounds

1.3 Cyclohexane The reaction of ozone with cyclohexane [52] is studied in a wide range of concentrations. The typical kinetic curves are given in Figure 1.6. The profiles of the product curve formation show that cyclohexanol (-OH), cyclohexanone (C=O), peroxide (ROOH) and acids are obtained in parallel reactions. The initial rate of alcohol formation is about 3 times higher than that of the ketone. The same ratio for this reaction has also been reported by other authors [9, 67, 97]. The ratio between the rates of product formation is Wal:Wket:WROOH = 4:1:1. The nature of the peroxides formed in the course of the reaction has been clariﬁed by studying the kinetics of iodine liberation during the oxidation reaction with HI solution in CHCl3:CH3COOH. The rate of iodine evolution was measured at L= 470 nm. In order to ignore the formation of peroxide compounds arising from the further oxidation of the reaction products (alcohol and ketone), we used reaction mixtures with very small conversion degrees, approximately 0.07%. The results obtained (Figure 1.7) justify the probable formation of H2O2, cyclohexylhydroperoxide (ROOH) and dicyclohexylperoxide (ROOR) in the reaction mixture, which react at different rates with HI (KI) [76].

10 -OH 8 C=O

[P], mM

6

4 ROOH

2

0 0

10

20

30

40

50

Time, min

Figure 1.6 Kinetics of basic product formation during cyclohexane ozonolysis at [RH] = 9.29 M, [O3] = 0.7 mM, 20 oC

The formation of the above-mentioned peroxides was conﬁrmed by the iodometric procedure modiﬁed by us. A volume of 0.5 ml saturated water solution of KI and 0.5 ml

36

Ozonolysis of low and high molecular weight saturated hydrocarbons CH3COOH were added to 0.5 ml of oxidate. The latter was kept in the dark at room temperature for 2 h. The quantity of the iodine liberated after 30 min corresponds to H2O2 and ROOH, and that after 24 h to ROOR. Further, the separate determination of H2O2 and ROOH has been carried out by treatment of the oxidate sample with an aqueous solution of catalase (a speciﬁc enzyme decomposing H2O2) [98]. Thus the titration of the iodine liberated after 30 min gives the concentration of ROOH and by the difference between the two titrations, the content of H2O2.

100 140

1

3

80

60 100

2

40

D / D, %

D / D0, %

120

80 20 60 0 0

20

40

60

80 100 120 1440

1460

1480

1500

Titration time, min

Figure 1.7 Dependence of the relative optical intensities DT/D (%) on the titration duration: DT, D, - determined at time T and after 24 h at 470 nm; 1 - 0.07% conversion; 2, 3 - reference mixtures (solutions) - tert-butylhydroperoxide (2) and di-tert-butylperoxide (3) = 1:1

The duration of the quantitative iodine liberation was determined using a model system comprising H2O2, tert-butylhydroperoxide and di-tert-butylperoxide and it was found to be 30 minutes for H2O2 and tert-butyl-OOH and 24 hours for di-tertbutylperoxide. These time durations in the dark treatment correspond to well-known literature data for their rates of interaction with HI (KI) [99-102]. The possibilities of the proposed procedure for the separate identiﬁcation of H2O2, alkylhydroperoxides and dialkylperoxides are illustrated by the data in Table 1.12. The yield of peroxides was found to be 1-15%. The authors of [78] reported a 45% yield but they, in fact, do not discuss it and have no explanation for this high yield.

37

Ozonation of Organic and Polymer Compounds In our opinion, it is most likely because of the higher experimental temperature - the chain radical oxidation might generate higher quantities of peroxides.

Table 1.12 Concentrations of H2O2, t-BuOOH and t-BuOOBu-t (s102 M) Compound

0 min

30 min

24 h

H 2O 2

1

1

-

H2O2*

1

0

-

t-BuOOH

1

1.13

-

t-BuOOH*

1

1.2

-

t-BuOOBu-t

1

0.07

1.0

t-BuROOBu-t*

1

0.08

1.0

1 = 0.5 + 0.5

0.49

1.08

H2O2:t-BuOOH = 1:1

Note: *titration carried out in the dark

The ratio between the initial rates of peroxide formation as shown by the kinetics of H2O2, ROOH and ROOR accumulation is 1:1:4. The relationships of the rates of product formation on the ozone content, depicted in Figure 1.8, exhibit a linear pattern. The acid numbers of the reaction mixture, determined by titration with an aqueousalcohol solution of NaOH and indicator phenolphthalein, are an order of magnitude higher than the content of adipic acid as determined by gas chromatography. This observation demonstrates the complex composition of the acids. In reference [97] it is considered, however, that the acid number is completely determined by the presence of adipic acid, but this statement obviously contradicts the chromatographic data obtained by us which show that adipic acid comprises a very small percentage of the total acid content. In a number of experiments we have found that the content of water is about 50% of the total yield. The same water yield was obtained in tetradecane ozonolysis [1].

38

Ozonolysis of low and high molecular weight saturated hydrocarbons

Sumof -OO-

30 25

HexOOHex

[P].104, M

20 15 HexOOH

10 5

H2O2

0 0

10

20

30

40

Time, min

Figure 1.8 Kinetics of peroxides formation, [RH] = 9.26 m, WO3 = 1 × 10-5 M/s-1, 20 oC On the basis of studies and analyses carried out, the most plausible mechanism of the reaction can be presented as shown in Scheme 1.4. }m (J) Rv + O2 + vOH RH + O3 = [Rv + HO3v (HOv + O2] }m cage

(1)

}m (1 - J) ROOOH; ROH +O2

RH + vOH = Rv + HOH

(2)

Rv + O2 = RO2v

(3)

}m (A) ROH+ R'C=O + O2 RO2v + RO2v }m

(6)

}m (1 - A) ROOR + O2 Scheme 1.4 In Scheme 1.4 the following reactions can be added: RO2v + RH = ROOH + Rv ROOH = ROv + HOv,

39

Ozonation of Organic and Polymer Compounds but as the concentration of the hydroperoxides in the reaction mixture is only about 2% and the experimental temperature is below 40 oC and the radical chain reactions are of minor importance in the oxidation process. The kinetic analysis of Scheme 1.4 leads to the following expressions for the rates of formation of alcohol, ketone and ROOR: WOH = (1 - J) WO3 + A k6 [RO2v]2 = (1 - J - 2AJ)WO3

(1.35)

WC=O = A k6 [RO2v]2 = 2AJ WO3

(1.36)

WROOR = (1 - A) k6 [RO2v]2 = (1 -A)JWO3,

(1.37)

where WO3 is the rate of ozone absorption. The experimentally obtained dependences of the accumulation rates of the alcohol, ketone and ROOR are in a good agreement with the equations derived from Scheme 1.4 (Figure 1.9). On the basis of Equations 1.35-1.37 obtained values for A and J were 0.7 and 0.3, respectively. From the rates of reaction product accumulation the stationary concentration of the peroxide radicals was determined to be 1.5 × 10-6 M. It has been suggested in references [11, 18] that ozone reacts with RO2v radicals: RO2v + O3 = ROv + 2O2

(4)

and then the alcohol will be obtained through the reaction: ROv + RH = ROH + Rv

(5)

However, in this case the dependence of WOH on WO3 should be linear in coordinates WOH/(WO3)1/2, which has not been experimentally observed (Figure 1.9). In addition W must be greater than the sum of the rates of all reaction product formation, a fact which has not been conﬁrmed by the experimental results. These two facts suggest that reactions (4) and (5) could be ignored.

40

Ozonolysis of low and high molecular weight saturated hydrocarbons

-OH, O3/O2 8

6

W.106

-OH, O3/He 4

C=O, O3/O2

2

ROOR, O3/O2

C=O, O3/He

0 0

2

4

6

W0.106, M.s

8

10

-1

6 ROOR, O3/He 5

W1/2.103, (M.s-1)1/2

4 3 ROOH, O3/O2, (100C)

2 1 0 0

1

2

3

W0.106, M.s

4

5

6

-1

Figure 1.9 Rates of product formation during cyclohexane ozonolysis depending on WO3 (20 oC)

If we accept that compared with cumene oxidation [14] the termination step generates RO radicals: RO2v + RO2v = 2ROv + O2 during this reaction, then an additional quantity of alcohol will be provided by reaction (5). Thus it may be expected that the dilution of cyclohexane with an inert solvent should decrease the rate of reaction (5), consequently the ratio alcohol:ketone. However the experimental results show that the ratio is not changed upon the cyclohexane dilution with tetrachloromethane thus rejecting the generation of oxyradicals from the peroxide radical recombination.

41

Ozonation of Organic and Polymer Compounds The yield of H2O2 is 2-3%. The formation of an insigniﬁcant quantity of H2O2 could be explained by the following reactions [97]: HOv + O3 m HO2v + O2 HO2v + RH m H2O2 + Rv Adipic acid is obtained in the monomolecular decomposition of ROv radicals: ROv m vCH2(CH2)4CHO m HOOOC(CH2)4COOH In addition one should also consider that in the total yield some products of the ozone reaction with cyclohexanone, cyclohexanol or cyclohexylperoxide are also of course included in tenths of a per cent. This shows that the reaction with ROv radical participation has an insigniﬁcant contribution to the total yield. We also carried out cyclohexane ozonolysis with reduced concentrations of O2 by its replacement with He. In this case, at equal initial ozone concentration its consumption rate increases two-fold, which could be due to the proceeding of the following reaction: Rv + O3 = ROv + O2

(7)

and the increase of the contribution of reaction (5). Considering reactions (7) and (5), WOH and WC=O will depend on WO3 in the following manner: WO3obs = WO3RH + WO3Rv

(1.38)

WO3Rv= k7.[Rv][O3]=2J k7 [O3] WO3RH (k2 [O2])-1

(1.39)

WOH={1-J+2AJ+2Jk7[O3](k2[O2])-1}WO3RH

(1.40)

WC=O=2AJWO3RH=2AJk2 [O2]{k2 [O2]+2Jk7[O3]}-1WO3RH (1.41) The dependences of the rates of product formation on WO3 using O3/He and O3/O2 demonstrate that at the same rate of reaction 1 the rates of product formation vary upon the replacement of O2 by He (Figure 1.9). This fact could be due to the occurrence of reaction (7) which apparently reduces the efﬁciency of the ozone reaction. The

42

Ozonolysis of low and high molecular weight saturated hydrocarbons comparison of the equations derived for the processes with and without reaction (7) provides a reasonable explanation of the relationships given in Table 1.13.

Table 1.13 The rates of cyclohexanol and cyclohexanone formation at various rates of ozone absorption (WO3 × 106, M-1.s-1) O3/He, WO3 = 7

O3/O2, WO3 = 3.5

O3/He, WO3 = 5

O3/O2, WO3 = 2.5

Cyclohexanol

6.0

3.0

4.8

2.2

Cyclohexanone

0.8

1.2

0.6

0.8

RH

3.6

3.5

2.4

2.5

v

3.4

-

2.5

-

Rate of formation

R

At low O2 concentration the rates of alcohol formation will be described by: WOHHe = (1 - J) WO3RH + WC=OHe + k5[ROv][RH]

(1.42)

The rate of ozone uptake in its reaction with R radicals can be estimated by solving the following equation: WO3Rv = WOHHe - WOHO2+ WC=OO2 - WC=OHe

(1.43)

The values obtained for WO3RH and WO3Rv are in a good agreement with the experimentally observed two-fold rise of WO3. In the case of ozonolysis by mixtures O3/O2 ROOR are obtained via reaction (6) (Figure 1.9), and with the O3/He mixture, because of the higher stationary concentration of RO2v radicals, the rate of the square termination of the alkoxy-radicals increases and hence WC=O (Figure 1.9). We have identiﬁed chlorocyclohexane in the chromatograms of the oxidates resulting from the ozonation of cyclohexane solutions in CCl4. This fact conﬁrms the generation of Rv radicals, which can further abstract the Cl atom from the solvent molecule via reaction (8): Rv + CCl4 = RCl + CCl3v

(8)

According to reference [14] the rate constant of the reaction at 20 oC is 1.59 M-1.s-1. We have estimated the stationary concentration of Rv to be 10-7 M from the dependence of the rate of cyclohexane formation on the CCl4 concentration.

43

Ozonation of Organic and Polymer Compounds The mechanism of ozonolysis includes a stage of radical generation which has also been assumed by other authors [10]. The electron spin resonance (ESR) signals, however, have been registered only for ozonolysis of polyolefﬁns and cumene [50], which are characterised by relatively low values of k6 and high [RO2v]st. We have conﬁrmed the existence of radical steps in the parafﬁns ozonolysis by carrying out the reaction directly in the ESR resonator. For this purpose we have developed and constructed a special ESR cuvette (Figure 1.10) [50].

1

3

2 4

5

Figure 1.10 A device for carrying out chemical reactions inside the ESR spectrometer. 1 - separating funnel with capillary for liquid reagent feeding; 2 capillary connected with a vacuum pump ensuring the necessary level of the liquid in the cuvette; 3 - capillary for the gas inlet; 4 - gas outlet; 5 - ESR cuvette

The registered ESR signals are shown in Figure 1.11. They have been interpreted by comparison with those obtained in the initiated oxidation of n-decane, cumene and polyvinylcyclohexane (PVCH) with oxygen. On the basis of the similarities in shape, g-factors and signal width, we have assigned them to RO2v radicals.

44

Ozonolysis of low and high molecular weight saturated hydrocarbons

80

Intensity, %

70 60

1

2

50 40 30 0

5

10

15

20

25

30

35

40

45

Magnetic field, [E]

100

Intensity, %

80

60

3 40

20

0 50

100

150

200

Magnetic field, [E]

Figure 1.11 ESR spectra: 1 - n-decane; 2 - cumene; 3 and 4 - PVCH at ambient temperature

45

Ozonation of Organic and Polymer Compounds The dynamics of ESR signal intensities during the PVCH ozonolysis is represented in Figure 1.12.

5

4

Rel. Intensity

3

2

1

0 0

100

200

300

400

500

T ime, s

Figure 1.12 Dynamics of RO2v radical formation during the ozonolysis of powder PVCH at 20 oC

The rise of the intensity of the RO2v singlet at the beginning is related to the opening of new surfaces and ozone interaction with the intermediate products of the reaction. The latter effect [50] has been considered to be one of the reasons for the observed course of the chemiluminescence (CL) intensity in the ozonolysis of 2,7-dimethyloctane, which has been found to be similar. The drop off in the ESR intensity is due to the consumption of the initial compound (100% conversion after 7 min). When the O3/He mixture was used, as would be expected, the ESR signal intensity is an order of magnitude lower as a result of the active proceeding of reaction (7). The kinetics of RO2v radical formation during cumene ozonolysis is given in Figure 1.13. The absolute concentration of RO2v radicals was determined using Rubine dye as a reference compound.

46

Ozonolysis of low and high molecular weight saturated hydrocarbons

1.0

[RO .2].105, M

0.8

0.6

0.4

0.2

0.0 0

50

100

150

T ime, s

Figure 1.13 Kinetics of RO2v-radical formation during cumene ozonolysis. W = 3.5 ×10-4 M.s-1, 20 oC

The decrease in the intensity of the ESR signal with time is connected with the consumption of cumene because of the high rate of ozone absorption. As the ﬁrst point in Figure 1.13 is registered at 0.6% conversion degree, the stationary concentration of RO2v radicals was determined by extrapolation to zero time and amounts to [RO2v] -5 st = 1.4 × 10 M. Under the experimental conditions applied the rate of the chain oxidation reaction is very low and so RO2v radicals would be consumed exclusively via the square law termination reaction. The rate of the latter, as estimated according to [ROv2]st and k6 = 1.26 × 104 M-1.s-1 [88] is 5.4 × 10-6 M.s-1. This value is 64 times lower than that calculated based on the assumption that WO3 = Wi (initiation rate). This fact conﬁrms that ozone is apparently not an initiator in the studied reaction.

1.4 Cumene It has been found out that the O2 oxidation of cumene takes place through a chainradical route yielding cumylhydroperoxide (CHP), dicumylhydroperoxide (DCHP), acetophenone (ACP), dimethylphenylcarbinol (DMPC), A-methylstyrene, acetone, methanol, formic acid and benzoic acid [14] as reaction products. The kinetic curves of product formation during cumene ozonolysis are shown in Figure 1.14.

47

Ozonation of Organic and Polymer Compounds

2.5

CH P

[P].103, M

2.0

1.5

DMPC

1.0

Ozonides

0.5

A cids 0.0 0

5

10

15

20

25

T ime, min

Figure 1.14 Kinetics of product formation during ozonolysis of cumene, [RH] = 7.17 M; WO3 = 8.6 ×10-8 M/s-1, 20 oC

The reaction products include ozonides, DMPC, CHP, DCHP, H2O and acids. The linear character at the plots and the absence of induction periods in the curves of ozonide, CHP and DMPC formation are an indication that these products are obtained in parallel reactions. The pattern of the kinetic curves is typical for the formation of intermediate products. In the case of DMPC the curve reaches a plateau in 10 minutes, and that of ozonide which at the beginning is identical to the DMPC curve, begins to go down after 15 minutes. This fact is apparently related to ozonide decomposition and polymerisation and from this part of the curve its rate of decomposition was calculated to be 3 ×10-3 s-1. The dependences of the initial rates of DMPC and ozonide formation on WO3 and of CHP on (WO3)1/2 are depicted in Figure 1.15. It is seen that, while the rates of DMPC and ozonide formation depend linearly on WO3, the rate of CHP formation is proportional to (WO3)1/2. These results suggest the occurrence of parallel steps of formation of the reaction products as well as radical stages in the reaction mechanism.

48

Ozonolysis of low and high molecular weight saturated hydrocarbons

2.5

CH P

[P].103, M

2.0

1.5

DMPC

1.0

O zonides

0.5

A cids 0.0 0

5

10

15

20

25

T ime, min

0.4

W CHP .105, M.s -1

0.3

0.2

0.1

0.0 0

1

2

3

4

5

W 01/2.103

Figure 1.15 (a) Dependence of the rates of DMPC and ozonide formation on WO3, 20 oC. (b) Dependence of the rate of CHP formation on (WO3)1/2, 20 oC

The chemiluminescence kinetic curve, shown in Figure 1.16, resulting from the RO2v radical recombination [55, 104] becomes a straight line in coordinates ln[(I01/2+I1/2)/ (I01/2-I1/2)]/t, where Io is the intensity of the chemiluminescence signal and I is the current intensity. The tangent of the curves has a slope which is equal to 0.5 × (Wi × k6)-1/2, where Wi is the rate of the initiation equal to A × WO3 (A<1) and k6 is the rate constant of the square law termination.

49

Ozonation of Organic and Polymer Compounds The obtained results could be described by the mechanism shown in Scheme 1.5:

RH + HOv = Rv + H2O (1) ROOv + RH = ROOH + Rv (2) 2ROOv = 2ROv + O2 (6) ROv + RH = ROH + Rv (5) Scheme 1.5

At quasistationary approximation [14, 28, 105] the rates of products formation will be represented as: WDMPC = (1 + 3A).B.W0

(1.44)

WCHP = k2 (2ABW0/k6)1/2 [RH]

(1.45)

W0 = WDMPC + WIDMPC + WCHP + WO3

(1.46)

where WIDMPC is the rate of DMPC formation at square law termination.

50

Ozonolysis of low and high molecular weight saturated hydrocarbons The values of A and B have been determined based on the slopes of the kinetic curves in Figure 1.15a and b and on the basis of the literature values for the rate constants k2 and k6 [95] and they are 0.04 and 0.23, respectively. It follows from the angle of the tangent on the ozonide curve (Figure 1.15a) that 75% of the effective collisions between ozone and cumene are with the benzene ring and only 25% are at the alkyl chain of the cumene molecule, which ﬁts very well the calculated value of B from Figure 1.16. Based on Figure 1.15a we have estimated that the value of WDMPC comprises only 7% of that of WDMPC; hence, the basic share of carbinol is formed via the ﬁrst reaction in the kinetic cage. Based on the slope of the curve in Figure 1.16 and using the literature values of k6 we have determined that 0.3% of WO3 and 1/6 of RO2 radicals yield products which are chemiluminescent. These results correlate well with the previously mentioned ESR data.

3.5

ln [( I 01/2 + I 1/2) / ( I 01/2 - I 1/2

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

5

10

15

20

25

T ime, s

Figure 1.16 Dependence of the chemiluminescence’s intensity on time at Wo = 4.2 ×10-5 M/s-1

1.4.1 Ozonation in the presence of transition metal compounds The data on the ozonolysis of cumene in the presence of transition metals are very scant [106, 107]. This reaction has been studied mainly in sulphuric acid medium and at low cumene concentrations. Having this in mind we carried out the ozonolysis reaction in both a pure and a 50% solution of acetic acid [57]. 51

Ozonation of Organic and Polymer Compounds As has been shown the main products of cumene ozonolysis are ozonides, DMPC and CHP. In acetic acid, the product composition remains the same, but the amount of the products is decreased by about 1.6 times. The rates of CHP and ozonide formation are about 8.8- and 1.6-fold lower, respectively, and the rate of DMC accumulation increases 1.6 times. The ratio of the products is found to be 2.1:1.9:1 ozonide:DMPC:CHP. The rate constant (k) characterising the ozone-acetic interaction at ambient temperature amounts to 1.5 × 10-5 M/s-1, that is about 3.5 × 10-4 smaller that that found for the ozone reaction with cumene (k = 0.35 M/s-1). It is known that acetic acid (AcAc) deactivates ozone, inhibits RO2v radicals and protects the benzene ring [9-101]. The deactivation of ozone in AcAc is also conﬁrmed by the change of the molar coefﬁcient in the UV spectrum compared with that in CCl4. The catalytic properties of Co2+ in hydrocarbon oxidation have been the subject of intensive investigations [107]. It has been established that during the cumeneAcAc ozonolysis in 1:1 (v:v) in the presence of Co(AcO)2 the oxidation reaction is accelerated (Figure 1.17). In contrast to the noncatalysed process in the catalysed process by transition metal salts the ozonolysis is characterised by: (1) absence of ozonide formation that is indicative of the absence of ozone interaction with the phenyl ring, and (2) the main product is DMPC, the accumulation rate of which is proportional to the concentration of Co2+ after 10 min. The initial rates of CHP formation do not vary with the changes in Co2+ but after the 15 min the rates increase with [Co2+]. It can be seen from Table 1.14 that if we take the ozonolysis of pure cumene as a reference reaction then the addition of AcAc results in autoretardation of the oxidation rate and leads to reduction of the product yield. The ratio [3P]/[O3} reaches a value of 6.9. These data indicate the participation of oxygen (accompanying ozone) in the formation of the reaction products. The course of the kinetic curve of DMPC formation on the Co(AcO)2 concentration is connected with its formation in reactions with participation of Co(AcO)2 (Figure 1.17). The catalytic properties of the metal ions under other identical conditions depend on their rate constant of interaction with ozone [102]. For that reason we have determined the values of k in the presence of different transition metals in the cumene-AcAc medium. The rate constants of ozone reaction with some transition metals ions at 20 o C in cumene:AcAc solution (1:1, v/v) are represented in Table 1.15.

52

Ozonolysis of low and high molecular weight saturated hydrocarbons

Table 1.14 The kinetics of formation of the sum of products (3P) and their ratio per unit of absorbed ozone [3P]/[O3] for the noncatalysed and catalysed reaction in the presence of AcAc (cumene:AcAc = 1:1) at ambient temperature Cumene

Cumene+ AcAc

5

0.3

0.2

0.8

0.8

0.9

1.0

1.1

1

0.7

2.7

2.7

3.0

3.3

3.7

0.7

0.4

1.4

1.4

1.8

2.2

3.3

1

0.6

2.0

2.0

2.6

3.1

4.7

1.0

0.6

2.1

2.1

2.9

4.0

5.9

1

0.6

2.1

2.1

2.9

4.0

5.9

1.4

0.8

2.8

2.8

4.5

6.1

8.7

1

0.6

2.0

2.0

3.2

4.3

6.2

1.7

1.0

3.5

3.5

6.2

8.1

11.7

1

0.6

2.1

2.1

3.6

4.8

6.9

2.0

-

4.2

4.2

8.0

10.1

-

1

-

2.1

2.1

4.0

5.1

-

[¤P] × 103 M [¤P]/[O3] [¤P] × 10 M

3

10

[¤P]/[O3] [¤P] × 10 M

3

15

[¤P]/[O3] [¤P] × 10 M

3

20

[¤P]/[O3] [¤P] × 10 M

3

25

[¤P]/[O3] [¤P] × 10 M

3

[Co2+]= 1 [Co2+]= 2 [Co2+]= 3 [Co2+]= 4 [Co2+]= 5 × 103 M × 103 M × 103 M × 103 M × 103 M

Time (min)

30

[¤P]/[O3]

Table 1.15 Rate constants of the ozone reaction with some transition metal ions Reagent

[Me+n] (mM)

k (M/s-1)

Co2+

1

500

2+

1

1500

0.5

64

6.52 × 103

0.032

Mn

3+

Cr

Cumene

53

Ozonation of Organic and Polymer Compounds

15

4.5 mM 12

[DMPC].10 2, M

3.6 mM 9

2.7 mM

6

1.8 mM 0.9 mM

3

0 0

5

10

15

20

25

30

35

T ime, min

0.16

3.6 mM

0.14

2.7 mM

[CHP].102, M

0.12 0.10 1.8 mM

0.08

0.9 mM

0.06 0.04 0.02 0.00 0

5

10

15

20

25

30

Time, min

Figure 1.17 Kinetics of DMPC and CHP formation on [Co(AcO)2], 20 oC

Upon ozonolysis of cumene in AcAc the value of WO3 is 2 × 10-6 M/s-1 and upon addition of Co(AcO)2 in concentrations 0.9, 1.8, 2.7 and 4.5 × 10-3 its value is 0.6, 1.1, 1.7 and 2.9 M/s-1, respectively. This clearly shows that ozone reacts ﬁrst of all with the Co2+ ions. In conformity with the obtained data we have proposed the following scheme of ozonolysis:

54

Ozonolysis of low and high molecular weight saturated hydrocarbons Mn+L + O3 m Mn+1L(O3 )

(0)

Mn+1L(O3 ) + RH m Rv + Mn+1L (ÎH) + O2

(1)

Mn+1L(OH) + RH m Mn+L + Rv + H2O

(1a)

Rv + O2 m RO2v

(2a)

RO2v + RH m ROOH + Rv

(2)

Rv + O3 m ROv + O2

(3)

m (A) 2 ROv + O2 RO2v + RO2v|

(6)

m (1 - A) ROOR + O2 ROv + RH m ROH + Rv

(4)

Scheme 1.6 Applying the quasisteady state approximation approach in relation to WDMPC we obtained: WDMPC = 2.[A/(1-A)].k0 .[Men+L].[O3].[RH]

(1.47)

Equation 1.47 has been derived without considering reactions (2) and (3) because of their insigniﬁcant contribution. It follows from Equation 1.47 that WDMPC is proportional to the concentration of the catalyst applied. By applying values from the literature of k2 and k6 to cumene oxidation [14, 103-105] and [RO2]st as determined from Figure 1.17 the value A is found to be 0.63 which is in good agreement with that reported by other authors [14]. From the analysis of Figure 1.18 and Table 1.15 and considering the concentrations of ozone, the same value of A is obtained. The good correlation of the kinetic results with the proposed Scheme 1.6 testiﬁes to its validity. The addition of cobalt and manganese acetates to the reaction mixture changes the general features of the product formation kinetics (Figure 1.19). Thus WO3 increases and ACP has been identiﬁed. The sum of the products exceeds by 7.3 fold that during

55

Ozonation of Organic and Polymer Compounds cumene ozonolysis. Ozonides are obtained upon the interaction of ozone with the benzene ring, and ACP is produced by the monomolecular decomposition reaction of ROv radicals. The main role of manganese is most likely in accelerating these two reactions. The catalytic properties of the studied metal salts are conﬁrmed by the ratio of the amount of the products formed in the catalysed and noncatalysed processes per unit of absorbed ozone. At Co:Mn = 5:1 this ratio becomes equal to 7.3 and it is greater by about 6% than that in the absence of manganese. The cumene conversion is increased but the selectivity of the process is reduced. The contribution of ozonides and ACP to the total sum of the formed products is 27%. The synergism of the simultaneous action of both salts (Figure 1.19) can be associated with the occurrence of the following reaction: Mn3+(AcO)2(O3-) + Co(AcO)2 = Co3+(AcO)2(O3-) + Mn(AcO)2

12 10

W DMPC .105 [M.s -1]

8 6 4 2 0

0

1

2

3

4

5

Co( A cO) 2.103 [ M]

Figure 1.18 Dependence of WDMPC on [Co(AcO)2] concentration

Nenchev and Shopov [108] stated that increasing the Mn concentration the catalytic effect should decrease mainly on the account of its noneffective interaction with the oxy-radicals. The data in Figure 1.19, however, show that such a trend is not observed as the variations in the concentrations of DMPC, CHP and ACP are insigniﬁcant. The curves, proﬁles are explained by the oxy-reduction interactions of the catalysts and by the radical intermediates.

56

Ozonolysis of low and high molecular weight saturated hydrocarbons

4

DMPC

[P].10 2, M (25min)

3

2

O zonides CH P

1

0

AP 0

20

40

60

80

100

[ Co] / [ Mn] , mol.%

Figure 1.19 Reaction product formation depends on the ratio Co/Mn

1.4.2 Ozonolysis in the presence of NiO The ozonolysis of cumene in the presence of heterogeneous additives, which are effective in ozone decomposition [14] is of interest from two points of view: (1) for elaboration of the mechanism of ozone interaction with the heterogeneous surface, and (2) for increasing the oxidation selectivity. Upon passing ozone (0.1 l/min) through powder NiO (33 m2/g) depending on the oxide concentration either partial or complete ozone decomposition is observed (Figure 1.20). The ozone concentration becomes zero at NiO amount of 140 mg. A badly resolved singlet is registered in the ESR spectrum, however, this disappears in the course of the reaction. This signal was assigned to O3- anion radicals. The UV spectra have a maximum at 320 nm which is attributed to the electron transfers in Ni2. The intensity of the latter goes down in the process of ozone decomposition (the grey colour of the fresh NiO turns black during the reaction process). We have estimated that the catalytic coefﬁcient, i.e., the number of decomposed ozone molecules per one NiO molecule is about 20. By special experiments it has been established that on one Ni2+ ion consequently about 133 ozone molecules of the gas phase are absorbed, but the probability of Ni2+ being transformed to Ni3+ ion is 1/7, i.e., only one out of seven ions is oxidised. These results indicate that the reaction of ozone with Ni2+ proceeds probably in two ways:

57

Ozonation of Organic and Polymer Compounds NiO + 2O3 m NiO + 3 O2 + Q NiO + O3 m ONi3+O3

4

1.0

3 2

0.8

ΔD:D 0

1 1 - 0.85 mM 2 - 0.44 mM 3 - 0.26 mM 4 - 0.08 mM

0.6 0.4 0.2 0.0 0

20

40

60

80

100

120

140

Catalyst, mg

Figure 1.20 Dependence of the ozone decomposed quantity $D/Dot on the NiO amount. $D = Do–Dt; Do - ozone concentration at the reactor inlet at ﬂow rate of 6 l/h with the formation of surface compounds of ONiO3+O3- type. They can react further with a new ozone molecule from a neighbouring centre, with NiO or with the oxides: ONi3+O3 +O3 m NiO2 + 2.5O2 ONi3+O3 + ONi3+O3 m Ni2O3 + 2O2 ONi3+O3 + NiO m Ni2O3 + O2 NiO2 + NiO m Ni2O3 If a hydrocarbon is added to this system, oxidation with participation of ONiO3+O3might be expected. For this purpose the cumene ozonolysis was carried out in the presence of NiO. Figure 1.21 illustrates that ozonides, DMPC and CHP are the main products of the cumene ozonolysis in the presence of NiO.

58

Ozonolysis of low and high molecular weight saturated hydrocarbons It can be seen that the rate of ozonide formation is the highest one and does not depend on [NiO] while the rates of DMPC and CHP formation are relatively lower and increase with [NiO]. As we have mentioned before the ozonides are obtained by the ozone interaction with C=C bonds in the benzene ring, and the ozonolysis of the hydrocarbon takes place via a H-atom abstraction mechanism. Our explanation of these experimental facts is related to the absence of ozonide formation on the surface and to the dominating homogeneous reactions as well as to heterogeneous reactions upon ozonolysis of the alkyl part. The latter is related to the ozone activation on the NiO surface via the formation of the surface compound Ni3+O3- which according to the scheme: Ni3+O3 m Ni3+ + O3 O3 + RH m Rv + HO + O2 Ni3+ + RH m Rv + Ni3+ + H+ oxidises the alkyl part of cumene. The contribution of these reactions to the formation of the reaction products is, however, small as the total rate of product formation is only from 5 to 17% higher compared with the rate of ozone consumption.

6

1, 2, 3

[P].10 3, M

5 4

3'

3

2'

2

1'

1

3" 2" 1"

0 0

5

10

15

20

25

T ime, min

Figure 1.21 Kinetics of product formation in the ozone reaction with cumene in the presence of NiO 20 oC. 1, 2, 3 - OZ; 1a, 2a, 3a - DMPC; 1aa, 2aa, 3aa - CHP; 1 0.05, 2 - 0.325, 3 - 0.635 wt% NiO

59

Ozonation of Organic and Polymer Compounds

1.4.3 Ozonolysis in the presence of Mo and V oxides We studied ozonolysis of cumene catalysed by Mo-V catalysts. The reactions were performed at ambient temperature in a glass reactor with a centrifugal pump with 33 ml cumene in the presence of 0.35 wt% mixtures of MoO2 and V2O5 on SiO2 (Figure 1.22) at various ozone concentrations. The catalysts shown in Table 1.16 were prepared by soaking of SiO2 in water solutions of ammonium para-molybdate and ammonium vanadate and subsequent treatment at 400 oC.

3

2

1

Figure 1.22 Reactor for kinetic studies; 1 - magnetic stirrer; 2 - reactor; 3 - cooler; 4 - centrifugal pump with rotation speed 100-1100 rpm

Figure 1.23a-c illustrates the kinetic curves of major product formation - CHP, DMPC and ozonides. The rate of CHP formation for sample no.5 with Mo:V = 1:1 ratio amounts to 1.6 x 10-5 M/s–1. The values of WCHP for samples nos.7 and 8 are close to that value. In case of sample no. 1 the initial rate of CHP formation is 0.4 x 10-5

60

Ozonolysis of low and high molecular weight saturated hydrocarbons M/s–1 and it is the same as that for the noncatalysed reaction (curve K). The untreated SiO2 (sample no.8) also has some catalytic properties. The integral areas below (under) the kinetic curves, corresponding to the amount of the product formed vary in the presence and in the absence of the catalyst. For example, the area below the kinetic curve of CHP formation in the catalysed reaction is greater compared with that of the noncatalysed one. The stationary concentration of CHP depends on the type of the catalyst and in relation to its catalytic activity the sample shows the following order 5>8>7>1>K. Curve K is characterised by a S-shape form wherein three periods are distinguished: (1) the start of the product accumulation, (2) stationary regime in relation to the product, and (3) autocatalytic process of product formation (Figure 1.23a).

Table 1.16 Catalysts sample with total amount of oxides 10% No.

MoO2:V2O5

S (m2/g)

1.

0:1

13

2.

1:99

10

3.

1:9

11

4.

1:3

5

5.

1:1

4

6.

9:1

5

7.

1:0

10

8.

0:0

14

Upon DMPC formation (Figure 1.23b) one can also notice some difference in the course of the kinetic curves in the presence (curves 1, 5, 7 and 8) and in the absence (curve K) of the catalyst. It has been found that the rate of DMPC formation is one and the same for all studied samples and it is 4.2 x 10-6 M.s–1. After 10 minutes curve K becomes constant and after 15 minutes it rises again, i.e., its proﬁle is typical of an autocatalytic process. The curves of DMPC formation in the presence of catalyst (1, 5, 7 and 8) are characterised by two sections - an initial one with a bigger slope and a second one with a smaller slope - and the transition point is in the interval 5-10 minutes. Obviously, WDMPC is slowed down in the advanced reaction because of the occurrence of secondary reactions associated with the DMPC depletion or are due to the catalyst deactivation. With respect to their activity the samples have the following sequence: 5>1>7>8>K.

61

Ozonation of Organic and Polymer Compounds

8

5

A

7

8

(CHP].10 3, M

6 5

7

4

1

3

K

2 1 0 -1 0

5

10

15

20

25

T ime, min

5 4

1 7

B

8

[DMPC].10 3, M

3

2

K 1

0 0

5

10

15

20

25

T ime, min 10

K

C

8

1

7

,3M

6

8

[OZ].10

4

5 2

0 0

5

10

15

20

25

T ime, min

Figure 1.23 (a-c) Kinetic curves of product formation during the catalysed cumene ozonolysis. (a) CHP, (b) DMPC, (c) ozonides; 1, 5, 7 and 8 are the number of catalysts according to Table 1.16; K - in the absence of catalyst, WO3 = 1 x 10-5 M/s–1 62

Ozonolysis of low and high molecular weight saturated hydrocarbons The kinetics of ozonide formation is entirely different from that discussed before (Figure 1.23c). The initial section of the curves is characterised by the appearance of an induction period up to 15 minutes. These results unambiguously point out that the catalyst directs selectively the process to the oxidation of the isopropyl substituent in the cumene molecule and only after the accumulation of a deﬁnite amount of product, which probably block the catalytic surface, the ozone attacks the benzene ring. In this case the order of activity of the samples is just opposite to that of CHP formation, i.e., 5<8<7<1
20 min

7

CHP

[P].10 3, M

6 5 4 3

DMPC

2 1

OZ

0

20

40

60

80

100

Mn, %

Figure 1.24 Kinetic curves of product formation depending on the Mo:V ratio

The catalytic effect of the studied samples is also demonstrated by the increase of the initial rate of oxidation that results in a rise of CHP yield. This increase in time can be represented by the following equation: [CHP] = a.t2 + b.Wi1/2.t

(1.48)

63

Ozonation of Organic and Polymer Compounds For example, the CHP amount formed in the presence of catalyst is 2.4-fold higher compared with that of the noncatalysed process. The selectivity of the studied catalysts consists in directing the process to oxidation of the isopropyl substituent in the cumene molecule.

1.4.4 Cumenehydroperoxide In order to assess the effect of secondary reactions occurring during cumene ozonolysis we investigated its reaction in the presence of CHP. The dependence of WO3 and [O3] is studied by means of stopped-ﬂow techniques at mixing time of 0.5 seconds.

3.5

Ozone

3.0

CHP

W 03.105, M.s -1

2.5 2.0 1.5 1.0 0.5 0.0 0.0

0.5

1.0

1.5

2.0

2.5

Conc., mM

Figure 1.25 Dependence of the rate of ozone absorption on [O3] ([CHP] = 2.5 M) and [CHP] ([O3] = 0.68 mM), 20 oC, CCl4

As is seen the rate depends linearly on the concentration of both reagents, thus, showing ﬁrst-order kinetics with respect to each one of them. The rate constant k was determined from the slope and it amounts to 14.5 p 0.2 M/s–1. The main reaction products were found to be DMPC and ozonides, while ACP was obtained in much smaller amounts (Figure 1.26).

64

Ozonolysis of low and high molecular weight saturated hydrocarbons

Figure 1.26 Kinetics of product formation during ozonolysis of CHP in CCl4, [O3] = 0.36 mM

Ozone reacts simultaneously with both the alkyl and phenyl moiety in the CHP molecule yielding DMPC, ACP and ozonides. In the ﬁrst case the attack on the OOH group is kinetically much more beneﬁcial resulting in the formation of cumene peroxide and hydroxyperoxide radicals. These radicals according to the classical concepts of oxidation, further undergo transformations giving rise to DMPC and ACP. Upon attacking the phenyl ring, ozone is incorporated into the double bond forming mono-, di- and triozonides. The formation of mono-ozonide is the limiting step as at that moment the aromatic character of the ring is destroyed, while the ozone interactions with dienes and oleﬁns occur with high rates and low activation energies [1, 25]. In view of this we have assumed the existence of the following mechanism shown in Scheme 1.7 for this reaction: ROOH + O3 m A (OZ) + B (RO2v + HOv + O2) +G (ROv + HO2v)

(1)

RO2v + RO2v m 2D (ROv) + (1-D) (ROOR)

(6)

ROv + ROOH m DMPC + RO2v

(4)

ROv m ACP + CH3v

(5)

Scheme 1.7

65

Ozonation of Organic and Polymer Compounds On the basis of the Scheme 1.7 and the available value of k for reactions (4) and (5) the theoretical ratio WDMPC:WACP is 25 ÷ 30:1. The experimentally observed value is 25:1 which is in a very good ﬁt to the theoretical one. Another interesting ratio has been obtained for WDMPC:WOZ = 2.5:1 (Figure 1.16) and the values of A = 0.4 and (B + G) = 0.6 have been found. The good correlation between the theoretical and experimental values of the WDMPC:WACP ratio suggests that G<<1, i.e., the free radical formation is insigniﬁcant. The rate constant of the ozone reaction with DMPC at ambient temperature has been determined in a separate kinetic experiment and k = 3.4 ± 0.1 M-1.s–1. At this value of k the rate of end-product formation becomes commensurable at [CHP]:[DMPC] = 1:4.3. The linear proﬁle of the kinetic curves of DMPC, ozonide and ACP formation depends on the time interval and the absence of induction periods can be regarded as kinetic evidence for the independent formation of the products in parallel reactions (Figure 1.26). Upon ozonolysis of cumene, 1 mol of absorbed ozone yields 1 mol of products, hence the reaction is not a chain one. The main products in the acetic acid medium (Figure 1.27) are ozonides, DMPC and ACP. The curve of CHP consumption is found to decrease exponentially (it becomes a straight line in semilogarithmic coordinates), the curve of ozonide formation passes through a maximum, DMPC is being accumulated linearly with the time and that of ACP formation is strongly accelerated after 50 minutes. The ratio of the initial rates of DMPC:ACP and DMPC:OZ formation are 1:1 and 1:5, respectively. Phenol (PhOH) which has been identiﬁed in the oxidate could be obtained from the wellknown heterogeneous decomposition of CHP in acidic medium giving rise to phenol and acetone (Scheme 1.8).

100

CH P

80

A cP

[P], mM

60

40

DMPC

OZ 20

0 -20

0

20

40

60

80

100

120

140

160

T ime, min

Figure 1.27 Kinetics of product formation during CHP ozonolysis in AcAc, [O3] = 0.8 mM

66

Ozonolysis of low and high molecular weight saturated hydrocarbons Ph C(Me)2 OOH + H+ Ph C(Me)2 O +OH2 Ph C(Me)2 O +OH2 m Ph C(Me)2 O+ + H2O Ph C(Me)2-O+ m Ph +O=C(Me)2 Ph +O=C(Me)2 + H2O m Ph O C(Me)2 +OH2 Ph O C(Me)2 +OH2 Ph O C(Me)2 OH + H+ Ph O C(Me)2 OH (H+) m PhOH + (Me)2C=O Scheme 1.8 The composition of products has been found to be different in ozonolysis in CHCl3 compared with that in pure cumene and in AcAc solution (Figure 1.28).

80

CH P

70 60

PhOH

50 [P], mM

40

OZ

30 20

DMPC

10

A cP 0 -20

0

20

40

60

80

100

120

140

160

T ime, min

Figure 1.28 Kinetics of product formation during CHP ozonolysis in CHCl3, [O3] = 0.8 mM

It has been established that the main products are again ozonides, DMPC and ACP but higher amounts of phenol are observed. In addition, the ozone reaction with CHCl3 yields inorganic chlorine-containing acids and the pH of the medium approaches values of 1-2. These acids (probably HCl and HOCl) are efﬁcient catalysts for the

67

Ozonation of Organic and Polymer Compounds CHP decomposition into phenol. The ratios of the rates are WDMPC:WACP = 2.5:1 and WDMPC:WOZ = 1:14. The kinetic curve of ozonides formation is indicative of its formation in consecutive reactions. The rates of ACP and PhOH formation rise just at the moment of the fall in the kinetic curve of ozonide formation (Figures 1.27 and 1.28), thus suggesting the participation of ozonides in their formation. The dependence of the initial rate of DMPC, ozonides and ACP on WO3 during the cumene ozonolysis in CCl4 is presented in Figure 1.29. The values of A = 0.4, B = 0.6, G<<1 and D = 0.04 calculated from this ﬁgure are in a good agreement with those obtained from the analysis of Scheme 1.7 and Figure 1.26.

18

DMPC

16 14

W.10 6, M.s -1

12 10 8

OZ

6 4

A cP

2 0 -2 0.0

0.5

1.0

1.5

2.0

2.5

3.0

W O 3.105, M.s-1

Figure 1.29 Dependence of the initial rates of product formation (W) on the rate of ozone absorption (WO3) in CCl4, at 20 oC

1.5 Polyethylene and polypropylene We have extended our investigations on ozonation of the C-H bonds in polymer analogues of parafﬁns - polyethylene, polypropylene, their mixtures and polystyrene, with the aim of elucidating the effect of the polymer structure on this reaction.

68

Ozonolysis of low and high molecular weight saturated hydrocarbons

dH/dt, mV

The ozonolysis of polyoleﬁns leads to a change in the structural-physical and dynamic parameters of the polymer matrix such as: the degree of crystallinity (K); density of the amorphous phase; and orientation factor and segmental mobility of the macromolecules [109-110]. The variations of these parameters in polymer mixtures have been little studied. This provoked us to study the effect of the ozone reaction with orientated high-density polyethylene (PE), orientated isotactic polypropylene (PP) and their mixtures on the melting point of crystallites (TT), the relative melting heat, and thus on the degree of crystallinity and the size distribution in the crystallites of the two components. The polymer blends were prepared from noninhibited PP samples with average weight mass (Mw) of 2.86 x 105, average number mass (Mn) of 6.23 x 104 and the ratio between them (Mw/Mn) of 4.6 and for PE samples with the corresponding values of 4.15 x 104, 2.71 x 104 and 1.53. The isotropic ﬁlms were prepared by pressing of granules at 190 oC and under pressure of 150 MPa on a cellophane support and tempered with water at 100 oC. The orientated ﬁlms of 12 p 2 Mm thickness and stretching degree (L = 9) were obtained by local heating up to 100-110 oC K and 120-130 oC. The melting point and the degree of crystallinity were evaluated by means of the differential scanning calorimetry (DSC) method (Figure 1.30).

50

100

150

200

250

Temperature, K

Figure 1.30 DSC curves of the PE:PP = 90:10 blend at L = 9

The values of (K) for PP and PE were estimated using the following values of the relative heats of crystallite melting, i.e., 284.2 and 135.9 kJ/g, respectively. The polydispersity of the crystallites was characterised by the half width of its peak of

69

Ozonation of Organic and Polymer Compounds melting $h. The orientated samples were subjected to ozonolysis in a thermostated ozone chamber. The reaction kinetics were followed by monitoring the variations of the IR spectral intensity at N = 1715 cm–1, characteristic of C=O. The frequency at 1455 cm–1 which remains constant during the course of the experiment, was used as an internal reference. The dependences of the oxidation degree per unit thickness of the sample on the mixture composition at various oxidation times are presented in Figure 1.31.

0.24

46 h 0.20

24 h

D1715, (C=O)

0.16

0.12

10 h 0.08

4h

0.04

0

20

40

60

80

100

PP, %

Figure 1.31 Kinetics of C=O accumulation in orientated ﬁlms from PE/PP mixtures with L = 9, [O3] = 1 mM

Two weak maxima are registered after 4 hours in the IR spectra of the samples containing 10 and 50% PP. Their appearance could be associated with the more porous structure leading to the better dissolving of ozone. This also suggests that the rate of oxidation should increase with increasing time of interaction. We will discuss further the changes of TT and K of PE and PP in the ozonised and fresh samples depending on the mixture composition. The relationship between TT of the crystallites in the initial samples from and those ozonised for 45 hours at ambient temperature are shown in Figure 1.32.

70

Ozonolysis of low and high molecular weight saturated hydrocarbons

170

0h PP

160

46 h

TT, 0C

150

140

0h PE 130

46 h 0

20

40

60

80

100

PP, %

Figure 1.32 Variation of the melting points of PE and PP crystallites in mixtures

As is seen TT goes down for the PE and PP samples, irrespective of the composition and duration of treatment. For PE this decrease is 2-6 oC depending on the composition of the mixture and in the case of PP it is considerably higher - 12 oC for the initial PP and 24 oC for the 1:1 mixture. As for as the ozonised samples of 0-30% PP content, PP crystallites are not observed and for those with PP 40wt% their amount is within the limits of accuracy of the DSC method. The lower melting point of the crystallite ﬁlms points to a reduced ordering of the crystal structure of PP and PE or to an increase in the relative free surface energy of the crystallites sides (Se) compared with those that were nonozonised. The disturbance of the crystallite ordering might be ascribed to the penetration of ozone. At PE ozonation the ozone penetration into the crystallites depth is much weaker since TT in the ozonised and nonozonised samples is similar (Figure 1.32) or it reacts only with the folded surface of the crystallites [115, 116]. The data in [117] support the latter supposition for it has been found that the treatment by 100% nitric acid of the orientated PE samples of stretching degree L = 10 leads to 5-8 oC reduction of TT of the crystallites. This decrease is related to the increase of Se of the crystallite walls as a result of this treatment. A similar decrease of TT (2-6 oC) has also been established upon ozonation of orientated PE with L = 9 (Figure 1.32) which gives us grounds to assume that similar to nitric acid, ozone also reduces TT on account of Se rise.

71

Ozonation of Organic and Polymer Compounds The ozone ability to penetrate through the PP crystallites is accelerated with the decrease of its content from 90 to 50% in the mixtures (Figure 1.32). The notable decrease of the melting point (12-24 oC) upon PP ozonolysis could not be explained by the increase in Se of the crystallites as they simply disappear. More probably the amorphisation is connected to the destruction of crystallites as a result of the ozone action. Actually, in the DSC curves of the ozonised mixtures with 0-40% PP content crystallites are not observed, while in the IR spectra the amount of the CH3 groups is not altered. Ozone reacts with the tertiary hydrogen bond in the amorphous and crystal part. The former leads to an increase in the number of defects which facilitates the subsequent ozone access. Thus the crystallites are destroyed and become further amorphised. The melting point appears to be a very sensitive parameter in investigating the behaviour of the ozonised samples. In Figures 1.31 and 1.32, an antibatic dependence is observed between the curves illustrating the dependence of the oxidation level on composition and TT of the crystallites in blends with various PE content. The periods of acceleration, retardation and maximum occurrence correspond to decrease, increase and minimum appearance depending on TT of the PE crystallites in the mixture composition. Figure 1.33 demonstrates the effect of composition of mixtures on the degree of crystallinity (K).

PP, 0 h

80

PP, 46 h

, %

60

40

PE, 0 h 20

PE, 46 h 0 0

20

40

60

80

100

PP, %

Figure 1.33 Dependence of degree of crystallinity on the composition of the polymer blends

72

Ozonolysis of low and high molecular weight saturated hydrocarbons First we have to note the decrease for the values of K for PE and PP with the decrease of their content in the mixture both for the nonozonised and the ozonised samples. However, in the case of the nonozonised samples the proﬁle of K is characterised by a range wherein it is constant. The value of K for PE in mixtures with PE content varying from 0 to 50% amounts to 60% and in those with a PP content of 50-70% it is 64%. The observation of the decrease in degree of crystallinity of the components in the system PE/PP is also conﬁrmed by other authors [117]. In the case of PE the values of K for the ozonised samples are found to be higher than those for the nonozonised samples. An exception are the samples containing 10 and 20% PE for which K drops abruptly. The increase, could probably be explained by the fact that ozone does not penetrate into the PE crystallites but rather reacts with PE in the transition zone on their surface. This reaction leads to breakdown of the macromolecules in the transition zone and thus the degree of crystallinity is enhanced on account of the fragment provided by the cut-off molecules in the transition zone. The authors of reference [118] have also observed a similar increase of K for PE during its ozonation. These changes of K are also demonstrated by the curves in Figure 1.33. It has been found that the values of K in the PP crystallites of ozonised samples are always higher compared with those in the nonozonised samples (Figure 1.33). This observation could be related to the fact that ozone penetrates into the crystallites and reacts further with tert-C-H bonds. Thus they are gradually crushed and transformed in an amorphous state. This phenomenon is characterised by a decrease of TT and K in the ozonised samples whereby the initial content of PP decreases to lower than 30% and the crystal structure is completely destroyed. It is well known that the polydispersity of crystallites can be characterised by the half width of the melting peaks observed by the DSC method. The changes of this parameter for the two components are illustrated in Figure 1.34. For PE, $h in the ozonised samples is higher than that of the nonozonised samples in the whole range of the mixture compositions with the exception of pure PE and the mixture with 1:9 content. The comparison of the relationship between $h and K allows us to assume that the increase in polydispersity of the PE crystallites can be explained by physical reasons determined by the mixture composition. Thus, in mixtures with a high PE content ranging from 30 to 100% a simultaneous increase of $h and K is observed. The polydispersity of the crystallites results from the rebuilding of the destroyed parts of the macromolecules from the transition zones. In mixtures with lower PE content, below 30%, the rise of polydispersity is accompanied by a drop in the degree of crystallinity.

73

Ozonation of Organic and Polymer Compounds

15

PP, 0 h

14

PP, 46 h

13

Δh

12

PE, 46 h

11 10

PE, 0 h

9 8 0

20

40

60

80

100

PP, %

Figure 1.34 Dependence of the half width of melting peaks on the mixture composition It has been found that the polydispersity of PP crystallites monotonously decreases with the decrease of PP content in the mixtures (Figure 1.34). In each case both the polydispersity and K in the ozonised samples are always lower than those in the nonozonised samples in the whole range of mixture compositions. Apparently, the decrease in PP polydispersity is associated with its initial reaction with the smallest crystallites. Upon more prolonged ozonation of PE and PP their amorphous phases become considerably altered as a result of the accumulation of oxygen-containing compounds of various molecular mass which can also destroy them.

1.6 Polystyrene When ozone is passed through a polystyrene (PS) solution in CCl4 the ozone is absorbed [58]. The rate constant at ambient temperature k has been determined according to Equation 1.14 and it is 0.37 M–1 s–1. The rate of ozone absorption (WO3) remains constant up to an uptake of 3.2 moles ozone per monomer unit. Further, at a consumption of 4.5 moles ozone it becomes 10 times lower. WO3 does not depend on the ozone ﬂow rate in it but increases linearly with the increase of ozone and PS concentration (Table 1.17).

74

4.2

0.31

W x 107 (M.s-1)

k (M–1 s–1)

0.01

0.75

4

0.6

[O3] x 10 (M)

[PS] (M)

v x 103 (l/s)

0.23

8.4

0.75

0.027

0.8

0.45

34

0.81

0.054

0.8

0.27

1.2

0.09

0.027

1.0

0.35

3.9

0.23

0.027

1.0

0.32

11.6

0.75

0.027

1.0

0.47

39

1.7

0.027

1.0

0.25

35.2

2.9

0.027

1.0

0.38

131

7.1

0.027

1.0

0.40

35

0.72

0.068

1.4

0.48

80

0.78

0.12

2.1

0.33

30.4

0.71

0.072

2.8

Table 1.17 Dependence of the rate constant on ozone and PS concentrations and on the ozone ﬂow rate (v) at 25 oC

Ozonolysis of low and high molecular weight saturated hydrocarbons

75

Ozonation of Organic and Polymer Compounds The IR spectra of the ozonised samples (Table 1.18) show that the content of tertCH2 groups hardly changes during ozonolysis as the intensity of the bands at 700 and 2930 cm–1 does not change practically. On the other hand, the intensity at 1500 cm-1, which is assigned to aromatic ring vibrations, decreases in the course of the reaction. This fact conﬁrms that the ozone interaction with the benzene ring gives rise to ozonides. The latter compounds have been identiﬁed by the occurrence of C-O vibrations in the range of 1100-1200 cm-1, as well as by the iodometric titration of the oxidates. It is known that the stoichiometry of the ozone reaction with benzene is 1:3. The kinetic curves represented in Figure 1.35 demonstrate the linear kinetics of ozonide formation. The rate of the ozonide formation at 25 °C has a magnitude of 0.29 p 0.01 M-1.s-1. This value correlates well with the rate constant of benzene ring consumption for cumene which is 0.28 p 0.02 M-1.s-1.

Table 1.18 The intensity of the IR absorption bands of the products of PS ozonolysis: [PS] = 27.4 mM; [O3] = 0.88 mM; at ambient temperature Time (min)

700 cm-1 (%)

1500 cm-1 (%)

2930 cm-1 (%)

-log(It/I0) at 1500 cm-1

0

36.1

23.0

36.0

0

10

35.4

21.2

35.8

0.035

15

36.0

20.1

35.9

0.059

25

34.5

19.3

35.1

0.076

30

34.9

18.0

35.8

0.106

The kinetics of hydroperoxide accumulation is illustrated by the curves in Figure 1.36; their proﬁle suggests the intermediate character of the hydroperoxides formed in this reaction. It is seen that at the initial stage of the reaction [ROOH] increases and WROOH depends linearly on [O3] (Figure 1.36, curve 2). The ROOH content at 40% conversion is about 6% irrespective of [PS] which is within the range 0.1-1.2%.

76

Ozonolysis of low and high molecular weight saturated hydrocarbons

1.0

a 0.8

[OZ], mM

0.6

0.4

0.2

0.0 0

50

100

150

200

250

300

Time, min 5

b 4

Woz.106, M.s-1

3

2

1

0 0

1

2

3

4

[O3]g.104, M 0.12

c

0.10

-1 lg(I/I0)1500 cm

0.08 0.06 0.04 0.02 0.00 0

5

10

15

20

25

30

Time, min

Figure 1.35 Kinetics of ozonide formation (a) at 25 oC, [PS] = 24.7 mM, [O3] -5 g = 1.2 x 10 M; (b) dependence of the rate of ozonide formation on ozone concentration at the reactor outlet, 25 oC, [PS] = 27.4 mM; (c) variation of the band intensity at 1500 cm-1, 25 oC, [PS] = 27.4 mM, [O3]g = 8.8 x 10-4 M 77

Ozonation of Organic and Polymer Compounds 1.6

a

1.4

[ROOH], mM

1.2 1.0 0.8

1

0.6 0.4 0.2 0.0

-0.2 0

50

100

150

200

250

300

Time, min b

30

2

WROOH.108, M.s-1

25 20 15 10 5 0 0.0

0.5

1.0

1.5

2.0

2.5

[O3]g.104, M

Figure 1.36 (a) Kinetics of ROOH formation (1), at 25 oC, [PS]0 = 60 mM, [O3]g = 1.4 x 10-4 M; and (b) dependence of the initial rate of ROOH formation on the ozone concentration at the reactor outlet (2), 25 oC, [PS]o = 27.4 mM

The kinetic curves of the variation of PS molecular weight at [PS] = 1.5% pass through a maximum (Figure 1.37, curve 1). It has been established that carbonyloxides take part in crosslinking processes [1]. For this reason we have studied the PS degradation in solutions with concentrations below 1.5%. Upon ozonolysis of diluted PS solutions with concentrations below 0.5% we have observed a decrease of the average molecular weight (Figure 1.37, curve 2). The rate of degradation is constant up to a conversion degree of 30% (Equation 1.49): Wd = (1/M -1/M0).t-1.[PS]0-1

(1.49)

It is linearly dependent on the ozone uptake rate and on temperature (Figure 1.38).

78

Ozonolysis of low and high molecular weight saturated hydrocarbons

1

1.4 1.2

Mn.10-5

1.0 0.8 2

0.6 0.4 0.2 0.0 0

20

40

60

80

100

120

Time, min

Figure 1.37 Kinetics of PS molecular weight variation with average number molecular mass 1.2 x 105 upon ozonolysis (1) [PS] = 3.5%, (2) [PS] = 0.3%, WO3 = 2 x 10-4 mol/(kg.s), at 25 oC

6 3

Wd.106, brakings/(kg.s)

5 2 4 3 2 1

1 0 0.0

0.5

1.0

1.5

2.0

Rate.104, mol/(kg.s)

Figure 1.38 Dependence of the rate of PS degradation on the rate of ozone consumption: 1 - 0 oC; 2 - 25 oC; 3 - 60 oC, [PS] <1%

79

Ozonation of Organic and Polymer Compounds The number of chain breaks in diluted solutions related to 1 mol absorbed ozone n = Wd/Wi does not depend on the polymer concentration but it rises with temperature (Table 1.19). Under these conditions the reactions, leading to an increase in the molecular mass do not exhibit any essential inﬂuence. This is also conﬁrmed by the proﬁles of the molecular mass distribution (MMD) of low and high molecular weight components (Figure 1.39). The decrease of the molecular mass and the rise of the number of broken chains (n) is accompanied by narrowing of the MMD (Figure 1.39a) and the ratio Mw/Mn approaches a value equal to 1 (Figure 1.39b).

100

3

4

2 1

7

80

W (M), %

5

6

60

a

40

20

0 0

10

20

30

40

50

M.w.104 0.7

0.7

0.6

0.5

0.5

0.4

0.4

0.3

0.3

- lg [M w/ M n-1]

Mw/Mn-1

b 0.6

0.2

0.2 0.0

0.2

0.4 2

0.6

0.8

1.0

n.10 , brakings/mol

Figure 1.39 (a) Integral curves of MMD at 25 oC, WO3 = 6.6 x 10-5 mol/(kg.s), [PS] = 1% and ozonation time of: 1 - 0 min, 2 - 15 min, 3 - 30 min, 4 - 45 min, 5 - 60 min, 6 - 90 min and 7 - 120 min; (b) MMD in the course of ozonation

80

0.4

0.47

n x 102

nav x 102

W x 10

2.2

0.88

Wd x 106

p 0.07

0.54

3.1

1.6

0.7

1.5

[PS] (%)

4

0

0

2.8

1.2

3.4

0.19

25

1.8

1.9

2.0

3.8

0.30

25

p 0.6

2.3

2.2

5.0

0.42

25

1.2

1.2

1.4

0.71

25

1.7

3.4

5.8

1.5

25

0.9 p 0.8

0.8

1.7

1.4

6.5

25

0.2

2.1

0.4

10.4

25

6.2

6.0

1.0

6.0

1.5

60

6.4

1.2

7.7

0.7

60

p 0.2

Table 1.19 Dependence of the rate of PS degradation on the solution composition and temperature (t)

t ( C)

o

Ozonolysis of low and high molecular weight saturated hydrocarbons

81

Ozonation of Organic and Polymer Compounds It has been established by means of ESR that the ozonolysis of PS in the solid phase generates peroxide radicals [126] which arise not in the direct ozone reaction with PS but are also secondary products. The kinetic scheme of the reaction, proposed by us, is based on the experimentally obtained data and it can be represented as shown in Scheme 1.9: (-CH2-(Ph)CH-)m + O3 m AP1 + (1-A)(P2 + P3 + …)

(1)

P1 + O3 m RO2v

(2)

(-CH2-(Ph)CH-)m + RO2v + O2 m ROOH + RO2v

(3)

RO2v + RO2v m B(2ROv) (6a)+ (1-B) ROOH + O2,

(6a)

where: P1, P2 and P3 are intermediate products Scheme 1.9 The reactions describing the ozonides formation and the ozone interaction with ROOH are included in Scheme 1.9: (-R-)mPh + O3 m monoozonide (slow) monoozonide + O3 m diozonide

diozonide + O3 m triozonide

(4)

m GROH + 2O2

(5a)

ROOH + O3 }| m (1-G) (RO2v + HOv + O2v)

(5aa)

The linear dependence of the rate of ozone absorption on its concentration as well as the stoichiometric coefﬁcient of the reaction in the range of 4.2-4.5 can be regarded as evidence for the absence of ozone absorption in a chain reaction. According to Scheme 1.9 the reaction of ozone consumption will be represented as: W = (k1 + 3k4)[PS][O3] + k2[P1][O3]

(1.50)

[P1] = A.[PS]0.k1.(k2 –k1)-1.{exp [-k1[O3].t – exp[–k2[O3].t}

(1.51)

and

82

Ozonolysis of low and high molecular weight saturated hydrocarbons It is apparent that reaction (2) has an insigniﬁcant effect in regard to the total balance of ozone consumption as only 6% of ROOH is obtained upon the absorption of 1 mol ozone. The value of the observed rate constant kobs is equal to k1 + k4 and then k1 = 0.13 M-1.s-1. The insigniﬁcant amounts of ROOH indicate that actually the oxidation of tert-H bonds does not take place via a chain reaction. During the initial stage of ozonation [P1] = A × k1 × [PS]o[O3] × t, as [RO2] ^O3 and WROOH = k3[RO2] × [PS] ^O3. This is also observed experimentally (Figure 1.36b, 2). The results suggest that the RO2 radical formation in PS is not dependent on the phase state of the occurring reaction. The recombination of RO2 radicals according to reaction (6a) gives rise to RO radicals. The macromolecule destruction takes place via a monomolecular decomposition of RO radicals: ROv m rv+ (Ra)2C=O (degradation)

(7)

In the case where the degradation process takes place via reaction (7), then W d ^W O3 . In order to elucidate the mechanism of the degradation process we carried out an initiated oxidation of PS {[AIBN] = 2%, 60 o C, Wi = 1 x 10 -5 mol/(kg.s)}, where AIBN is azoisobutyronitrile. For the inhibited oxidation W d has been found to be 9 x 10 -7 breakdowns/(kg.s) and upon ozonation a similar value for Wd has been obtained at WO3 = 1.5 x 10-5 mol/ (kg.s) (Figure 1.38, 3). If we assume that the initiated oxidation and the ozonolysis differ only in the pathway of RO2 radical generation, then the mechanism of destruction and the kinetic correlation should be similar. The experimental obtained for values the ratio of Wi/WO3 accounting for the occurrence of radical steps in the ozonolysis are: 0.05 at 0 oC, 0.2 at 25 oC and 0.69 at 60 oC (Figure 1.38). The activation energy of degradation Ea calculated on the basis of Figure 1.38 amounts to 7.6 kcal/mol and correlates well with the activation energy of radical isomerisation, Ea [120]. The ratio Wi/W could be estimated using the values of k1 and k4 and the quasistationary approximation conditions in respect to P1: Wi/W = 0.43AB

(1.52)

From the experimentally measured ratio Wi/W = 0.2 at 25 oC it follows that AB = 0.47. Because of the lack of any data for the magnitude of B for PS we have used the known value of B for cumene oxidation (low molecular weight analogue of PS) that is 0.86 and then we have obtained 0.55 for A. The latter value demonstrates that the major part of RO radicals have been generated not via reaction (6aa) but as a result of the direct ozone attack on the PS macromolecule. Thus it can be reasonably expected that the degradation will occur via C=C bonds scission, for which [1, 25] the rate of their reaction with ozone is known, i.e., 105 83

Ozonation of Organic and Polymer Compounds M–1s-1 and similar to polydienes they would be responsible for the PS degradation. However the amount of C=C bonds in PS is of the order of (3 p 0.5) × 10-2 mol/kg and at average number mass of PS - 10-4 mol/(kg.s) - which means that one macromolecule contains 1-2 double bonds. Upon statistical distribution of C=C bonds along the macromolecule length, the degradation at WO3^10-4 mol/(kg.s) should ﬁnish in 5-10 minutes. However we failed to observe a marked decrease of the molecular weight within this time interval. Further, Wd should begin to go down. Actually the rate of degradation remains constant even after 150 minutes when the conversion is already 30%. Apparently, the C=C bonds in PS are not effective in this respect, mainly because they are terminal double bonds. This supposition has been conﬁrmed by measuring the concentration of C=C bonds which decreases during ozonation with the simultaneous rise of PS molecular mass. If we take into account that the degradation proceeds via a random law then the equilibrium width of MMD should approach value of 2 only after 2-3 breakdowns. However this is in contrast to the experimentally obtained data illustrated in Figure 1.39b. It is seen that MMD depends on the conversion degree and approaches 1. This effect could be explained by the variation of the ratio of the values of the elementary constants which characterise the scission of C-C bonds in the macromolecule. A decrease of Wd has been registered during the transition to lower molecular polymers as is the case with fractionated PS and with polymers with broad MMD. This might be the explanation for the decrease of the effective Wd upon ozonolysis and the narrowing of the MMD. The mechanism of alkane ozonolysis will be discussed further from the quantum chemical point of view in Chapter 5.

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Ozonolysis of low and high molecular weight saturated hydrocarbons 64. S.K. Rakovsky, D.R. Cherneva, A.A. Spozhakina and K. Novakov in the Proceedings of the 8th International Symposium Hetercyclic Catalysis, Varna, Bulgaria, 1996, p.407. 65. R.I. Martinez, International Journal of Chemical Kinetics, 1982, 14, 237. 66. I.N. Komissarovva, V.D. Komissarov and E.T. Denisov, Izvestiya Akademii Nauk Seriya Kimicheskaya, 1978, 9, 1991. 67. T.M. Hellman and G.A. Hamilton, Journal of the American Chemical Society, 1974, 96, 1530. 68. F.E. Stary and D.E. Emge and R.W. Murray, Journal of the American Chemical Society, 1976, 98, 1880. 69. I. Vedeneev, L.V. Gurvich, V.N. Kondrat’ev, V.A. Medvedev and E.L. Frankevich, Energy Breaking of the Chemical Bonds, Ionisation Potencials, and Electron Afﬁnity, Publishing House of the Akademy Nauk SSSR, Moscow, 1962, p.90. 70. V.V. Voevodskii, Physics and Chemistry of Elementary Chemical Processes, Nauka Publishers, Moscow, 1969. 71. D.A. Frank-Kamenetzkii, Diffusion and Heat Transfer in Chemical Kinetics, Nauka Publishers, Moscow, 1987. 72. T.K. Sherwood, R.L. Pigford and C.R. Wilke, Mass Transfer, McGraw-Hill, New York, NY, USA, 1975. 73. P.V. Danckwerts, Gas-Liquid Reactions, McGraw-Hill Book Company, New York, NY, USA, 1970. 74. G. Astarita, Mass Transfer with Chemical Reaction, Elsevier, Amsterdam, The Netherlands, 1967. 75. G.S. Hammond, Journal of the American Chemical Society, 1955, 77, 334. 76. I.C. Hisatsune, L.H. Kolopajlo and J. Heicklen, Journal of the American Chemical Society, 1977, 99, 3704. 77. W.G. Alcock and B. Mile, Journal of the Chemical Society: Chemical Communications, 1976, 5. 78. U. Burkert and N.L. Allinger, Molecular Mechanics, ASC Monograph No 177, ACS Washington, DC, USA, 1982.

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Ozonolysis of low and high molecular weight saturated hydrocarbons 96. M.G. OEvans and M. Polany, Transactions of the Faraday Society, 1936, 32, 1333. 97. L.G. Galimova, V.D. Komissarov and E.T. Denisov, Izvestiya Akademii Nauk Seriya Kimicheskaya, 1973, p.307. 98. M. Dioxin and E. Ebb, Ferments, Mir Publishing House, Moscow, Russia, 1966, p.277. 99. V. Karnogitzkii, Organic Peroxides, Inostrannaja Literatura Publishers, Moscow, Russia, 1961, p.115. 100. F. Kritchﬁld, Analysis of Basic Functional Groups in Organic Compounds, Mir Publishers, Moscow, Russia, 1965, p.188. 101. E.J.E. Hawkins, Organic Peroxides, Khimia Publishers, Moscow, Russia, 1965. 102. R.M. Johnson and J.W. Siddiqi, The Determination of Organic Peroxides, Pergamon Press, New York, NY, USA, 1970, p.15. 103. E.T. Denisov, Rate Constants of the Homolytical Liquid-Phase Reactions, Nauka Publishers, Moscow, 1971. 104. V.Ya. Shlyapintoh, O.N. Karpuhin, L.M. Postnikov, I.V. Zaharov, A.A. Vichutinskii and V.F. Tzepalov, Chemiluminescent Methods of Investigation of the Slow Chemical Proceses, Nauka Publishers, Moscow, Russia, 1966. 105. E.T. Denisov, Kinetic of Homogeneous Chemical Reactions, Visshaya Shkola Publishing House, Moscow, Russia, 1988. 106. V.A. Yakubi, B.A. Ponomarev, N.F. Tupalo, G.A. Galstyan, L.P. Petrenko, S.M. Tyutyunnik and P.P. Karpuhin in Theory and Practis of Liquid Phase Oxidation, Ed., N.M. Emanuel, Nauka Publishers, Moscow, Russia, 1974, p.303. 107. G.A. Galstyan in Proceedings of the 2nd Ozone Conference, Moscow, Russia, 1977, p.40. 108. L.K. Nenchev and D.M. Shopov, Neftekhimia, 1978, 18, 2, 256. 109. S.G. Karpova, A.A. Popov, S.N. Chvalun, Yu.A. Zubov and G.E. Zaikov, Vysokomolekulyarnye Soedineniya Seriya B, 1985, 27, 9, 686.

91

Ozonation of Organic and Polymer Compounds 110. S.G. Karpova, A.A. Popov, S.N. Chvalun, Yu.A. Zubov and G.E. Zaikov, Vysokomolekulyarnye Soedineniya Seriya A, 1986, 28, 7, 1404. 111. A.A. Popov, A.V. Rusak, O.A. Ledneva and G.E. Zaikov, Vysokomolekulyarnye Soedineniya Seriya A, 1986, 28, 9, 1836. 112. N.N. Blinov, A.A. Popov, E.S. Popova, A.V. Butusov, A.N. Neverov, G.E. Zaikov and G. Irinyi, Polymer Degradation and Stability, 1988, 22, 7. 113. A. Cipitoin, A. Constantinesku and V. Dobresku in the Proceedings of the IUPAC Macro’83, Budapest, 1983, Section 4, p.727. 114. K.H.H. Patel, Journal of Polymer Science: Polymer Physics, 1975, 13, 2, 303. 115. A.A. Popov, N.N. Blinov and G.E. Zaikov, DokladyAkademii Nauk SSSR, 1986, 286, 4, 922. 116. V.I. Selihova, L.A. Ozerina, L.N. Ozerin and N.F. Bakeev, Vysokomolekulyarnye Soedineniya Seriya A, 1986, 28, 2, 342. 117. D.J. Priest, Journal of Polymer Science Part B: Polymer Physics Edition, 1971, 9, 10, 1777. 118. S.G. Karpova, Solid Phase Polyethelene Oxidation, Institute of Chemical Physics, Moscow, Russia, 1985. [PhD Thesis] 119. I.S. Gaponova, V.M. Gol’dberg, G.E. Zaikov, A.A. Kefely, G.B. Pariiskii, S.D. Razumovskii and V.Ya. Toptigin, Vysokomolekulyarnye Soedineniya Seriya A, 1978, 20, 2037. 120. J.A. Howard and K.V. Ingold, Canadian Journal of Chemistry, 1969, 47, 3797.

92

2

Ozonolysis of oxygen-containing organic compounds

The ozonolysis of oxygen-containing compounds is a promising reaction that takes place under mild conditions and yields compounds of a higher oxidation level than that of the starting compounds. It may ﬁnd various applications in chemical and pharmaceuticals industries, ﬁne organic synthesis, etc. [1-2]. Moreover, this reaction is extremely important from an ecological point of view for the utilisation and puriﬁcation of waste industrial waters from hydroxybenzene production, through their partial or complete oxidation [3-19]. The importance of this reaction for theory and practice gave us a basis for making systematic investigations [20-28].

2.1 Alcohols Current concepts on the ozonolysis of alcohols are discussed in several monographs [1, 2, 20, 29]. During recent years a number of authors have tried to elucidate the kinetics and mechanism of this reaction and its application for the selective preparation of ketones and aldehydes in high yields under mild conditions [29-34]. It has been established that the rate of ethanol ozonolysis does not change when the hydroxyl group is deuterated [33] and its value is 4.17 times higher than when the methylene group is deuterated. On this basis, together with the data from the analysis of the product composition and the kinetics of their formation the authors have suggested that the H-atom abstraction from ozone is the rate-determining step of the reaction. This conclusion is conﬁrmed by the data in Table 2.1 as reported by various authors [31-33]. The ratio of the relative reactivities of tertiary:secondary:primary alcohols is 1:12:241 according to reference [31], or 1:156:817 according to reference [6]. On the basis of the studies on the reaction of ozone with methyl, ethyl and 2-propyl alcohol, the following reaction mechanism is proposed (Scheme 2.1) [30]: CH3OH + O3 [HOCvH2...HO3v]# m HOCH2OOOH, and in the presence of a base: CH3OH + O3 + (B) m HOCHO + O2H + BH+ 93

Ozonation of Organic and Polymer Compounds CH3CH2OH + O3 [CH3HCv(OH)...HO3v]# m CH3HC(OOOH)OH m CH3OOH + H2O2 and/or CH3CHO + O2 + H2O (CH3)2C(OH)H + O3 [(CH3)2Cv(OH)...HO3v]# m (CH3)2C(OH)OOOH n -O2 CH3C(OH)=CH2 + H2O +O2 (CH3)2CO + H2O n +O3 CH3COOH + HCHO Scheme 2.1

Table 2.1 Kinetic parameters of the ozone reaction with aliphatic alcohols at 25 oC k (M-1.s-1)

log A

E (kcal/mol)

Reference

t-Butanol

0.01

4.6

9.0

[32]

t-Butanol

0.05

-

-

[31]

Ethanol

0.35

6.7

9.8

[32]

Ethanol

0.25 (22 °C)

6.7

9.8

[33]

n-Butanol

0.54

7.3

10.3

[32]

n-Butanol

0.39

iso-Propanol

0.89

Cyclopentanol

1.35

Alcohols

[31] 7.3

10.0

[32] [31]

It is supposed that an intermediate ion or radical pair is formed whose recombination in the kinetic cage yields A-hydroxy-hydrotrioxide. The latter leaves the cage and passes into the solution volume. The authors of reference [30] found that the product composition of 2-propanol ozonolysis depends on the method of propanol puriﬁcation. It has been established that the ozonolysis of 21 alcohols always leads to the formation of the corresponding aldehydes and ketones in 53-83% of the yields [34].

94

Ozonolysis of oxygen-containing organic compounds From the interesting results given above and the assumptions for the reaction pathway, in our opinion, the mechanism is still unclear and new data should be provided for its elaboration. In this connection we have carried out intensive experimental and theoretical studies of this reaction, the results of which are summarised in Figures 2.1-2.3 and Tables 2.2-2.4. Figure 2.1 demonstrates the results from ozone consumption in solutions of methanol and iso-propanol by means of the stopped ﬂow technique with time of mixing below 0.2 seconds. It is seen that irrespective of the type of alcohol being ozonised, the kinetic curves coincide and ﬁrst-order kinetics are observed under the experimental conditions applied. Calculations of the kinetic curves of ozone reaction with methyl alcohol (MeOH), ethyl alcohol (EtOH) and iso-propanol (i-PrOH) gave the following values of the rate constants: 0.057, 0.17 and 1.13 M-1.s-1, respectively. The same values were also obtained by carrying out the reaction in a bubble reactor. The values of k calculated according to Equation 1.4 are given in Table 2.2 and are presented graphically in Figures 2.2 and 2.3.

1.0

[O3].104, M

0.8

0.6

1 2

0.4

3 0.2

0.0 0

20

40

60

Time, s

Figure 2.1 Kinetics of ozone consumption in the reaction of ozone with: 1 methanol, 22 oC, 0.74 M; 2 - ethanol, 3 oC, 0.24 M; and 3 - isopropanol, 3.5 oC, 0.037 M

95

Ozonation of Organic and Polymer Compounds

Table 2.2 Kinetic parameters of ozone reaction with MeOH in carbon tetrachloride (CCl4) and pure MeOH solutions: 22 oC, W = 0.167 s-1; v = 1.67 x 10-3 l/s, maximum rate of ozone inlet - 1.67 x 105 M.s-1 [MeOH] (M)

[O3]0 × 105 (M)

[O3]g × 105 (M)

$[O3] × 105 (M)

W × 106 (M.s-1)

k (M-1.s-1)

0

10

0

10

0

-

0.247

10

8.32

1.68

2.80

0.057

0.439

10

7.35

2.65

4.43

0.056

0.618

10

6.64

3.36

5.61

0.058

0.740

10

6.22

3.78

6.31

0.055

0.987

10

5.53

4.47

7.46

0.057

1.0

10

5.49

4.51

7.53

0.056

2.0

10

3.79

6.21

10.37

0.055

3.0

10

2.89

7.11

11.87

0.057

4.0

10

2.34

7.66

12.79

0.058

5.0

10

1.96

8.04

13.42

0.054

10.0

10

1.09

8.91

14.88

0.056

24.7

10

0.47

9.53

15.91

0.058

0.740

8

4.98

3.02

5.04

0.057

0.740

6

3.73

2.27

3.79

0.055

0.740

4

2.49

1.51

2.52

0.056

0.740

2

1.24

0.76

1.27

0.058

Processing the data from Table 2.2, columns 5, 2 and 3, according to Equation 1.3 yields the linear dependencies shown in Figure 2.2a and b. The rate of W on $O3 has also been found to be linear. The dependence of W on [ROH] (column 1) is a curve which approached a limit at W = 1.67 × 10-5 M/s-1. The nonlinear character of this relationship is in accordance with the complexity of Equation 1.3 (Figure 2.3) as when [ROH] md, [O3] m0 and $O3m [O3]o.

96

Ozonolysis of oxygen-containing organic compounds

7 6

W.106, M.s -1

5 4 3 2 1 0 0

2

4

6

8

10

8

10

[O3]0.105, M

7 6

W.106, M.s -1

5 4 3 2 1 0 0

2

4

6 5

[O3]0.10 , M

Figure 2.2 Dependence of the rate of ozone consumption on the inlet (a) and outlet (b) ozone concentration

The values of k calculated by Equation 1.4 for each point of the curve are equal to 0.057. Hence, when using the bubbling method when the rate is of the ﬁrst order in relation to ozone the dependencies of W on [O3]o, [O3]g and $[O3] should be linear and that on [ROH] should have the proﬁle of the curve in Figure 2.3.

97

Ozonation of Organic and Polymer Compounds From the analysis of the data obtained (Tables 2.3 and 2.4) it follows that the rate constant and the activation energies are strongly dependent on the alcohol structure.

Table 2.3 Dependence of k on temperature in oC for ozone reaction with three types of alcohol k (M-1.s-1)

0 oC

10 oC

20 oC

25 oC

30 oC

MeOH

0.008

0.021

0.049

0.072

0.108

t-BuOH

0.005

0.013

0.029

0.045

0.064

EtOH

0.14

0.28

0.54

0.74

1.10

n-PrOH

0.19

0.36

0.67

0.89

1.18

n-BuOH

0.15

0.30

0.56

0.76

1.10

i-PrOH

0.93

1.61

2.71

3.46

4.39

s-BuOH

0.88

1.54

2.58

3.29

4.18

c-HexOH

0.92

1.59

2.65

3.37

4.27

BuOH: butanol PrOH: propanol HexOH: hexanol

The interaction of ozone with MeOH possessing primary A-H atoms and tert-BuOH with only primary C-H bonds has been found to be the slowest, and the value of k at 20 oC per one A-H atom in the ﬁrst case is 1.62 × 10-2 M-1.s-1, and in the second case, related to one primary H atom, is 3.22 × 10-3 M-1.s-1. The difference in the values of these constants is due to the fact that while the OH group in MeOH directly affects the A-H atom, for the tert-BuOH, which does not possess any A-H atoms, the effect of the OH groups is carried through one S-bond and it is considerably weaker. As a result of this the reactivity of tert-BuOH becomes similar to that of methane and neopentane. The interaction of ozone with EtOH, n-PrOH, n-BuOH takes place at a higher rate and the values of k per one A-H atom amount to: 0.27, 0.34 and 0.28, respectively. The higher rate of the ozone reaction with these alcohols is associated with the presence of secondary A-H atoms in their molecule, which have lower bond energy than the primary ones. The enhanced reactivity of n-PrOH compared with that of EtOH could be ascribed to the donor effect of the second CH3 group while the donor effect of the C2H5 group in n-BuOH is weaker than that of the methyl group and thus k is lower. It has been found that the reaction

98

Ozonolysis of oxygen-containing organic compounds of ozone with i-PrOH, s-BuOH and s-HexOH alcohols possessing tert-H atoms with the lowest energy bond is the fastest with k equal to 2.71, 2.58 and 2.65, respectively, i.e., in fact they are the same. The ratio of the reduced values of k at 25 oC for methanol:ethanol:sec-butanol are 1:15:137. Simultaneously the values of Ea decrease with the decrease of the A-H atoms bond energy (D) and their ratio is 1:0.78:0.61 [11, 14]. This fact can be regarded as important evidence for the mechanism of A-C-H-atom abstraction by ozone.

7 6

W.106, M.s -1

5 4 3 2 1 0 0

2

4

6

8

10

8

10

[O3]0.105, M

7 6

W.106, M.s -1

5 4 3 2 1 0 0

2

4

6

[O3]0.105, M

Figure 2.3 Dependence of the rate of ozone consumption on methanol concentration according to Equation 1.4

99

Ozonation of Organic and Polymer Compounds

2.1.1 Application of Estimation of Reaction Mechanism (ERM) We have tried to elaborate the mechanism of alcohol ozonolysis by means of ERM, as described in Chapter 1. The calculations of A were performed considering the two possible structures of the activated complex (AC): linear (LC) and cyclic (CC) (Figure 2.4).

_ HO H

C

+

H LC

O H

O

O

O

O

H

H

O

O

O

HO

C H

H CC'

O

O H

C

.

H

H CC

Figure 2.4 Structure of the probable activated complexes in the reaction of ozone with aliphatic alcohols: LC - linear with free fragment rotation and CCa and CC cyclic complexes without free rotation

The good agreement between the experimental and theoretical values for A allows the determination of the AC structure and on this basis the selection of the most likely mechanism. Simultaneously the value of the pre-exponential was calculated by the collision method and by comparing its value with that calculated by the AC theory the steric factor -p was determined (Equation 2.1) [36-40]: k = p.Z0.exp(-Ea/RT),

(2.1)

where: Z0 = P.(rA+rB)2 × (kT/Pm*)1/2; Zo is the collision factor; rA and rB are the van der Waals radii of the reagents; k is the Boltzmann constant; T is the absolute temperature; and m* is the reduced mass. The pre-exponentials for the reaction of ozone with alcohols, calculated according to the activated complex method (ACT) and collision theory (CT) are presented in Table 2.5.

100

Ozonolysis of oxygen-containing organic compounds

Table 2.5 The values of A calculated by ACT with LC and CC and by CT, the sums (Rfr) and energy (Efr) of inner rotation, steric factors (p), van der Waals radii of the molecules (r) and the ratio between the calculated and experimental values of pre-exponentials ALC /Aobs cal

MeOH

EtOH

n-PrOH

i-PrOH

n-BuOH

s-BuOH

t-BuOH

c-HexOH

ACC × 10-4 (M-1.s-1)

52

8.3

3.7

3.5

2.4

2.2

6.3

1.6

ALC × 10-4 (M-1.s-1)

56

15

6.7

6.3

4.4

4.0

4.9

2.9

Ffr × 10-2

7.1

9.8

12

11

13.8

12.6

12.9

12.6

ALCcal × 10-7 (M-1.s-1)

40

15

8.1

6.8

6.1

5.0

6.3

3.6

r (Å)

2.24

2.57

2.91

2.88

3.13

3.23

3.13

3.23

CT

A × 10-11 (M-1.s-1)

2.6

2.6

2.7

2.7

2.8

2.8

2.8

2.7

p × 105

22

5.8

2.5

2.3

1.6

1.4

1.8

4.1

ALCcal/ Aobs

1.0

3.8

3.4

11

3.1

8.3

1.1

7.2

0

730

670

1368

701

1172

52

1081

Afr, (cal/mol)

Note: ALC is the pre-exponential calculated with LC without free fragment rotation (it is absent at Efr r4 kcal); ACC - calculated with CC; ALCcalc = at free fragment rotation, i.e., when exp(-Efr/RT) = 1 or Efr = 0 kcal; Act - calculated according to the collision theory at 27 0C and r - calculated by the Alinger method (pcmod4 program) at the radius of the ozone molecule of 2A; Efr - the calculated values of the rotation energy at ratio of ALCcal/Aobs = 1.

An interesting fact is the good agreement between the values of ALC and ACC. The values of ACC with the ﬁve- and seven-member cyclic form of AC are actually similar as they differ by not more than 1%. However the free rotation produces an increase in ALCcal by three orders of magnitude compared with those of ACC. The steric factor has a value which is in agreement with the liquid phase reaction. The ratio between the calculated and experimental values of ALC which increases in the sequence

101

Ozonation of Organic and Polymer Compounds primary:secondary:tertiary alcohols is 1:3.4:8.8, i.e., the coincidence in this sequence decreases. This is connected with the fact that Efr is not zero and they increase with the changes in the alcohol structure in the same sequence. It has been found that the values of ALCcalc. and Aobs. coincide when the values of rotation energy have the values in line 10. We have obtained the same values by means of the mopac6 program and therefore the values in line 10 can also be considered as calculated. The good agreement between ALCcalc and Aobs. suggests that the mechanism of the ratedetermining step of the ozone reaction with alcohols is associated with the formation of linear AC and abstraction of the A-H atom from the alcohol. The experimental and theoretical results conform well with Scheme 2.2.

R1R2(OH)C-H...O3

R1R2CHOH + O3

# R1R2COH + HO3 (HO + O2) kinetic cage

-O2

R1R2C(OOOH)OH

R1R2C(OOH)OH R1R2C(OH)OH

-H2O2

-H2O

-HO2

R1R2C(OH)OH

-H2O

R1R2C(O)OH

R1R2CO R2COOH + R1

R1R2CO

R1R2CO

Scheme 2.2 Ozone attacks the A-H atom forming a LC which further undergoes decomposition to a radical (or ion) pair in one kinetic cage. The A-hydroxy alcohol, A-hydroxyperoxy alcohol and A-hydroxytrioxy alcohol being unstable leave the cage and decompose rapidly to the corresponding aldehyde or ketone liberating water, hydroperoxide and oxygen or lead to the formation of hydroperoxy and alkoxy radicals. The latter can further undergo monomolecular decomposition.

2.2 Ketones The investigations of the reaction of ozone with ketones are of interest in relation to the theory of reactivity, the ozone chemistry, preparation of oxygen-containing compounds, and the degradation and stabilisation of organic materials. In particular, the ozonation of methylethylketone (MEK) [41-43] yielded acetic acid (AcAc), diacetyl and hydroperoxide as the main reaction products in CCl4. It has been assumed that the process proceeds as a nonchain radical reaction (RO2). Similar products are also

102

Ozonolysis of oxygen-containing organic compounds formed when the reaction takes place in water. However, in this case ozone interacts with the enol form of MEK [41]. The addition of nitric acid accelerates enolisation that is shown by the increase of the oxidation rate [43]. The decrease of the oxygen content in the ozone-oxygen mixture results in an increase of the oxidation rate that is due to the change of the radical leading the oxidation, from RO2v to ROv. The ozonation of isomeric decanones indicates that the reactivity of ketones depends on the length and structure of the alkyl chain. In this case the main products are A-ketohydroperoxides, monocarboxylic acids, keto alcohols and alkylbutyric lactones. However, it should be noted that the conclusions in these studies were based on product analysis at very high degrees of conversion whereby the secondary reactions play an important role in the product formation. This gives us grounds to doubt whether the proposed mechanism describes adequately the initial steps of the reaction. The rate constant and the activation energy of interaction between cyclohexanone and ozone are 1.6 × 10-2 M-1.s-1 and 11.2 kcal/mol, respectively. The reactivity of ozonised methyl derivatives of cyclohexanone is considerably higher than that of the H form. An increase in rate of interaction between the above derivatives and ozone is also observed depending on the site of substitution and it follows the sequence: 3-Me<4-M2< 2-Me [44-46]. It is evident that so far no systematic investigations on the ozonolysis of ketones have been carried out. In our studies on the ozonolysis of ketones we combined the kinetic approach and product analysis with theoretical methods. The results from the ozonolysis of some aliphatic ketones are shown in Table 2.6. The following six types of ketone have been the subject of our investigations: (1) Acetone containing only A-C-H bonds, (2) Ketones with general formula CH3CO(CH2)nCH3, where n = 1, 4, 5, 6 and 9, which contain secondary A, B, G C-H bonds, (3) Ketones with the general formula CH3(CH2)m CO(CH2)nCH3, where m = 1, n = 4, which contain A, Aa-secondary C-H bonds and cyclic cycloheptanone which contains A, B and G secondary C-H bonds, 4-4-methyl or 4-tert-butyl cyclohexanone with G-tert-C-H bond, (4) 2-Naphthylmethylketone with various substituents, and (5) Acetylacetone containing A-C-H bond activated by two keto groups. The rate constant of keto-enol tautomerism for the latter is considerably higher than that of the other ketones studied: CH3-CO-CH2-CO-CH3 CH3-CO-CH=C(OH)-CH3 103

Ozonation of Organic and Polymer Compounds

Table 2.6 Kinetic parameters of the reaction of ozone with some aliphatic ketones at 21 oC k0 × 103 (M-1.s-1)

kst × 103 (M-1.s-1)

krel

log A

Ea (kcal/ mol)

CH3COCH3

3.5

7.7

0.2

10.823

18

CH3COC2H5

12

24

0.7

10.63

17

CH3COC5H11

23

67

1

10.54

16.5

CH3COC6H13

45

58

2.4

10.46

16

CH3COC7H15

448

54

2.3

10.31

15.8

CH3COC10H21

42

113

2.2

10.58

16.2

C2H5COC5H11

20

74

1

11.48

15.5

29000

-

-

87

80

4.1

9.00

17

CH3CO2-Naph

22000

-

-

5.77

6

4-CH3-c-C6H9O

117

101

6.2

8.97

12

4-t-Bu-c-C6H9O

166

123

8.1

7.93

11.8

Ketone

CH3COCH2COCH3 c-C7H12O

-

Note: ko is calculated according to Equation 1.4; krel is the relative rate constant determined by gas chromatography relative to ethylpentylketone. 2-Naph = 2-Naphthylmethylketane

The values of k, depending on the ketone structure, particularly on the content of primary, secondary or tertiary A-C-H bonds, vary in a wide range. The ratio of their reactivity is 1:3.4-24.8:33.4-47.4. In contrast to parafﬁns these values are more similar, which is associated to the activation of A-C-H bonds by the keto group. Ozone is sensitive not only to the activation to the A-C-H bonds but also to the keto-enol equilibrium. The rate of its reaction with acetylacetone is by three orders of magnitude higher than that with monoketones. Ketones with different substituents, for example, 2-naphthylmethylketones also show higher reactivity than the aliphatic ketones. It is concluded that the reaction centre in this case is the naphthyl ring and not the A-C-H bond in the methyl group. Another interesting observation is the higher values of Ea compared with those of parafﬁns (by 2-3 kcal/mol). As ozone is an electrophilic agent and the activation of the A-C-H bonds from the keto group favours the nucleophilic attack, the increase of Ea should be reasonably expected.

104

Ozonolysis of oxygen-containing organic compounds Figure 2.5 shows a typical kinetic curve illustrating the change of ozone concentration at the reactor outlet during ozonolysis of ketones in solution. The horizontal line describes the inlet ozone concentration and the curve corresponds to the change of its concentration at the reactor outlet. It is seen that the addition of ketone results in an abrupt fall of the ozone concentration followed by a steady-state condition ([O3]g1). After some time the ozone concentration rises again and a second steady-state region is attained ([O3]g2).

1.5

[O3]0 [O3]g

2

4

[O3].10 , M

1.0

0.5

[O3]g

1

0.0 0

1

2

3

4

5

6

7

Time, min

Figure 2.5 Ozonolysis of 0.712 mmol cycloheptylketone in 10 ml CCl4, 21 oC, v 0.1 l/min

The shape of the kinetic curve of ozone consumption (Figure 2.5) is complicated and supposes that ozone reacts with more than one compound [1, 21, 22]. The two horizontal sections of the curve are the result of the interaction of ozone with two compounds the rate constants of which differ considerably. The areas over the curve correspond to the amount of absorbed ozone. At the known concentrations of the reacted compounds the stoichiometry coefﬁcients and other kinetic parameters can be calculated. This type of the kinetic curve for ketones is associated with the keto-enolic equilibrium and to the ability of ozone to react with the C=C bonds 106 times faster than with C-H bonds with nearly zero activation energy [1].

105

Ozonation of Organic and Polymer Compounds The processes corresponding to the ozone-gas curve (Figure 2.5) are the following: (1) ozone starts its reaction with ketone (probably with the enol form) and an abrupt fall of its concentration is registered, (2) when the rate of ozone supply becomes equal to the rate of the chemical reaction, the ﬁrst step is formed, (3) the end of the step is connected with the consumption of the enol form and the ozone concentration begins to rise, (4) at this moment ozone begins to react with the keto form and the second step is formed whereby the rate of ozone supply has become equal to the rate of the second reaction. The sharp transition between the two steady-state regions testiﬁes that the rate of restoration of the keto-enol equilibrium is considerably lower than the rate of ozone interaction with the keto form. The general scheme of the keto-enol tautomerisation is shown next: HO- or H+

CH3C(OH)=CH2

CH3COCH3

+

OH

fast

1. CH3COCH3 + H+

CH3CCH3

+

OH

OH

CH3-C-CH2:H + H2O

slow

CH3-C=CH2 + H3+O O

O -

_

slow

2. CH3-C-CH2:H + OH

[CH3-C -H2O

O -

[CH3 C

CH2]

OH _ CH2]

fast

CH3

C

CH2

The concentration of the enolic form can be found from the area below the kinetic curve in Figure 2.5 and then the equilibrium constant of enolisation could be measured. It is known that the stoichiometry of the reaction of ozone with C=C is 1:1 and thus the amount of the enolic form can be judged by the amount of absorbed ozone, i.e., in this particular case the concentration of the enolic form is 0.134 mM. As the initial concentration of ketone is known, the equilibrium will be the difference between the initial concentration and that of the enolic form. Then the equilibrium constant can

106

Ozonolysis of oxygen-containing organic compounds be easily calculated using the formula: Ke=[Enol]e/[Ketone]e, and it amounts to 1.9 × 10-2. From the curve in Figure 2.5 one can also determine the rate constants of the ozone interaction with the two tautomeric forms. On the basis of the parameters of the ﬁrst steady-state part and by using Equation 1.4, the value of the rate constant with the enol form is found to be kE= 2.9 × 102 M-1.s-1 and from that the second rate constant with the keto form is kK = 0.048 M-1.s-1. This demonstrates the importance of the method of ozone titration that makes possible the measurement of the equilibrium constant and the rate constants of ozone interaction with the two tautomeric forms in one kinetic experiment. It is evident that the rate constants of ozone interaction with ketone as measured by the steady-state method (Figure 2.6) through mixing of ozone and ketone solutions at [K] o>>[O3]o are higher than those found by the bubbling method (Table 2.6). This is not difﬁcult to explain because in the former case the values obtained represent the total effective constant of interaction of the enol and the keto form. The results from the calculation of the equilibrium constants of keto-enol tautomerism for some aliphatic ketones in CCl4 are given in Table 2.7. In this table the data for acetylacetone and 2-naphthylmethylketone are not presented because in the former case the rate of reaching the equilibrium is commensurable with the rate of ozone interaction and in the latter case the ozone reacts with the double bonds in the naphthyl ring. The equilibrium constants do not differ from those found within the temperature range of 21 oC to 3 oC and agree with data from the literature [47-50]. The kinetics of ozone reactions with ketones are also determined by gas chromatography. The relative rate constants shown in Table 2.6, column 4 demonstrate that only acetone and methylketone possess lower reactivity than the standard. It is seen that the rate constants calculated from the relative values and the value of the standard constant correspond to those found by the bubbling method. The main products of ozone interaction with methylethylketone are 2-hydroxymethylethylketone, diacetyl, peroxides - alkyl and hydro, acetaldehyde and AcAc.

107

108

Ke × 10

4

Ketone

1.2

CH3COCH3 1.2

CH3COC2H5 46

CH3COC5H11 29

CH3COC6H13 96

CH3COC7H15

130

CH3COC10H21

Table 2.7 Equilibrium constants of keto-enol tautomerism of some ketones in CCl4 solution at 21 oC determined by titration with ozone 190

c-C7H12O

Ozonation of Organic and Polymer Compounds

Ozonolysis of oxygen-containing organic compounds

0.06 4

lg([O3]0/[O3]t)

3 0.04

2

0.02

1

0.00 0

20

40

60

80

100

Time, s

Figure 2.6 Semilogarithmic anamorphosis of the kinetic curve of ozonolysis of: 1 - methylethylketone; 2 - methylpentylketone; 3 – ethylpentylketone; and 4 – cycloheptylketone; 21 oC, ketone concentration 28 mM

On the basis of the kinetic results obtained and the product analysis we suggest Scheme 2.3 for the proceeding of ketone ozonolysis:

R1CHCOR2 + O3

H...O3 # [R1CHCOR2]

a. R1CH(OH)COR2

R1CHO + R2CHO

H

[R1CHCOR2 + OH + O2 , (HO3)]

a, b, c

+b R1COCOR2 + R1COCH(OH)R2 + O2 b. R1CH(OO)COR2

+b +RH

c. R1CH(OOOH)COR2

-O2

2R1CO + 2R2CHO

2 R1CH(O)COR2 + O2 R1CH(OOH)COR2

R1CHO + R2C(OH)O

R1CH(OH)COR2

Scheme 2.3

109

Ozonation of Organic and Polymer Compounds Ozone attacks the A-H atom forming a LC which further undergoes decomposition to a radical (or ion) pair in one kinetic cage, or leads to the formation of hydroperoxy and alkoxy radicals. The latter can further undergo monomolecular decomposition. The intermediate formation of LC is assumed in the ﬁrst stage, followed by breaking of A-C-H leading to the formation of a radical (or ion) pair. Then A-hydroxyketone, A-peroxyketone radicals and A-hydroxytrioxyketone through C-C bond breaking can decompose to two aldehydes. The A-peroxyketone radical reacting with the initial ketone can be transformed into A-hydroxyperoxyketone which is decomposed through breaking of the C-C bond to aldehyde and acid or the two radicals recombine giving rise to diketone and A-hydroxyketone or two A-oxyketone radicals. The latter reacting with the initial ketone are transformed into A-hydroxyketone or through a monomolecular decomposition and breaking of the C-C bond to an alkoxy radical and aldehyde. A-Hydrotrioxyketone is decomposed to A-hydroxyketone and it in its turn to two aldehydes.

2.3 Hydrotrioxides Hydrotrioxides (HTO) are deﬁned as compounds containing an HOOO group. HTO of oxygen-containing compounds have been studied and described in the literature in detail. They include those of ethers, acetals, alcohols and aldehydes [51-63]. They are obtained by ozonation of the corresponding organic compounds, for example: benzaldehyde, 2-methyltetrahydrofuran, methylisopropyl ether, di-iso-propyl ether, etc., at low temperatures (<–60 oC to –80 oC). HTO are unstable compounds and on heating they decompose and liberate oxygen. The nuclear magnetic resonance (NMR) method has been used to conﬁrm their structure including the unstable HOOO group [53]:

O CH3 O

OOOH

;

H 3C

CH3 .

C

C-OOOH ; H3CO

OOOH

The NMR signal of the proton in the HOOO group appears at D = 13.1 ppm, while that in HOO group at D = 9 ppm [53, 57]. Benzoic acid and singlet oxygen were the main products of decomposition of benzaldehyde HTO. This reaction, due to the high yield of 1O2, appears to be a suitable source for its preparation [55]. The proton signals of the HOOO group in the acetals below:

110

Ozonolysis of oxygen-containing organic compounds

appear in the vicinity of D = 12-14 ppm [30, 31] and the proton signal of HTO of ethanol at 13.7 ppm is a doublet [62]. The kinetic parameters of decomposition of HTO of benzaldehyde, 2-methyltetrahydrofuran and methyl-iso-propylether are found to be dependent on whether the decomposition is performed in a solvent or in a pure substrate [53]. Thus in pure HTO a slight downﬁeld shift (by <0.15 ppm) of the peak of HOOO is observed [64-66]. The doublet at <–50 oC is transformed into a singlet [56, 57, 62] with the rise of the temperature to ambient. This is explained by the participation of a proton in an exchange reaction with hydrogen bond formation. The lower values of A and Ea found for the decomposition of pure HTO result from the hydrogen bond formation and HTO stabilisation in TS: O

C O

#

H

O

H

O

O

O

OH

O

C

+ O2

C

O

O

The kinetic parameters of the decomposition of 23 HTO obtained by different authors are given in Table 2.8 [53-71]:

Cl CH3 O

O

C

C

C O

HOOO

O

HOOO

1

5

C

OCH3

OOOH

C

OOOH

8 OCH3 C

H 3C

OOOH OCH3

OCH3 10

OOOH

OCH3

OCH3 H 3CO

OCH3 9

C

F

7

6

C

OOOH OC2H5

OCH3 Cl

OCH3

OC2H5

C OOOH OOOH

O

HOOO 4

OCH3

C

C

3

2 O

O

OOOH CH3

11

111

Ozonation of Organic and Polymer Compounds OCH3 H 3C

C

OC2H5 H 3C

OOOH

C

OCH3

OC2H5

12

13

H 3C

C

O

16

21

20

C O

Ph

O

OOOH

O

C CH3

O

OC2H5 19

OOOH

O

OOOH C

H

O

O

18

O

OOOH C

C

O

17

OOOH

O

H3CCCHC3H7

OH

OCH3 15

OOOH

OH

O

H 3C

14

H3CCHOOOH

OOOH C

CH3

OOOH C

H 3C

H 3C

OOOH

OOOH

H 23

22

In studying the decomposition of A-methylbenzylmethylether HTO, Kovac and Pleshnichar have established that the main products are acetophenone, methanol, methylbenzoate and traces of water and hydroperoxide [53, 54]. They have explained the formation of these products as shown in Scheme 2.4.

CH 3

CH3 #

O

H O

C H 3C

O

O

O

C H 3C

O

H 3C

H

O

C

O I

I

OCH 3

OCH 3

C

C

OO' + 'OH 2

CH 3

OH + 1O2

CH 3

II

II

-CH 3

C

O

;

+RH II

CH 3

+RH I

OOH

CH 3

Scheme 2.4

112

OH

CH3

OCH 3 C

OCH 3 C

O + CH 3OH

+

1O2

Ozonolysis of oxygen-containing organic compounds

Table 2.8 Kinetic and activation parameters of the decomposition reaction of HTO in pure substrate (-), ether and dichloromethane (DChM) HTO

Solvent

T1/2 (min)

t, 0C

k × 104 (s-1)

log A

Ea (kcal/ mol)

1

Ether DChM

43 58

–40 –30 –20 –30

160 660 640 120

14.8 20.6

12.2 16.6

2

Ether

-

–40

270

-

-

3

d6-Acetone

162 50 37 21 13

–35 –23 –20 –17 –12

0.7 2.3 3.1 5.4 10

16

10.4

4

DChM Ether

85 63

–20 –30 –40

380 80 110

18.6

14.7

5

Ether

14 33 82 180 41 82 216

0 –9 –2– –34 –27 –33 –39

8.6 3.5 1.7 4.6 2.8 1.4 5.3

10.8 16.5

5.4 11.1

6

Ether

58

–30 –35 –40

660 290 120

18.9

15.9

7

Ether

53

–30 –35 –40

760 280 130

19.8

16.7

8

Ether

–30 –35 –40

630 310 110

19.6

16.4

–35 –40

320 140

18.4

15.4

–25 –30 –35

440 200 100

17.2

13.7

–20 –25 –30 –35

2100 810 310 140

21.5

17.8

–20 –10 –10 –10 –20

270 560 400 290 90

20.8

15.6

63 9

Ether 49

10

Ether 69

11

Ether 49

12

DChM Ether

84

113

Ozonation of Organic and Polymer Compounds

–10 –20 –10 –20 –50 –40 –29 –20 –10

1100 370 760 280 0.1 2 120 370 1000

14.6 13.1 22.7

11.1 9.8 18.2

3 17 58 330 8 18 60 138

–6 –30 –50 –68 –12 –19 –27 –33

40 6.9 2 0.35 10 6.5 1.9 0.83

7.9 17.4

4.1 11.7

15

2.6 13 16 52 114

–10 –22 –26 –33 –38

50 8.8 7.3 2.2 1

16.5

11.5

16

150 18 7

–30 –10 0

0.76 6.2 20

11.7

6.4

17

19 29 46 120 288

–30 –40 –45 –60 –70

6.9

3.8

4 2.5 0.9 0.4

18

2 3 10 23

0 –10 –20 –31

60 40 10 5

11.5

6.9

19*

3 10 23 38

–10 –20 –30 –40

40 10 5 3

12.4

8.0

20*

168

–30

0.7

-

-

21*

84

–30

1.4

-

-

12

–30

10

-

-

38

–30

3

-

-

13

Ether

14

Ether

22* 23* Note: *our data.

114

DChM

Ozonolysis of oxygen-containing organic compounds We have established that the decomposition reaction of HTO in the temperature range of (–70 oC to 0 oC) obeys Equation 2.2: -d [HTO]/dt = kd . [HTO],

(2.2)

where kp is the decomposition rate constant. The values of the rate constants of the HTO decomposition indicate that electronaccepting substituents to the benzene ring affect the process in a rather complex way (Table 2.9).

Table 2.9 Activation parameters of the decomposition of hydrotrioxides of some A-methylbenzylalkylethers OR X

C

OOOH CH3

Solvent

kd × 102 at –30 o C (s-1)

Ea (kcal/mol)

log A

X=H; R=CH3

EtAc Ether

0.5 8.9

15.2 17.7

11.4 14.9

X = Cl; R = CH3

Ether

12

16.7

14.1

X = Br; R = CH3

Ether

7.6

16.8

14.1

X = CH3; R = CH3

Ether

49

18.1

16

X = Br; R = C2H5

Ether

6.9

17.6

14.7

HTO

The decomposition reaction of the synthesised HTO of 1,1-diethoxyethane yields ethylacetate (0.75-080 mol per 1 mol of absorbed ozone), ethanol (0.80-0.85 mol), water (0.06-0.12 mol), acetaldehyde, acetic acid ethylformiate, diethylcarbonate, dissolved gases, probably methane and ethane, and traces of 2-hydroxydiethylether

115

Ozonation of Organic and Polymer Compounds and A,A-diethoxydiethylperoxide and hydroperoxide. It was assumed that ethylacetate, ethanol, and singlet oxygen are formed as follows: C 2H 5 H

O

H 3C

O

C O

O

CH3(O)C2H5 + C2H5OH + 1O2 .

O

C 2H 5

The rest of the decomposition products were assumed to be formed by the interaction of the products generated in the decomposition reaction radicals with the initial and intermediate reaction products:

C 2H 5 O

H 3C

O

C O

CH3C(OC2H5)2O + O2H

H

O

O

CH3C(OC2H5)2OO + OH

C 2H 5 CH3C(OC2H5)2OO + RH 2 CH3C(OC2H5)2OO CH3C(OC2H5)2O + RH CH3C(OC2H5)2OH

CH3C(OC2H5)2OOH + R [CH3C(OC2H5)2O]2 + O2 CH3C(OC2H5)2OH + R CH3C(O)OC2H5 + C2H5OH

The comparatively low values of the activation energy of the thermal decomposition of some HTO (Table 2.9) can be explained by the fact that the reaction takes place simultaneously in several directions. The reaction step of radical decomposition was suppressed and became negligible with the decrease in temperature. Ozonolysis of 1,1-diethoxyethane in the –20 oC to +60 oC range leads mainly to the formation of ethylacetate, ethanol, the corresponding HTO and water. The radical steps of the process yielded ROOH and water and, hence, they were responsible for the formation of the latter of about 40% at 50 oC whereas in practice it was below 1% at 5 oC. These results were also conﬁrmed by the data in references [34, 37]. According to these authors, the share of the free radical route is 1.2% at –60 oC for the HTO of

116

Ozonolysis of oxygen-containing organic compounds 1,1-diethoxyethane while it varies in the range of 0.3-3.7 between –30 oC and 0 oC for the decomposition of HTO of 2-propanol. Formation of nitroxyl radicals has been observed upon decomposition of HTO of ethanol, isopropanol, 1,1-diethoxyethane and benzaldehyde in the presence of diphenylamine and 2,2,6,6-tetramethylpiperydine [66].

Table 2.10 Effect of temperature on the formation of water and HTO in the ozonolysis of 1,1-diethoxyethane (1) and 2-methyl-1,3-dioxolane at 12 mM absorbed ozone and initial concentration of acetal 0.1 M Acetal

t (oC)

HTO (mM)

H2O (mM)

HTO/O3

H2O/O3

1

60 50 40 20 5 –20

5.7 4.5 3.6 2.3 0.8 0

6.1 5.0 4.2 2.2 07 0

0.48 0.38 0.3 0.19 0.07 0

0.51 0.42 0.35 0.19 0.06 0

2

50 40 20 5 –20

4 3 2.1 0 0

4.3 3.3 2.2 0 0

0.33 0.25 0.18 0 0

0.36 0.27 0.19 0 0

According to Pryor and co-workers [55] the values of the activation parameters of cumene HTO decomposition (Ea and lgA) are 16 and 10 kcal/mol, respectively, whereas other authors reported 24 and 16 kcal/mol, respectively [67]. These parameters are very close to those of benzaldehyde HTO [53] and acetals [56, 57]. The main products of the cumene HTO decomposition are dimethylphenylcarbinol (DMPC), acetophenone (AcP), singlet oxygen and hydroperoxide (ROOH). The assumption that the decomposition process takes place via the chain-radical route was conﬁrmed with the help of 2,5-di-tert-butyl-4-methylphenol (Ionol). In this case the activation energy is strongly reduced. The product formation could be explained by [70]:

117

Ozonation of Organic and Polymer Compounds CH3

CH3

C-O-O-O-H

C-O

CH3

CH3

CH3

CH3

C-O + InH

C-OH + In

CH3

CH3

+ OOH

CH3

CH3

C-O (O2H) + In

C-OIn (InO2H) .

CH3

CH3

Then the process occurs without an inhibitor: ROv + ROOOH m ROH + ROOOv ROOOH + vO2H m ROOOv + H2O2 ROOOv m ROv + 1O2 ROv m AcP + vCH3 We found that the thermal decomposition of HTO is accompanied by an intensive chemiluminescence in the visible and infrared (IR) regions [51, 60, 65], which has also been observed by other authors. Carbonyl compounds and singlet oxygen are assumed to be chemiluminescent emitters. The rate constant of the thermal decomposition determined by chemiluminescence (kCL) turned out to be very close to that measured by the NMR (kNMR) method through monitoring the disappearance of the signal at D = 13 ppm (Table 2.11).

Table 2.11 Rate constants of decomposition determined by chemiluminescence and by NMR for some HTO HTO (CH3)2C(OOOH)OH (CH3)2C(OOOH)OCH(CH3)2 CH3C(OOOH)(OC2H5)2

118

t (oC)

kCL × 103 (s-1)

kNMR × 103 (s-1)

0

2

2

–26

12

18

0

1.3

0.9

Ozonolysis of oxygen-containing organic compounds It should be noted, however, that the decomposition rate determined by chemiluminescence in the IR range of the spectrum is lower than that in the visible region [60]. The reason for this difference is not yet clear. Our attempts and those of other investigators to use HTO as oxidising agents have given satisfactory results in a number of cases [1, 2, 73-78]. It is known that the oxidation of sulﬁdes to their respective sulfoxides using HTO of ethanol, 2-propanol, 1,1-diethoxyethane and benzaldehyde take place at a high rate and selectivity. Among them the latter two compounds were the most active [68]: +R'OOOH R2S

R2S=O

R = CH3, C4H9, C7H15

It has been found that dimethylsulﬁde was readily oxidised by the HTO of 1,3dioxolane to dimethyl sulfoxide (DMSO) (Table 2.12).

Table 2.12 Oxidation of dimethylsulﬁde by HTO of 1,3-dioxolane (1) and 2-methyl-1,3-dioxolane (2), at –30 oC and mol ratio [ROOH]/[DMC] = 1:4 HTO

[ROOOH]0 (mM)

[DMSO] (mM)

Yield (%)

1

0.7 0.6

0.6 0.6

86 100

2

0.5 0.6

0.3 0.4

60 67

Initially the HTO forms a monoether quantitatively. It was also demonstrated in the case of dimethyl and dibutyl sulfoxides that HTO can oxidise sulfoxides to the respective sulfones: R2S=O + RaOOOH m R2SO + RaOOH The HTO of benzaldehyde and 2-propanol were the most effective. The yield of sulfone per 1 mol of converted HTO was increased with the increase of the thermal stability of HTO (Table 2.13).

119

120 13 20 92 63 25 30 50 32 26 46 29 30

(CH3)2C(OH)(OOOH)

(CH3)2CHOC(CH3)2(OOOH)

CH3CH(OC2H5)(OOOH)

C6H5C(O)(OOOH)

[HTO]0 × 102 (M)

CH3CH(OH)(OOOH)

HTO 1.6 1.0 19 16.4 6.5 5.4 7.6 1.2 2.5 0.8 21.6 8.2

0.9 2.9 1.1 3.3 1.2 5.7 1.1 1.2 1 5 0.9 3.3

13.8 6.9 100 19 21 5.3 44 11

(CH3)2SO (C4H9)2SO

(C4H9)2SO 27 9.2 30.6 9

(C4H9)2SO (C4H9)2SO

(C4H9)2SO

[R2SO2] × 102 (M)

[R2SO]0 × 102 (M)

[HTO]0/ [R2SO]0

R2SO

Table 2.13 Oxidation of dialkylsulfoxides by HTO

0.67 1

0.09 0.087

0.17 0.11

0.18 0.86 0.31 1

0.12 0.15

[R2SO2]/ [R2SO]0

0.74 0.27

0.1 0.02

0.15 0.04

0.21 0.26 0.16 0.18

0.12 0.05

[R2SO2]/ [HTO]0

Ozonation of Organic and Polymer Compounds

Ozonolysis of oxygen-containing organic compounds Another direction in HTO application is the oxidation reaction of phosphines. According to reference [76] the HTO of alcohols, ethers and acetals convert readily, selectively and quantitatively triaryl- and trialkyl-phosphines to respective phosphites: (RO)3P + RaOOOH m (RO)3P=O + RaOOH The data from the oxidation process of triphenylphosphine by HTO of ethanol and propanol, obtained by the authors of [50] are listed in Table 2.14.

Table 2.14 Stoichiometric ratios at the oxidation of triphenylphosphine by HTO of 2-propanol and ethanol in ether solution at –50 oC [76] HTO

[HTO]0 (M)

[HTO]f (M)

[Ph3P]0 (M)

[Ph3P]f (M)

$[Ph3P]/ $[HTO]

2-Propanol

0.52 0.19 0.09 0.1

0.37 0.08 0 0

0.14 0.14 0.018 0.33

0 0 0.01 0.1

0.9 1.3 1.9 2.0

Ethanol

0.10 0.08

0 0

0.23 0.29

0.04 0.13

1.9 2.0

Note: In 2-propanol, the R in HTO is (CH3)2C(OH) and in ethanol, the R is CH3CH(OH); index f is ﬁnal concentration.

It is evident that the reaction stoichiometry depends on the initial concentration of the reagents and it attains its ultimate value of 2 at low HTO concentration and excess of phosphine. In this case HTO is converted to hydroperoxide which in its turn yields alcohol. It has been found that the titration with phosphines can be used as an analytical method for HTO determination. Its reliability is supported by the close values of the rate constant of A-hydroxyhydrotrioxide decomposition determined by the above method (–30 oC, kd = 6 × 10-4 s-1) and by the NMR method (–30 oC, kd = 4 × 10-4 s-1). It has been shown that pyridine is oxidised by HTO at –60 oC to N-oxide (60-80% yield). The oxidation of other organic compounds to corresponding products can also be accomplished by HTO. In this case, however, catalysts and higher temperatures should be applied.

121

Ozonation of Organic and Polymer Compounds Upon ozonation of 2-methyl-1,3-dioxolane in the temperature range of –60 oC to +50 oC it has been found that 1 mol ozone gives 2 moles of glycol monoester. It was presumed that the intermediate HTO can also oxidise the substrate (Table 2.15).

Table 2.15 Synthesis (–60 oC to +50 oC) and decomposition (–30 oC) of HTO of 2-methyl-1,3-dioxolanes in ethylacetate Acetal

[Ac]0 (mM)

[Ac]f (mM)

Time (min)

$[Ac] (mM)

[O3] (mM)

[Monoester] (mM)

2-Methyl1,3-dioxolan

22.4 11.2 11.2 11.2 5.6a 3.7a

18.0 9.4 9.4 9.7 3.8 1.9

45 10 15 10 15 20

4.4 1.8 1.8 1.5 1.8 1.8

1.8 0.7 0.8 0.6 0.9 1.0

4.2 1.2 1.6 1.5 1.5 1.8

The decomposition reaction of the HTO indicated above proceeds in the following way: O

OOOH C

O

C

+ R

H

O O

2 HO(CH2)2(O)C(O)R R

R = H, CH3

This suggestion was conﬁrmed in the oxidation of 2-methyl-1,3-dioxolane by the HTO of 1,3-dioxolane. Besides ethyleneglycol-monoformiate, ethyleneglycol-monoacetate was found in the mixture of reaction products:

Ring opening on the decomposition of HTO formed by the ozonolysis of nonsymmetric dioxanes (2-methyl, 2,4-dimethyl and 4,4-dimethyl-1,3-dioxane) may take place via two routes resulting in the formation of two isomeric esters:

122

Ozonolysis of oxygen-containing organic compounds

CH3

C

R"

O

O

C R'

R'C(O)O(CH2)2C(CH3)(OH)R" (a)

R"

+ O3

O

O

CH3

C

C H

R'

R'(O)OC(CH3)(R")(CH2)2OH (b) OOOH

R' = R" = H; R' = CH3, R" = H; R' = H, R" = CH3

The ratio of the products of type (a) and (b) upon ozonolysis of 1,3-dioxycyclanes at various temperatures are shown in Table 2.16.

;

O

O

OOOH

H

CH3

CH3

CH3

R"

H3C

1

;

O

O

O

O

OOOH

H

OOOH

CH3

3

2

Table 2.16 Products of ozonation of 4-methyl-1,3-dioxane (1), 2,4-dimethyl1,3-dioxane (2) and 4,4-dimethyl-1,3-dioxane (3) at various temperatures HTO

a/b, –50 oC

a/b, +15 oC

a/b, +50 oC

1

2:1

2:1

2:1

2

2.5:1

2:1

2:1

3

7:1

3:1

2:1

Note: a - monoesters with acylated primary groups. a/b: Ratio of products a and b

The main products of the reaction were monoesters with acylated primary groups. The more selective decomposition of the HTO of 4,4-dimethyl-1,3-dioxane (Table 2.16) is due to the steric repulsion between the OOOH group and the axial CH3 group at the 4C atom of the ring in the intermediate HTO:

H

O O

CH3

OOOH H3C

123

Ozonation of Organic and Polymer Compounds The above discussion shows that the chemistry of organic trioxides is a new rapidly developing branch of organic chemistry and any progress in this ﬁeld is essential for the development and improvement of the theory or reactivity and methods of organic synthesis [79-96]. 1,3-Dioxolanes, in particular those substituted in the fourth position, have found wide application as solvents, plasitiﬁers and biologically active substances. In this connection we have tried to develop a new method for the preparation of the above compounds in satisfactory yields and to obtain a number of new derivatives. The two-phase catalytic ‘liquid-liquid’ and ‘liquid-solid’ systems were applied.

2.4 Synthesis of oxolanes 2.4.1 Derivatives of 4-hydroxymethyl-1,3-dioxolane A four-necked ﬂask, supplied with a reﬂux condenser, a magnetic stirrer, thermometer and a funnel was charged with 10 ml aqueous NaOH and 5 mM Katamin AB and with continuous stirring, 0.05 M 4-hydroxymethyl-1,3-dioxolane was added dropby-drop [24]. The mixture was heated up to 15 oC and the corresponding chloride (0.05 M) was added drop-by-drop for 30 minutes. Then the mixture was diluted with water and extracted with ether. The ethereal solution was dried over MgSO4, the diethyl ether removed and the mixture subjected to vacuum distillation. Thus, the following oxolanes were obtained: 4-Propyloxymethyl-1,3-dioxolane; NMR spectrum [CCl4, D ppm of hexamethyldisilazane (HMDS)]: 0.7 - 1.0 (t) CH3; 1.3-1.7 (qt) CH3-CH2; 3.2 - 3.5 (m) CH2-O-CH2; 3.6-4.2 (m) 5H, -OCH2-CH2-CH2O; 4.7 (s) -O-CH2-O-ring. 4-Benzyloxymethyl-1,3-dioxolane; (CCl4, D ppm of HMDS): 3.6 - 4.2 (m) 5H, -OCH2CH2-CH2O; 4.35 (s) OCH2-Ph; 4.7 - 4.8 (s) O-CH2-O- ring; 7.0-7.3 (m) C6H5. Phenylsulfone of 4-hydroxymethyl-1,3-dioxolane: (CCl4, D ppm of HMDS): 3.6 - 4.4 (m) 5H -OCH2-CH2-CH2O; 4.7 - 4.9 (s) 2H, O-CH2-O-ring; 7.5-8.0 (m) C6H5. Note: t = triolet, qt = quartet, m = multiplet and s = singlet

2.4.2 Phenyl ethers of 4-hydroxymethyl-1,3-dioxolane A four-necked ﬂask, provided with a reﬂux condenser, a magnetic stirrer, thermometer and a funnel was charged with the phenol, 0.1 M of solid KOH, a catalytic quantity of 18-crown-6 and acetonitrile or DMSO as solvent and under continuous stirring 0.05 124

Ozonolysis of oxygen-containing organic compounds M 4-hydroxymethyl-1,3-dioxolane was added drop-wise. The mixture was stirred for 3 h at 70 oC followed by ether extraction and the extract was dried over MgSO4. The diethyl ether was distilled off and the residual mixture subjected to vacuum distillation. Thus, 4-(2,5-dichlorophenoxymethyl)-1,3-dioxolane was obtained with the following NMR characteristics: (CCl4, D ppm of HMDS): 3.2-4 (m) 5H, -CH2CH2-CH2O; 4.8-4.9 (s) 2H, -CH2-O-ring; 6.6-7.2 (m) C6H5.

2.4.3 Alkyl ethers of 4-hydroxymethyl-1,3-dioxolane The synthesis of alkyl ethers of 4-hydroxymethyl-1,3-dioxolane (B) was carried out by the interaction of 4-hydroxymethyl-1,3-dioxolane (A) with alkylhalides (RX): propylchloride, butylchloride, octylchloride, butylbromide and hexylbromide. In the presence of an aqueous alkaline solution with the application of interphase catalyst - Katamin AB - the following products were obtained: CH2OR

CH2OH O

O

+ RX

90oC

O

O

1.5 h

where R = C3H7 (I), C4H9 (II), C6H13 (III), C8H17 (IV), with the following physical constants: boiling point t in oC, at mm Hg, nD20 and yield in % - 42(5), 1.4313, 55; 58(4), 1.4320, 57; 80(6), 1.4359, 50 and 132(4), 1.5213, 44, respectively. In the series of normal alkychlorides the yields of monoethers decrease with the increase of the molecular mass of the substituents. The substitution of chlorides by the corresponding bromides also leads to an increase of the yields up to 65-70%. The arylmethylethers of 4-hydroxy-1,3-dioxolane were obtained under similar conditions. Benzyl and p-chlorobenzylchloride react with 4-hydroxy-1,3-dioxolane to give the corresponding ethers: CH2OCH2PhX

CH2OH 70oC O

O

+ XPhCH2X

O

O

where X = H(V) or Cl(VI) in yields of 84 and 70%, respectively. The physical constants for (V) are 132(4), 1.5231, R = 1.1832 g/cm3, and for (VI) 160(7), 1.5242, 1.0119, respectively.

125

Ozonation of Organic and Polymer Compounds In some case it seems more preferable to use epichlorohydrin derivatives. In interphase catalysis (solid KOH, catalyst - tetrabutylammonium bromide and 90 oC) it became possible to obtain benzyl ether of 4-hydroxymethyl-1,3-dioxolane: CH2OCH2Ph

CH2Cl 90oC O

O

+ PhCH2OH

O

O

The yield, however, does not exceed 25%. A series of aryl ethers were obtained by the reaction of 4-chloromethyl-1,3-dioxolane with substituted phenols in the presence of various solvents: Z

CH2 Cl O

+

O

CH2 O

Z

HO

O

Y

O

Y X

X

where X, Y and Z are: CH3, Cl, H (VII); H, H, H (VIII); Cl, H, Cl (X); Cl, H, H (XI); H, CHI, H (XII); and H, Cl, H (XIII). Depending on the conditions and the type of substituent the yield of monoethers varies from 10 to 80% (Table 2.17). The C4-C8 aliphatic alcohols do not react with 4-chloromethyl-1,3-dioxolane under interphase catalysis conditions. The results obtained show that it is better to conduct the synthesis of 4-alkoxymethyl-1,3-dioxolanes on the basis of 4-hydroxymethyl-1,3dioxolane and the corresponding alkyl halides. Derivatives of 4-hydroxymethyl-1,3dioxolane-arylsulfonates were obtained by the interaction of 4-hydroxymethyl-1,3dioxolane with arylsulfochlorides (aqueous-alkaline solution, Katamin AB): Y CH2 OH

Y

CH2 OSO2

15oC O

O

+

ClSO2

O

O

X

X

where X and Y are: H, H (XIV); CH3, H (XV); CH3, CH3 (XVI); Cl, H (XVII). The yields of the compounds in (%) and their melting points in (oC) are as follows: 88, 55-57; 80, 75; 50, 90-92 and 30, 37. 126

Ozonolysis of oxygen-containing organic compounds Compounds XIV-XVI are new. The data reveals that 4-hydroxymethyl and 4-chloromethyl1,3-dioxolanes can be successfully applied in the synthesis of cyclic acetals, containing alkyl and aromatic substituents.

Table 2.17 Synthesis of aryl derivatives Substrate

Solvent

Catalyst

bp (oC) (mm Hg)

R (g/cm3)

nD20

Yield (%)

VII

NaOH/ H2O

Katamin AB

154 (5)

1.2381

1.5330

10

VIII

NaOH/ H2O

Katamin AB

112 (7)

1.0548

1.5150

42

IX

CH3CN

18crown-6

165 (4)

-

1.5445

15

X

CH3CN

18crown-6

150 (5)

-

1.5341

10

XI

DMSO

18crown-6

118 (5)

-

1.5194

50

XII

DMSO

18crown-6

75-77

-

20

XIII

DMSO

18crown-6

113 (3)

1.5231

80

1.1832

2.4.4 5-Nonylene-1,2,4-trioxolane 1,2,4-Trioxolanes are the main products at the ozonolysis of oleﬁns [1, 2]. In contrast to peroxides [100], and 1,2,3,4-tetraoxolanes their thermal properties have not been thoroughly investigated. The synthesis of 5-nonylene-1,3,4-trioxalane-1-deceneozonide was accomplished by ozonation of 10 ml solution of 0.5 M 1-decene at a temperature of –70 oC [25]. The yields of the ozonide was 60%. Its structure is: Its thermal properties were investigated by means of differential scanning calorimetry (DSC). A highly intensive exothermal peak with maximum (Tm) in the range of

127

Ozonation of Organic and Polymer Compounds 110-130 oC was observed on the thermogram of 1-decene ozonide. It has been found that this peak depends on the heating rate. The analysis of the thermogram data are presented in Table 2.18.

Table 2.18 DSC analysis of the thermal decomposition of 1-decene ozonide Heating (oC/min)

Tm (K)

$H (kJ/mol)

Ea (kJ/mol)

n

o

3

110 C

341

129

1.02

5

117 oC

349

127

0.98

10

122 oC

353

131

1.01

15

128 oC

345

129

0.99

o

0

130 C

347

128

0.98

Average value

-

349 p 9

129 p 4

1 p 0.06

Note: $H, Ea and n are the enthalpy, the activation energy of decomposition and the order of the reaction, respectively.

It is seen that $H for the ozonide is 349 kJ/mol which is about 62% higher that the respective value for dicumene peroxide - 215 kJ/mol [110] (Table 2.19).

Table 2.19 Activation parameters of the reaction of thermal decomposition of organic peroxides Substrate

Ea (kJ/mol)

log A

Reference

RO-OR

155 p 5

15.3 p 0.5

[115-118]

Cumene-DPO

140 p 8

-

[101]

Acetone-DPO

153 p 5

13.8 p 1.1

[101]

Benzoil-DPO

128

-

[101]

Pynacoline-DPO

128

12.6 p 1.3

[101]

Ethylene-ozonide

115.1

13.6

[105]

Hexene-1-ozonide

99.2

10.8

[105]

Heptene-1-ozonide

95.4

10.2

[105]

Note: R = Me, Et, n-Pr, i-Pr, sec-Bu, t-Bu radicals; PO = peroxide; DPO = diperoxide.

128

Ozonolysis of oxygen-containing organic compounds A modiﬁed version of the method of Ellerstein [110] is used to determine the values of the activation energy and the kinetic order of the reaction. Calculations were made according to Equation 2.3 proposed by Crane and co-workers [111]. T2(S/h) = (Ea/R) – nT2(h/r)

(2.3)

where S = dH2/dT2, h = dH/Jdt = dH/dT, r = $H-Hp(T), $H is the total enthalpy of the reaction, Hp(T) is the partial enthalpy evolved up to T, t is the reaction time, J is the heating rate. From the linear plot in coordinates of Equation 2.4 (correlation coefﬁcient of 96%) the values of Ea and n were determined form its intercept and slope, respectively. The Kissinger relationship between the peak maximum temperature (Tm) and Ea for data obtained at different heating rates was used as an alternative method for Ea determination [112, 113]: d(lnJ)/d(1/Tm) = -Ea/R - 2Tm

(2.4)

In this case the plot in coordinates J/(1/Tm) gives a straight line with a slope Ea/R when Ea/R>> 2Tm. The value of Ea was determined as 132 kJ/mol with a correlation coefﬁcient of 0.999. Taking into account the ﬁrst-order kinetics of the thermal decomposition of 1-decene, the value of A in the Arrhenius equation was calculated from the following expression (Equation 2.5) [114]: A = Ea.J.n.(RTm2)-1.exp(Ea/RTm),

(2.5)

according to which A = 1.7 × 1015 s-1. Thus the Arrhenius equation for the thermal decomposition reaction of 1-decene can be written as shown in Equation 2.6: k = 1.7.1015.exp(-129000/RT), s-1

(2.6)

The thermal data allows analysis of the scheme for the decomposition of the ozonides: O

O

A CH2

C8H17CH2 O

O

C8H17CH2

O

CH2 . O

O

O

CH2

B C8H17CH2 +

+

O

.

129

Ozonation of Organic and Polymer Compounds Kinetic data on the thermolysis of three different classes of peroxide compounds are summarised in Table 2.19. It is known that the homolytic scission of the peroxide bond [59, 64, 65] is the rate-limiting step in the thermal decomposition of dialkylperoxide and ketodiperoxides. In the case of 1,2,4-trioxalanes, it has been reported that they can decompose through a synchronous step along the ether and peroxide C-O bonds yielding carbonyloxide and carbonyl compounds [66, 67]. One of the arguments in favour of this suggestion (route B) is the lower values of Ea and A for ozonides than those for peroxides. However, the proximity of the Ea and A values of 1-decene ozonide with those of alkyl peroxides agree with the hypothesis of homolytic decomposition (route A). Thus it can be concluded that depending on the conditions and ozonide type either of the two possible routes for the decomposition reaction can dominate, but are always in a dynamic equilibrium.

2.5 Ethers Investigations of ozone reactions with ethers began as early as the last century [119123]. Results of these studied are summarised and reported by Bailley in his review [74]. The main products formed during ozonolysis of aliphatic ethers are alcohols, aldehydes, esters, acids, HTO, hydrogen and organic peroxides, singlet oxygen and water. Price and co-workers [125] propose the so-called ‘insertion’ mechanism, according to which ozone is inserted into the A-C-H bonds at the ﬁrst step through a 1,3-dipolar addition, thus forming unstable HTO. This mechanism was also supported by the data of Erickson and Bakalik [126], Bailey [2] and Stary and co-workers [126] who measured the activation parameters of the decomposition of a series of HTO obtained by the ozonolysis of some ethers. Giamalva and co-workers [127] summarised the possible mechanisms known to date, i.e., interaction with the ether oxygen atom, 1,3dipolar insertion of ozone into the A-C-H bonds, homolytic abstraction of the A-H atom and heterolytic abstraction with carboanion and carbocation formation. These authors clearly indicate the predominance of the one-step mechanisms with transfer of a hydrogen atom, hydride anion or cation. As was mentioned above the low-temperature ozonolysis of ethers yields HTO which however are stable at very low temperatures. At normal temperatures the composition of the products is found to be different from that at the decomposition of HTO. These two reasons led us to carry out extended studies on the ozone reaction with ethers in order to establish the real mechanism of these reactions at normal temperatures. It was found from Figure 2.7 that the kinetic constant of diethyl ether (DEE) ozonation at –5 oC is 0.9 M-1.s-1 and the stoichiometric coefﬁcient is 1. The calculated stoichiometric coefﬁcient of the ozonation reaction with other ethers was also unity. The kinetics of ozonolysis was studied with the example of

130

Ozonolysis of oxygen-containing organic compounds n-dibutylether (DBE). The kinetic curves of the reaction product formation and the initial ether consumption are shown in Figure 2.8

1.2 [O3]0

A 1.0

[O3].104, M

0.8 0.6 [O3]t B

0.4

C 0.2 0.0 0

1

2

3

4

5

6

Time, min

Figure 2.7 Kinetic curve of ozone concentration at the reactor outlet 10 ml, 92 mM DEE at –5 oC

0.10

DBE n-BuOH

0.08

[P], M

0.06

n-PrCHO

0.04

0.02 n-PrC(O)O-n-Bu

0.00 0

5

10

15

20

Time, min

Figure 2.8 Kinetics of product formation and consumption of DBE during ozonolysis at ambient temperature, [O3]o = 2.34 × 10-4 M

131

Ozonation of Organic and Polymer Compounds The following products were identiﬁed by gas chromatography-mass spectrometry in the reaction mixture after 10 minutes: 130 * 57 41 87 56 55 101 39 130 43 45 * n-dibutylether, 74 * 56 41 43 42 55 39 57 45 40 41 * n-butanol, 72 * 44 43 41 72 57 42 38 37 40 71 * butanal, 88 * 60 73 41 42 43 45 39 55 61 88 * butyric acid; 144 * 71 89 56 43 41 57 60 73 55 42 * butyl butyrate and 164 *& 247 * - chloro-containing compounds like: CH3(CH2)2CH(Cl)O(CH2)3CH3 (Mw 164.4) and CH3(CH2)2CH(CCl3)O(CH2)3CH3 (Mw 247) The rate of DBE decomposition (7.4 ×10-5 M/s-1 determined from Figure 2.8) is almost equal to the sum of butanal (6.5 × 10-5 M/s-1), butanol (6.71 × 10-5 M/s-1) and butyl butyrate (1.0 × 10-5 M/s-1) formation rates. The kinetic curves of butanol and butanal formation have almost the same slope and they start without any induction period. This could mean that they are formed in parallel reactions from a common precursor. The rate of butylbutyrate accumulation, as demonstrated by its kinetic curve, is approximately 7 times lower, thus indicating its formation in a parallel reaction to butanol (butanal) formation, most probably from the same precursor. The ratio product amount:initial DBE after 10 minutes is found to be 1:1. This means that the share of the auto-oxidation process is negligible although we have identiﬁed some butyric acid by its IR spectrum (1765 cm-1 (monomer) and E = 1470 M-1.cm-1) resulting from the butanal oxidation. The possible existence of a common precursor raises the question about its nature. Such a precursor could be either A-hydroxyether (EOH) or A-hydrotrioxyether (EOOOH) which through intermolecular disproportionation can further produce simultaneously aldehyde and alcohol: CH3(CH2)2-HC

O-(CH2)3CH3

CH3(CH2)2CHO +

CH3(CH2)2CH2OH

(2)

H

O O

CH3(CH2)2-HC O

132

-O2

O

O-(CH2)3CH3 H

CH3(CH2)2CHO +

CH3(CH2)2CH2OH (2')

Ozonolysis of oxygen-containing organic compounds Probably, the intermolecular reaction of EOOOH disproportionation is preferable, because of the six-member ring transition state, while EOH disproportionation occurs via a strained four-member-ring transition state, which is energetically unfavourable. If EOH is assumed to be the precursor, then butylbutyrate should be the major reaction product as a result of the rapid oxidation of EOH by ozone: O O

O

H

H

CH3(CH2)2C

O

-H2O, -O2

CH3(CH2)2C(O)O(CH2)3CH3

(3')

O(CH2)CH3

which is not observed experimentally. When EOH is assumed to be a precursor, it is difﬁcult to imagine its one-step transformation into an ester without any additional assumptions. The solvent, CCl4, could not also affect this transformation bearing in mind its weak oxidising properties and nonspeciﬁcity. The formation of the ester from EOOOH in one step can be easily presented through a four-member ring transition state: H H

O

CH3(CH2)2C

O

O -H2O2 CH3(CH2)2C(O)O(CH2)3CH3 (3)

O(CH2)CH3

The formation of four-member transition state will be more unfavourable than the six-member transition state. This conclusion is in agreement with the kinetic data, namely, the rate of ester formation is approximately 7 times lower than that of alcohol and aldehyde formation. Direct evidence for EOOOH formation was found only after prolonged ozonation for 24 h at –78 oC [129]. The NMR spectrum of the oxidate has a signal at D = 13.52 ppm, which is attributed to OOOH. The order of the reaction in [DBE] and [O3] is unity (Figure 2.9) and the rate law can be written as W = k × [DBE][O3]. Based on the results discussed above, a scheme of the ozone reaction with aliphatic ethers is proposed in Scheme 2.5.

133

Ozonation of Organic and Polymer Compounds Ether + O3 m EOOOH

(k1)

EOOOH m Alcohol + Aldehyde + O2

(k2)

EOOOH m Ester + H2O2

(k3) Scheme 2.5

12 10 10 8 8

1

6

6

4

4

2

2

2

0

0 0

1

2

3 5

4

5

6

-1

W0.10 , M.s

Figure 2.9 Dependence of the reaction rate of DBE ozonation (21 oC) on the concentration of ozone (1) at [DBE] = 10 mM and DBE (2) at [O3] = 0.1 mM

The formation of chlorine-containing compounds (1-2%) can be explained by the presence of radical intermediates in the reaction mixture. This means that EOOOH will also decompose via the radical route: EOOOH m EOv + vO2H

(k4)

EOv + EH m EOH + Ev

(k5)

Ev + O2 m EO2v

(kpa)

134

Ozonolysis of oxygen-containing organic compounds Ev + CCl4 m ECl + CCl3v

(kpa)

EO2v + EH m EOOH + Ev

(kp)

Ev + CCl3v m ECCl3

(kta)

2EO2v m nonradical products

(kt)

The validity of the mechanism indicated above was conﬁrmed by the good agreement between the experimental points on the DBE decomposition curve, and product accumulation and the theoretical curves calculated according to the scheme and given in Figure 2.8. For example, the curve describing the DBE consumption is obtained from Equation 2.7: [DBE]t = [DBE]0.exp(-kat)

(2.7)

where ka is the constant measured by the stop-ﬂow method - 1.26 ×10-3 s-1, the corresponding bimolecular constant is 6.3 M-1.s-1.

2.5.1 Application of the ERM In order to elucidate the intimate mechanism of ozone interaction with ethers we have applied the experimentally theoretical approach developed by us which was described in Chapter 1. It is based on a comparison of the experimental and calculated values of A assuming linear (LC) and cyclic (CC) forms of the activated complex (AC). The kinetic constants of ozone reaction with diethylether (DEE), dichlorodiethylether (DClDEE), di-iso-propylether (i-PrE), di-n-butylether (n-ButE), di-iso-amylether (i-AmE) and di-n-amylether (n-AmE) in CCl4 are given in Table 2.20. The rate constants for DEE, n-ButE, i-AmE and n-AmE have similar values. All these ethers have close A-C-H bond energies and similar electronic environment. The value of the rate constant for i-PrE ozonation is also close to those mentioned above, while the constant for DClDEE is 100 times lower value. In DIPE the presence of tert-C-H bonds, according to our discussion in Chapter 1, should contribute to their higher reactivity because of the lower energy of these bonds. The presence of oxygen, however, and the more difﬁcult stabilisation of the transition state due to steric factors, makes this interaction slower. The very low rate constant of DClDEE ozonation can be attributed to the strong electron-accepting properties of the chlorine atom. Arrhenius parameters calculated on the basis of the data in Table 2.20 are summarised in Table 2.21.

135

136 -

-

-

-

-

i-PrE

n-ButE

i-AmE

n-AmE 2.2

1.9

1.9

1.6

0.009

1.0

–5 oC

2.8

2.5

2.1

1.9

0.016

1.2

3.5 oC

-

-

-

-

-

1.6

4 oC

-

-

-

-

-

2.2

10 oC

4.2

4.1

4.3

3.4

0.028

2.4

13 oC

3.1/3.0

21 oC

7.8/7.5

7.8/7.0

6.3/6.3

5.4/5.1

0.035/0.04

Note: In column 9, after the solidus the values of the constants determined by stop-ﬂow method are given.

-

0.9

0.5

-

–6 oC

–15 oC

DClDEE

DEE

Ether

Table 2.20 Rate constants of ozonation of ethers at various temperatures (oC) k p5% M-1.s-1

-

-

-

-

-

3.6

24 oC

Ozonation of Organic and Polymer Compounds

Ozonolysis of oxygen-containing organic compounds Comparison of the data obtained with reference data shows a good coincidence in relation to the values of the rate constants at 21 oC. Only a slight difference in the activation energies was observed. Perhaps, it is the heat of ozone dissolution in CCl4 that needs to be taken into account as the reason for these differences.

Table 2.21 Arrhenius parameters of ozone reaction with some aliphatic ethers k (M-1.s-1)

log A

Ea (kcal/mol)

3.1

6.4 (7.3)

7.9 (9.9)

0.035

4.9 (5.5)

8.5 (10.1)

5.4

6.3 (7.9)

7.6 (10.0)

n-ButE

6.3 (6.0)

6.6 (7.4)

7.8 (9.7)

i-AmE

6.8

6.6

7.8

n-AmE

7.8

6.5

7.6

THydP

(1.3)

(6.8)

(9.7)

THydF

(12.3)

(6.6)

(8.2)

Ether DEE DClDEE i-PrE

Note: The values in brackets are taken from reference [79]; THydP and THydF are tetrahydropyrane and tetrahydrofurane, respectively.

All literature reaction schemes describing the ozonation of ethers proceed by two geometric forms of the activated complex - LC and CC: R' R' R

C OR"

H

+

O3

R

R'

O3 C

H

R"O

or

R

C

# H...O3

OR" CC

LC

The CC has a more compact structure, without any possibility for free rotation. It can be a transition state for the following reaction: EH + O3 m EOOOH m products

(a)

where EOOOH if formed in one step via 1,3-dipolar insertion.

137

Ozonation of Organic and Polymer Compounds The LC structure is an open one, allowing free rotation around H-O and O-O bonds. LC can be a transition state for the reactions: EH + O3 m Ev + HO3v m EOOOH m products

(b)

EH + O3 m E+ + HO3 m EOOOH m products

(c)

in which an H atom or hydride anion abstraction occurs and EOOOH is formed during the second step. The necessary parameters for estimating the A values are presented in Table 2.22.

Table 2.22 Heats of formation ($H) in kcal of initial, intermediate and ﬁnal products, van der Waals radii of the ethers and free rotation energy (Efr) around H-O bond Ether

r (Å)

$H, EH

$H, EOOOH

$H, Ev

$H, E+

Ea (kcal/mol)

DEE

3.18

–68

–66

–32

135

1.3

DClDEE

3.51

–74

–73

–41

132

2.2

i-PrE

3.48

–76

–74

–42

125

1.4

n-ButE

4.03

–81

–79

–47

120

1.3

i-AmE

4.31

–87

–86

–53

115

1.3

n-AmE

4.45

–86

–85

–51

116

1.3

The calculated values of A are shown in Table 2.23. The good agreement between the calculated and experimental values for A conﬁrms the conclusion that the geometry of the transition state is a linear (LC) and abstraction mechanisms (a) and (c) seem more probable. In order to evaluate which mechanism occurs we used the thermodynamic parameters shown in Table 2.22. The calculated heats of formation of the individual compounds ﬁt well to the experimentally measured values [36, 37, 51, 66]. Using different expressions for the Hamiltonians, we have obtained the following values for $H: O3 –33.3 kcal against data from the literature of (34.1); HOOOH –17.8 (–17.1); HOOO –9.2 (–17.8); HOOO –20.3 (–24.9); HO –5.2 (–25.8) [66].

138

Ozonolysis of oxygen-containing organic compounds

$H = –$H (EH) – $H (O3) + $H (EOOOH)

(a)

$H = –$H (EH) – $H (O3) + $H (Ev) + $H (HO3v)

(b)

$H = –$H (EH) – $H (O3) + $H (Å+) + $H (HO3 ) – e2/rip – Es

(c)

where e2/rip = 5.4 eV and Ea = 17 kcal for tetrachloromethane were taken from reference [66].

Table 2.23 The calculated pre-exponents (A) and steric factors (p) log A, CT

log A, CC

log A, LC

p × 106, CC

p × 106, LC

AcalcLC/ Aexp

DEE

11.66

4.02

6.40

0.02

5.5

1

DClDEE

11.66

3.62

4.99

0.009

0.2

1.23

i-PrE

11.67

4.28

6.35

0.04

4.8

0.91

n-ButE

11.72

3.99

6.09

0.02

2.3

0.31

i-AmE

11.75

3.90

6.00

0.01

1.8

0.25

n-AmE

11.74

3.77

5.87

0.01

1.3

0.23

Ether

Note: the values of A in columns 3 and 4 are per one equivalent A-C-H atom.

The calculated values for $H are shown in Table 2.24.

Table 2.24 Calculated heats of investigated reactions according to mechanisms a, b and c Ether

a (kcal/mol)

b (kcal/mol)

c (kcal/mol)

DEE

–32

–16

0

DClDEE

–33

–19

6

i-PrE

–32

–18

1

n-ButE

–32

–18

1

i-AmE

–32

–18

2

n-AmE

–33

–17

2

From thermodynamic point of view, (a) is the preferred mechanism and out of (b) and (c), (b) is more likely.

139

Ozonation of Organic and Polymer Compounds

2.6 Hydroxybenzenes The reaction of ozone with mono- and dihydroxybenzenes has provoked a particular interest [1, 2, 129-132], namely, because of its great importance for environment protection, chemical stabilisation and the theory of reactivity. The ozonation of phenol, pyrocatechol, resorcinol and hydroquinone has been studied in different solvents - aqueous and organic, aimed at the elaboration of the kinetic parameters and product composition [133-139]. The rate constants at room temperature of phenol and resorcinol ozonation in water are 1.3 × 103 M-1.s-1 and >3 × 105 M-1.s-1, respectively, whereas the rate constants of benzene, toluene and anisole ozonation in organic media are 2, 14 and 2.9 × 102 M-1.s-1 [129]. Gurol and co-workers [130] found that the relative rates of pyrocatechol/phenol and resorcinol/phenol ozonation in water medium are 220 and 70, respectively. Provided that the rate constant of phenol ozonation is known [129], the calculated values of the rate constants of pyrocatechol and resorcinol ozonation are 2.86 × 105 M-1.s-1 and 9.1 × 104 M-1.s-1, respectively. However, if the reaction is carried out in organic solvents the values are quite different. For example, in CCl4 and at room temperature the following values have been obtained for: benzene - 0.06, ethylbenzene - 0.2, anisole - 1.1, phenol - 230 and pyrocatechol - 3.2 × 103 M-1.s-1 [136-139]. One of the possible explanations for the different values of the rate constants of hydroxyphenol ozonation obtained by the various researchers could be the great inﬂuence of the water on this reaction, for example for phenol they vary in the range 100-180 M-1.s-1, 500 M-1.s-1 for pyrocatechol and 300 M-1.s-1 for 3,6-di-tert-butylpyrocatechol. Pryor and co-workers [128] have reported that the rate constants of ozone reaction with A-tocopherol in CCl4 and water are 5.5 × 103 M-1.s-1 and 1 × 106 M-1.s-1, with A-tocopherol acetate - 1.45 × 102 M-1.s-1 and that with A-tocopherolquinone it is 1.15 × 104 M-1.s-1. Product analysis of pyrocatechol ozonation in aqueous medium shows that 3 moles of ozone are readily absorbed to give CO2 (24.8%), CO (6%), formic acid (32.5%) and glyoxal (4.2%). As CO2 and formic acid are the main reaction products, it seems very likely that most of the pyrocatechol undergoes anomalous ozonolysis [135]. Radical formation was observed during 2,6-di-tert-butylphenol ozonolysis. When the ratio of absorbed ozone to phenol is 0.8, 50% viscous yellow oil and 3,5, 2,6-di-tert-butyl-o-quinone were identiﬁed in the products [138]. In 3,6-di-tertbutyl-pyrocatechol ozonation the corresponding quinone was found to be the main product after complete consumption of the initial substrate [139]. Side products like 2,5-di-tert-butyl-, 3-hydroxy-p-quinone and 3,6-di-tert-butyl-1,2-phenylacetal5-hydroxy-3,6-di-tert-butyl-p-benzophenone were also found. The rate constant of this reaction amounts to 3 × 102 M-1.s-1. The same constant with the corresponding pyrocatechol with O-hydroxy-acetylated groups is estimated to be a two orders of magnitude lower value. A mechanism has been suggested to involve the formation of either a 1,3-cyclic activated complex between two hydrogen atoms from the OH

140

Ozonolysis of oxygen-containing organic compounds groups and one ozone molecule or P- or S-complexes with the benzene ring. The different values of the literature constants and the various mechanisms proposed for this reaction testify the necessity of further research of its kinetics and reaction pathway. In this connection we have studied the ozonolysis of the following hydroxybenzenes (Scheme 2.6): t-Bu t-Bu

OH

t-Bu

(II)

OH t-Bu

(III)

t-Bu O

O

O

O

t-Bu

t-Bu

(V)

S=O

O

O

O

O CHPh

CH2

C(CH3)

t-Bu

(IV)

t-Bu

t-Bu

t-Bu

OH

OH

OH

OH (I)

t-Bu

OH

OH

(VI)

t-Bu

(VII)

(VIII)

t-Bu

t-Bu O

O P O

HO t-Bu

t-Bu (IX)

Scheme 2.6 The probable mechanisms of ozone interaction with dihydroxybenzenes are presented in Scheme 2.7: OH R

TS

+ O3

PRODUCTS

OH H

TS

OH O +

R

O

#

H OH

O

H O

CC OOOH OH

C

R

R

OH

OH

O

A

H O O

LC-I O3

O R OH

LC-II

R

LC-I, CC

#

O

R

OH

TS

O

O

O

O-

OH A

B R

LC-II

O

O

O

O OH

Ozonide

O3 O R O

#

O CX 2 + O 3

R

CX2

D

Ozonides

O Sor P-complex

Scheme 2.7

141

Ozonation of Organic and Polymer Compounds Mechanism A with a cyclic complex (CC) formation in the transition state was suggested by Konstantinova and co-workers [139]. Mechanism B with linear complex LC-II in the transition state was suggested and discussed by Bailey [2]. The interaction of ozone with C-H bonds with the formation of trioxide [127] as a possible parallel reaction is indicated in mechanism C. The acetylated forms of dihydroxybenzenes can react only via attack on the benzene ring according to mechanism D or C. Formally, mechanism D can be regarded as an extended version of mechanism B, involving the formation of TS similar to P- or S-complexes. We suggest a new mechanism, an extended version of the Razumovskii mechanism whereby the transition state is linear with LC-I structure. Upon ozonation of any of the investigated catechols directly in the electron spin resonance cell or after freezing the reaction products in liquid nitrogen no signals were detected. The kinetic curve of the change of ozone concentration at the bubbling reactor outlet (Figure 2.10, curve 1) is characterised by three different regions: AB - fast ozone consumption after the addition of pyrocatechol, BC - steady-state part when the rate of the chemical reaction becomes equal to the rate of ozone supply, and CD - ozone concentration begins to rise due to the pyrocatechol consumption. The BC part of the curve allows calculation of the rate constant, and from the area below the curve ABCD - the stoichiometry of the reaction. The straight line designated {O3}0 is the ozone concentration at the reactor inlet. Curve 2 presents the o-quinone formation with the reaction time. Its proﬁle suggests the intermediate formation of o-quinone. The kinetic curves of the product formation in pyrocatechol ozonolysis, its consumption and o-quinone consumption during its ozonation in a separate experiment are given in Figure 2.11. Calculated from the kinetic curves in Figure 2.11, rate constants of pyrocatechol and o-quinone consumption were 3.2 × 104 M-1.s-1 and 7.1 × 104 M-1.s-1, respectively. The initial rate of o-quinone formation had almost the same value as that of pyrocatechol consumption. The small variation of the constants is due to the participation of pyrocatechol in parallel reactions. Actually, during the reaction small amounts of open-chain products were identiﬁed. Pyrocatechol ozonation at 15% conversion gave the following yields: o-quinone 85%, pyrogallol - 3%, ozonide - 10%, muconic acid -2%, maleic acid and fumaric acids and polymer products - 1%. The ratio between the amount of absorbed ozone and the consumed pyrocatechol was calculated as 6. Similar ratio values were obtained for the other hydroxybenzenes with free hydroxy groups.

142

Ozonolysis of oxygen-containing organic compounds

12

[O3]0 10

D

A

1

8 6 4 2

B

C

0

2 0

2

4

6

8

10

12

14

Time, min

Figure 2.10 Kinetic curves of ozone absorption at reactor outlet (1) and o-quinone accumulation (2), at ambient temperature, [PC] = 0.227 mM in CCl4 (10 ml) during bubbling of ozone with 0.1 l/min ﬂow rate

1. 0

1

0. 8 0. 6

3

0. 4

2

0. 2 0. 0 0

50

100

150

200

Time, s

Figure 2.11 Kinetics of pyrocatechol consumption (1), o-quinone accumulation (2) and o-quinine consumption (3), ambient temperature, [O3] = 1 × 10-5 M

143

Ozonation of Organic and Polymer Compounds It was found that with the increase of the conversion from 0 to 100% the amount of open-chain products continuously increases while that of the remaining products passes through a maximum. The individual compounds identiﬁed by 13C-NMR are: muconic semialdehyde and acid - 20%, maleic and fumaric semialdehyde and acid - 40%, glyoxal, formic acid, oxalic acid, carbon dioxide and polymer products - 40%. The reaction rate of dihydroxybenzene ozonation follows ﬁrst-order kinetics in relation to each reagent (Figure 2.12, A and B). Table 2.25 presents experimental kinetic data on the ozonation of the investigated dihydroxybenzenes at various temperatures.

Table 2.25 Kinetic data of dihydroxybenzene ozonolysis in CCl4, 20 oC Substrate

k (M-1.s-1)

log A

Ea (kcal/mol)

I

3230

5.35

2.5

II

3100

4.91

1.9

III

3100

5.07

2.1

IV

3200

4.80

1.7

V

500

7.14

6.0

VI

749

7.17

5.8

VII

828

7.06

5.6

VIII

598

6.84

5.5

IX

111

6.81

6.4

Benzene

0.06

8.32

12.8

Toluene

0.40

7.05

10.3

Anisole

10

-

-

Phenol

160

7.31

7.0

A drastic difference between the values of the kinetic parameters of catechols I-IV and V-IX in which the OH groups are acetylated is observed. The rate constants of catechols I-IV manifested 4-28 times higher values than those of catechols of V-IX. On the other hand the pre-exponentials demonstrated by about two orders of magnitude lower values with the former compounds. The activation energies are 2.05 ± 0.5 and 5.9 ± 0.5 kcal/mol, respectively, or the acetylation of the HO groups leads to an increase in EA by about 4 kcal. Probably, ozone reacts predominantly with the hydrogen atoms of the HO groups and, to a very small extent, with the benzene

144

Ozonolysis of oxygen-containing organic compounds ring. The ratio of these ozone interactions varies from 94:4 to 80:20 depending on the hydroxybenzene nature. The lower activation energies of catechols I-IV with respect to those of V-IX are more consistent with the formation of an activated complex of contact type in the transition state, i.e., with structure LC-I (Scheme 2.7). This assumption could also be conﬁrmed by analysis of the kinetic parameters of the ozonation of anisole, benzenes, phenol and toluene. The reaction of ozone with benzene proceeds with a low speciﬁc rate of 0.06 M-1.s-1 and relatively high activation energy equal to 12.8 kcal/mol. The methyl derivatives of benzene, i.e., toluene, reacts 6.7 times faster and the Ea is decreased by 2.5 kcal/mol, and the rate of anizol ozonation is even higher - 10 M-1.s-1. The exchange of ketyl group by a hydroxyl one leads to signiﬁcant acceleration of the ozonation rate and approaches values of 160 M-1.s-1, and the activation energy is reduced by 5.8 kcal/mol. Probably, in this case the mechanism of the reaction is changed and ozone interacts predominantly with the hydrogen atoms of the OH groups. The decrease in the reactivity of anisole compared with that of phenol (^16 times) supports this assumption.

12

A

W0.105, M.s -1

10 8 6 4 2 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

[O3]t.105, M 3.0

B

2.5

W0 / [O3]t, s -1

2.0 1.5 1.0 0.5 0.0 0

2

4

6

8

[P].104, M

Figure 2.12 Dependence of the rate of pyrocatechol ozonation on the concentration of ozone (a) and pyrocatechol (b)

145

Ozonation of Organic and Polymer Compounds Analysis of the kinetic parameters of catechols in acetylated form and those of benzene, toluene, anisole and phenol shows that the rate constants increase and the activation energies go down with increasing numbers of electron-donating substituents (Table 2.25). The high values of the pre-exponential factors could be associated with the high values of the energy of free rotation in the activated complex as a result of the steric hindrances caused by the presence of the bulky tert-butyl groups. This means that when the hydroxyl groups are acetylated, the activated complex should have a LC-II like structure and the ozonation would lead only to the formation of ozonides and open ring products. The kinetic parameters of some dihydroxybenzenes were determined in ozonation in aqueous medium (for those which are water soluble - I-IV, VIII and IX). However, we found that because of diffusion limitations in the bubbling reactor we were not able to measure constants higher than 1 × 104 M-1.s-1. In fact, the literature values for some of these constants are of this order, but they have been measured by a modiﬁed stop-ﬂow technique [27]. The higher reactivity of dihydroxybenzenes in aqueous medium is connected with the change of the ozonation mechanism from a hidden radical to an ion one. However, this question requires special study and discussion. The analyses made so far show that the reaction path of the ozone reaction with dihydroxybenzenes depends strongly on their nature and it proceeds via a transition state with activated complex - LC-I or LC-II

2.6.1 Application of ERM For detailed analysis of the reaction mechanism of the ozone reaction with catechols we have used the ERM method. The necessary parameters for calculations are presented in Table 2.26. In addition, we calculated the heats of formation of pyrogallol - –117.6 kcal/mol (–129 kcal/mol [140]), 3-trihydroxy pyrocatechol - –59 kcal/mol, H2O3 - –21.2 kcal/ mol (–17.7 kcal/mol [140]) and ozone - 34.1 kcal/mol. Thus, the heats of the ozonation reaction according to the different mechanisms were calculated and are as follows: A - 33.1; B and D - 46.1 and C - 32.1 kcal/mol. All the mechanisms are exothermic and, therefore thermodynamically favourable. In this case only the magnitude of the activation energy and the entropy beneﬁts will determine the reactivity of these compounds and the reaction pathway. Calculated pre-exponentials (Table 2.27) were compared with those obtained experimentally (Table 2.25) and it is seen that A have lower values (^10 times) for the cyclic form of

146

Ozonolysis of oxygen-containing organic compounds the activated complex if compared with the experimental values. At the same time the values of A for LC are about 200 times higher. The values of A for CC are the highest as predicted by the theory and they are lower than the experimental values. This suggests that the reaction takes place via a cyclic complex. The values of A calculated for LC in Table 2.27 were obtained without taking account of the energy of free rotation. The latter was calculated as a sum of the rotation around H-O and O-O axes (by MOPAC6) and amounts to 3.1 kcal. This means that the real values in column 4 are about 200 times smaller. The comparison between the A values corrected in this way and the experimental data for the compounds I-IV reveals full agreement.

Table 2.26 Symmetry numbers (S),van der Waals radii (r), heats of formation ($H) of dihydroxybenzenes and their corresponding quinones and ozonides Substrate

S

r (Å)

$H (kcal/mol)

$Hquinone (kcal/mol)

$Hozonide (kcal/mol)

I

2

3.02

–62 (–85)

–39 (–43)

–73

II

1

3.82

–88

–66

–99

III

2

4.46

–109

–86

–125

IV

1

4.39

–110

–90

–123

V

2

5.07

–116

–125

VI

2

4.56

–90

–103

VII

1

5.37

–63

–86

VIII

1

4.77

–73

–77

IX

1

5.30

–135

–140

Benzene

6

2.86

19 (13)

21

Toluene

1

3.37

12 (3)

11

Anisole

1

3.45

–16 (–26)

–20

Phenol

1

2.96

–21 (–38)

–24

Note: The experimental values are in brackets [140].

147

Ozonation of Organic and Polymer Compounds The reaction pathway of the ozonation reaction of compounds V-IX is quite different as the transition state includes the formation of a S or P-complex. For these compounds the formation of AC in the transition state is impeded due to the breaking of their aromatic character but simultaneously the free rotation is facilitated, which may be even zero, due to the action of the principle of the lowest energy. If the energy of free rotation is assumed to be very low then the values of A calculated for LC-II would coincide with the experimentally found ones. Such a coincidence is observed for compound V in column 5 (Table 2.27).

Table 2.27 Calculated pre-exponential values for ozone reaction with dihydroxybenzenes Substrate

log A

log A,

log A,

log A,

Acalc/

log p

TC

CC

LC

calc.

Aobs

1

2

3

4

5

6

7

I

11.318

4.346

7.69

5.39

1.10

5.928

II

11.422

3.905

7.25

4.95

1.10

6.472

III

11.500

4.070

7.42

5.12

1.12

6.380

IV

11.490

3.780

7.12

4.82

1.05

6.670

V

11.572

4.017

7.36

7.36

1.66

4.212

VI

11.511

4.056

7.40

7.40

1.70

4.102

VII

11.801

3.934

7.28

7.28

1.66

4.521

VIII

11.533

3.717

7.06

7.06

1.66

4.473

IX

11.575

3.689

6.88

6.88

1.10

4.695

Benzene

11.319

5.124

8.47

8.47-

1.41

2.849

Phenol

11.322

4.196

7.54

7.54

1.70

3.782

Note: p is the collision factor.

The basic conclusion for the analysis of the results obtained for this reaction is that the kinetics and mechanism of the ozonation reaction of dihydroxybenzenes depend strongly on their structure and the type of the reaction medium.

148

Ozonolysis of oxygen-containing organic compounds

2.7 Carbohydrates and model compounds The selective oxidation of carbohydrates has received increasing attention in recent years [141-151]. This stems from interest in the transformation of carbohydrates to new valuable and ecologically friendly oxygen-containing compounds from available starting materials. Hence, a detailed clariﬁcation of the processes related to oxidation, the right choice of the catalysts and the deﬁnition of optimum conditions are of great importance [143, 145, 146, 148]. These reactions are invaluable for structure determination and transformation of polyhydrates and other natural products. The formation of two carboxylic groups alters signiﬁcantly the carbohydrate (polymer) properties making it water soluble and imparting to it ion-acceptor properties. This polymer can be used for preparing new classes of detergents and ion-capture agents. The most interesting reaction is oxidative cleavage of C2-C3 bonds [147]:

OH

OH

O

O

-OOC

HO

-OOC

OH O

n

O

n

Formation of two carboxylic groups dramatically changes the properties of carbohydrates making them water soluble and they become ion capturers. If combined with hydroxylation of the double bond, the oxidation can be used as an alternative method for direct cleavage of double bonds by ozone [1, 2]. The oxidation of C2-C3 bond is accomplished by applying stoichiometric oxidants such as Pb(AcO)4 in organic solvents and HIO4 in aqueous media. Occasionally, other oxidising agents have been used such as ceric ammonium nitrate, sodium bismuthate, chromium trioxide and trivalent or pentavalent [152-161] iodine compounds. The mechanism of the C-C bond cleavage of vicinal diols with periodine acid can be represented as follows:

149

Ozonation of Organic and Polymer Compounds _

H

O

OH +

O I

O

_

O

O

I O

OH

O

O

OH

O A

_ + IO3 + H2O

O O

_

_

O O

O O

O

I O

O

OH I

+ H2O

O

OH

O

O O

_ + IO3

The cleavage of C-C bonds between two adjacent OH groups in the linear diols is a good source for aldehydes [162-164]. The efﬁciency and selectivity of the ozone reaction with C=C bonds in organic compounds is well known [1, 2]. Rate constants are of the order of 14-106 M-1.s-1, in contrast to 10-2-10 M-1.s-1 for the ozone reaction with C-H bonds. The rate determining step of the alcohol ozonolysis is the H-atom abstraction in the A position in relation to the OH group. The relative reactivity of the primary:secondary:tertiary alcohols with respect to ozone lies in the following sequence: 1:15:138 and corresponds to the C-H bond energies. The main products of alcohol ozonation are the corresponding aldehydes or ketones. The carbonyl compounds are in equilibrium with their enol forms which brings about their further decomposition via an ozone reaction with the C=C bond of the enol form according to Scheme 2.8. C

OH

O3

C

O

H+

C

OH

C

OH

-H2O; -O2

C

OH

HO-

C

OH

OH C

OH

O3

C

O

C

OH

H2O

C

O

OH

Scheme 2.8

150

+ H2O2

Ozonolysis of oxygen-containing organic compounds The monomer glucose unit contains the following A-hydrogen atoms: tert-C-H bond at C2 and C3 adjacent to the OH group; at C4 - adjacent to acetal group; at C1 - aldehyde hydrogen atom; at C5 -with alkyl substituents; and sec-C-H bond at C6. The energy of the aldehyde hydrogen atom is slightly lower than that of the tert-C-H bonds (difﬁcult to determine the difference) and considerably lower than that of the sec-C-H bond. Obviously, the hydrogen atom in the aldehyde group will be the most reactive. Among the tert-C-H bonds those with higher electron density will be more reactive. They will be presented in the following order of decreasing reactivity: C5>C4>C3=C2. The sec-C-H bond at C6 appears to be the most inert in the ozonolysis reaction. The preliminary analysis reveals that ozone will attack predominantly the H atom at the C1 atom in starch with the formation of carboxylic groups and opening of the glucoside ring, and to a lesser extent, give rise to carbonyl compounds at the C5, C4, C3 and C2 atoms which can be easily converted to acids. However, a selective cleavage of the C2-C3 bond could not be expected to occur. Conventionally, the oxidation reactions of carbohydrates are carried out in an aqueous medium. It is known that the solubility of ozone in water is low and requires very good stirring. Moreover, at a pH above 7, ozone begins to react rapidly with the hydroxyl anions in the solution yielding HO radicals [129, 165-167]. The latter reduces the selectivity of the oxidation process (Scheme 2.9): HO2 O2 + H+

(1)

O3 + HO m HO2v (O2 ) + O2

(2)

O3 + (H+)O2 m (H+)O3 + O2

(3)

O3 + HOv m HO2v (O2 ) + O2

(4)

Initiation:

Propagation:

Termination: Any recombination between HOv, HO2v, O2

(t)

where (1) pKa= 4.8; (2) k2 = 48; 70 M-1.s-1. O3 + O2 m O3 + O2

(5)

151

Ozonation of Organic and Polymer Compounds O3 + H+ HO3v

(6)

HO3v m HOv + O2 ,

(7)

where k5 = 1.6 × 109 M-1.s-1; k6 = 5.2 × 1010 M-1.s-1, k-6 = 3.7 × 104 s-1; k7 = 1.1 × 105 s-1 Scheme 2.9 The study of this reaction is impeded by the low solubility of polymeric carbohydrates (starch) in water and in this connection we have used model compounds for elucidation of the reaction mechanism. The ozonolysis of methanol, ethanol, isopropanol and other alcohols proceeds via A-H-atom abstraction by ozone leading to the formation of a carbonyl group which is subsequently oxidised to a carboxylic one. It has been found that the ethanol ozonolysis is promoted by adding small amounts of hydroperoxide [168] because the steady concentration of the HO radicals is increased. Upon ozonolysis of isopropanol in an aqueous medium at pH 8, controlled by titration with NaOH, ambient temperature and 36% conversion, the following products in the high-performance liquid chromatography (HPLC) chromatogram were identiﬁed: acetone - 25%, AcAc - 6% and formic acid - 5% (Figure 2.13).

iso-Propanol

10

Ozone

[P], mM

8

6

4 Acetone 2 NaOH 0 0

20

40

60

80

Time, min

Figure 2.13 Kinetics of product formation during the ozone reaction with isopropanol at pH 8. The products were identiﬁed by means of HPLC and 13 C-NMR

152

Ozonolysis of oxygen-containing organic compounds The mechanism of the reaction could be expressed as shown in Scheme 2.10: (CH3)2C'OH + HO3' (HO' + O2)

(CH3)2CHOH + O3

-O2 (CH3)2C(OOOH)OH

(CH3)2C(OH)OH

-H2O

(CH3)2C(O')OH

-HO2'

(CH3)2CO CH3COOH + CH3'

Scheme 2.10

2.7.1 2,3-Butanediol Butanediol was oxidised in aqueous medium without pH control and at constant pH 10 and the main kinetic results are given in Figure 2.14.

25 Ozone

pH=10 20

[P], mM

15 NaOH Butandiol 10

5 Acetoin AcOH

0 0

20

40

60

80

100

120

Time, min

Figure 2.14 Kinetics of product formation during ozonolysis of 2,3-butanediol at pH 10. The products were identiﬁed by means of HPLC In the HPLC chromatogram of the starting material two peaks were recorded from the two stereoisomers. New peaks were found after ozonation: keto alcohol, acetoin,

153

Ozonation of Organic and Polymer Compounds AcAc, acetaldehyde, formaldehyde and formic acid. At higher conversions the peaks of the keto alcohol and acetoin disappeared whereas the peak from AcAc increased its intensity. The observed rate of butanediol consumption is higher than the sum of rates of product formation determined by HPLC. Probably at higher conversions of ozone the HO radicals oxidised the reaction products to carbon dioxide and water. In this connection calcium carbonate was found in the trap. The stoichiometry of the ozonation reaction with respect to ozone is unity and the rate constant of the reaction is 0.2 M-1.s-1. At low conversions up to 10-120% the main products of the reaction are acetoin and AcAc. It has been found that at pH 8 the rate of acetoin formation is lower than that without pH control. The scheme proposed for the ozonation reaction is shown in Scheme 2.11:

HO CH3

OH

C

C

H

H

HO

+ O3

HO C

CH3

CH3

HO

H

CH3

C

OH

O

OH

C

C

C

C

CH3

CH3

CH3 C O'

D

OH C

-H2O;-O2

O

O

C

C

CH3

C

H

CH3

+

'C

CH3

H

+RH F

B

F

G

G

Scheme 2.11

154

C

OH

O

E

C

E

CH3

CH3

OH

HOO

CH3

CH3

HO

-HO'

D

CH3

HO

H

-H2O2

+O3

C

H

D

HO

E

H

CH3

+ RH

O

A

C

'OO B

-H2O

-HO2'

CH3

OH

C

CH3

H

-H2O

H

HO

OH

A -O2

CH3 + HO3' + (HO', O2)

H

C

HOOO

OH C

C .

CH3

OH

C

CH3

HO CH3

+O3 CO2, HCHO, CH3CHO...

Ozonolysis of oxygen-containing organic compounds

2.7.2 1,2-Cyclohexanediol A portion (100 ml) of 10 mM 1,2-cyclohexanediol was ozonised with stirring at ambient temperature. The pH of the medium was kept constant at 7, 8, 9 or 11. In the experiments without pH control its change was measured during the reaction duration. In all cases formation of keto alcohol, diketone and open-chain products was observed (Figure 2.15).

Figure 2.15 Initial rates of product consumption and formation upon 1,2cyclohexanediol ozonation (Sub) at various pH and without pH control (NC). The products cyclohexanol-2-on (KetAlc) and adipic acid (AdAc) were identiﬁed by HPLC and 13 C-NMR, and the total acid number by NaOH

Upon ozonolysis without pH control the main products were KetAlc and AdAc. The value of pH decreases rapidly at the beginning from 7 to 2.2. In the 13C-NMR spectra of the reaction mixture peaks were recorded at 179.5, 180.1 and 179.4 ppm which could be ascribed to adipic acid semialdehyde (179.5 and 179.4 ppm) and AdAc (180.1 ppm). Moreover, at conversions up to 20% other products with open chains like glutaric, succinic, malonic, AcAc and formic acid were also identiﬁed. The rate

155

Ozonation of Organic and Polymer Compounds of cyclohexanediol is found to be 2-3 times higher than the rate of ozone uptake, the latter corresponding to the sum of the rates of KetAlc and AdAc formation. This fact suggests that its decomposition occurs via another route. The ratio between the rate of formation of cyclic and open-chain products decreased from 2 to 0.5 with increasing pH from 4 to 11. Simultaneously the rate of alcohol consumption increases 4 times. When the pH was controlled from 7 to 11, the rate of consumption of NaOH and ozone corresponds to the rate of cyclohexanediol decomposition. In the NMR spectra the diversity of peaks corresponding to carboxylic carbons reaches its maximum at pH 11 and peaks at 167.8, 174.7, 184, 178.3 and 184.8 ppm were recorded. All of the above implies that the mechanism of cyclohexanediol ozonolysis in aqueous media is changed from stoichiometric A-C-H-atom abstraction in acidneutral media to a chain hydroxy-radical oxidation in alkaline media.

2.7.3 Mannitol and its derivatives The kinetics of product formation during the ozonolysis of mannitol and 1,2,5,6-diO-iso-propyl-mannitol:

HO

O

HO

O

HO

HO OH

OH

OH

O

OH

O

is shown in Figures 2.16 and 2.17. In the ozonation of mannitol at pH 7 the inner OH groups were converted to carbonyl groups or the C-C bonds broke leading to the formation of products with a reduced number of carbon atoms compared with that of the starting compound. For example, carbonyl and carboxylic compounds, CO2, H2O were obtained. The rate of ozone absorption and that of mannitol consumption coincide; the sum of the rate of acid formation and keto compounds is also close to these values.

156

Ozonolysis of oxygen-containing organic compounds

3.0 pH=7

Mannitol 2.5

Ozone

2.0 1.5

[P], mM

3,4-C=O

1.0

NaOH

0.5 0.0 -20

0

20

40

60

80

100

120

140

160

Time, min

Figure 2.16 Kinetics of product formation during the ozone reaction with mannitol at pH 7. Products were identiﬁed by HPLC and 13C-NMR

7

NaOH

pH=4

6

[P], mM

5

Ozone

4 3 PM 2 Mannitol

1 0 -1 0

50

100

150

200

250

300

350

Time, min

Figure 2.17 Kinetics of product accumulation during 1,2,5,6-O-di-isopropylmannitol ozonolysis. The products were identiﬁed by HPLC and 13C-NMR

The acetylation of the 1,2,5,6-OH groups at pH up to 7 causes hydrolysation of the compound thus obtaining mannitol and acetone (Figure 2.17). At pH above these values the selectivity of ozone interaction with the third and fourth A-H atom is increased.

157

Ozonation of Organic and Polymer Compounds If the ozonation (Scheme 2.12) was carried out without pH control the pH changes from 7 at the beginning to 2.8 at 20% conversion. A good correlation has been found between the rate of PM decomposition, ozone consumption and the rates of product (acetone, ketones and acids) formation. Ozonation at constant pH 4, kept constant through titration with 0.1 N NaOH, shows similar kinetic characteristics, i.e., monoketone at the third and fourth position is formed which is further converted into 3-4-diketone. It has been established that the rate of the reaction was accelerated by 4-5 times in an alkaline medium.

O

O

O O

O

O HO HO

+

O3

OOOH

-H2O

OH

-O2

OH

O OH

O O

O

O

O

O

A

.

O A

-HO2

OH

O HO

B

+ O

O O

;

A

-HO

B

+RH; -H2O2

;

A

+RH -H2O

B

Scheme 2.12 Ozonation of 1,2,5,6-O-di-iso-propylmannitol

2.7.4 A-D-Glucose In the ozonation of A-D-glucose it was found that ﬁrstly ozone attacks the H atom of the aldehyde group at the ﬁrst C atom, and after that the H atoms of the tert-C-H bonds at C2, C3, C4 and C5. Upon ozonation without pH control, the values of pH changes from 7 to 2 in the course of the reaction (Figure 2.18). The yield of gluconic acid was 60% at 5% conversion (C1 signal at 177.2 ppm). The rest of products at this moment was about 40%: D-arabino-hexulose (C2 - 178.4 ppm), D-ribo-hexos3-ulose (C3 - 177.8 ppm), D-xylo-hexos-4-ulose (C4 - 176.9 ppm), D-xylo-hexos-5ulose (C5 - 176 ppm), D-gluco-hexodialdose (C6 - 175.9 ppm), oxalic (167.2 ppm) and formic acids. The pattern of product formation during glucose ozonolysis at pH from 7 to 9 was similar to that in an acid medium, but with a different ratio of product distribution. In that case the main product was gluconic acid reaching values up to 90-95% (Figure 2.19).

158

Ozonolysis of oxygen-containing organic compounds

13

C NMR - 177.2, 167.2, 178.4, 177.8 ppm

10

10

D-glucose

8 pH

6

6 NaOH

4

2

pH

[P], mM

8

4

2 Gluconic acid

0

0 0

20

40

60

80

100

120

140

Time, min

Figure 2.18 Kinetics of product formation during ozonolysis of D-glucose without pH control. Products were identiﬁed using HPLC and 13C-NMR 13

C NMR - 180.1, 173.0

pH=7

D-glucose

10

Ozone

[P], mM

8

6

NaOH

4

Gluconic acid

2

0 0

50

100

150

200

250

Time, min

Figure 2.19 Kinetics of product formation during the reaction of ozone with D-glucose at pH 7. Products were identiﬁed by means of HPLC and 13C-NMR

In an alkali medium sodium salts were formed and the peaks of the carboxylic carbon atoms were slightly shifted. The rate of the reaction rises 1.5-2 times compared with that in an acid medium. Among the other products, products were identiﬁed from the ozone attack on C2, C3, C4 and C5 - 177.4, 176.2, 174.7 and 172.6 ppm. Thus 159

Ozonation of Organic and Polymer Compounds analysing the results it can be assumed that ozone attacks all tert-C-H bonds in the glucose molecule with almost equal probability but the rate of cleavage depends on the energy bond and the electron density. The variations in pH change the rate of ozone interaction with the hydroxyl anions and thus changes the oxidation carrier species from ozone to hydroxyl radical.

2.7.5 A-D-Methyl-glucose The change of H in the hydroxyl group at C1 for a methyl group is used for protection of this group and for modelling of a part of the polymeric carbohydrate chain. From this point of view A-D-methyl-glucoside (MG) can be regarded as a suitable low molecular model for elaborating the reactivity and mechanism of ozonolysis. Figure 2.20 demonstrates a pattern of the ozone reaction with MG at pH 8. The reaction was carried out in an alkaline medium to prevent the hydrolysation of MG in acid medium and the transformation of the methoxy group to a hydroxyl one. It is seen that MG is almost quantitatively converted into glucose although the ozonation was carried out in an alkaline medium. This could be attributed to the occurrence of oxidative hydrolysation under the action of ozone. After this ozone and HO-radicals react with it yielding acids, identiﬁed by the 13C-NMR signals (Figure 2.20). We determined that the rate of MG decomposition is 3.4 × 10-7 M/s, the rate of acid groups accumulation as 3.12 × 10-7 M/s, the rate of ozone absorption 2.43 × 10-7 M/s and the initial rate of glucose formation as 1.9 × 107 M/s. 13

C NMR - 180.1, 172.5, 174.6 ppm

pH=8

6

NaOH

5 Ozone

[P], mM

4 3 -Me-D-glucoside

2 1

D-glucose

0 -50

0

50

100

150

200

250

300

350

Time, min

Figure 2.20 Kinetics of product accumulation during A-D-methyl-glucoside at pH 8. The products were identiﬁed by HPLC and 13C-NMR 160

Ozonolysis of oxygen-containing organic compounds

2.7.6 B-Cyclodextrine and starch The ozonolysis of B-cyclodextrine and starch was carried out in solution and suspension and the kinetic results are presented in Figures 2.21 and 2.22.

13

C NMR - 172.4, 181.1, 179.8, 174.5, 177.1 ppm

16

pH=7

14

Ozone

12

[P], mM

10 8 6

NaOH

4 2 -cyclodextrine 0 -2 0

50

100

150

200

Time, min

Figure 2.21 Kinetics of product formation during ozone reaction with B-cyclodextrine at pH 7. Products were identiﬁed by HPLC and 13C-NMR

B-Cyclodextrine is an excellent model of polysaccharides that contains 10 glucoside units arranged in one cycle, in which the OH groups in the 1 and 4 position are blocked (acetylated). The OH groups at C2, C3 and C6 remain free. Upon ozonolysis of aqueous B-cyclodextrine solution at pH 7 its consumption rate amounts to 1.67 × 10-7 M/s. This value is almost equal to the rate of ozone consumption while the rate of titration by NaOH is found to be by about 25% of these rates. Ozone converts the B-cyclodextrine molecule into a linear one with formation of carboxylic groups. The latter were identiﬁed by the appearance of chemical shifts at 172.4, 181.1, 179.8, 174.5 and 177.1 ppm in their 13C-NMR spectra. The underlined values can be assigned to the carboxylic atoms at C2-C3 atoms. At ozonation of starch suspension (Figure 2.22), ﬁrstly it is oxidised and only after that does it dissolve. A series of peaks which could be ascribed to the carboxylic acid formation has been observed in the NMR spectra. A very good agreement was obtained between the rate of starch consumption, ozone uptake and rate of titration by NaOH.

161

Ozonation of Organic and Polymer Compounds

13

C NMR - 178.2, 177.4, 174.9, 172.6, 162.6 ppm

3.0 Starch

2.5

[P], mM

2.0 1.5 1.0 Ozone 0.5 NaOH 0.0 0

10

20

30

40

Time, min

Figure 2.22 Kinetics of product accumulation during starch ozonolysis at pH 7. Product identiﬁcation was carried out using HPLC and 13C-NMR

The observation that the suspension is dissolved and the coincidence between the amount of absorbed ozone and the acid number of the oxidate suggest that the ozonation method can be used for transformation of natural polyhydrates to new valuable products. The mechanism of the reaction of ozone with model compounds (vicinal alcohols), as shown above, is based on the interaction between ozone and the A-H atom in the alcohol molecule, the cleavage of C-C bonds being a very rare and random process. The latter was observed during the monomolecular decomposition of oxy-radicals generated in the course of some of the conversion steps of intermediate products. The possibility of improving the selectivity of the C-C bond cleavage could be based on oxidation of carbohydrates using catalytic systems and speciﬁc reagents.

2.8 Catalytic ozonolysis and oxidation The selectivity of the C-C bond cleavage at vicinal OH groups is improved by applying speciﬁc reagents like RuO4 [29]. Usually Ru(VIII) is obtained in situ via Ru(III) oxidation by suitable oxidants, for example NaOCl in alkaline medium. After that RuO4 interacts with the alcohol forming a Ru-diol complex according to the following scheme (Scheme 2.13):

162

Ozonolysis of oxygen-containing organic compounds RuCl3 + 3NaOH Ru+3

+O3

Ru+3 + 3OH- + 3Na+ + 3Cl-

Ru+4

-O3-

+O3 -O2

Ru+6

+O3 -O2

Ru+8

O OH OH

-H2O + RuO4

H

O

H

O

Ru O

-RuO3

HO

O

+O3

HO

O

O

Scheme 2.13 Our idea was to use ozone instead of hypochloride. In this connection we have measured the rate constant of Ru(III) oxidation by ozone which at ambient temperature at pH 7 has the value of 15 M-1.s-1. Unfortunately, from a kinetic point of view this value is extremely low to compensate the rate of RuO4 interaction with the alcohol or carbohydrate and to ensure high Ru(VIII) concentration. Because of this, the ozonolysis was carried out in the following manner: the black solution of 1 wt%. RuCl3.H2O in 50 ml water was ozonised at pH 7 until it turned yellow (RuCl4). After that the substrate solution was added drop-wise. In this way all alcohols and carbohydrates mentioned above, were ozonised. AcAc was the main product of 2,3-butanediol ozonolysis (Figure 2.23).

NaOH

pH=8

20

15

Butanediol

Ozone

10

AcAc 5

0 0

75

150

225

300

Time, min

Figure 2.23 Kinetics of 2,3-butanediol ozonolysis (50 ml water), in the presence of 0.1 g RuCl3.H2O, ozone ﬂow rate - 0.1 l/min and pH 8. Product identiﬁcation was performed by HPLC and 13C-NMR 163

Ozonation of Organic and Polymer Compounds It is seen that (Figure 2.23) the initial rates (up to 90 minutes) of butanediol consumption, ozone absorption, AcAc formation and titration by NaOH are almost the same as shown by the equal slopes of the curves. However, after 90 minutes other products were also formed and the acidity of the reaction mixture begins to rise as shown by the sharp increase in the titration curve (by NaOH). Upon ozonolysis of 1,2-cyclohexanediol, mannitol and its derivatives via the method mentioned previously, a selective C-C bond cleavage between the vicinal HO groups with carboxylic group formation was observed only to 20-30% conversions. At higher conversions the number of products is increased, namely because of the occurrence of secondary processes. In the cases with straight-chain alcohols two monoacids are formed from one alcohol molecule and with cyclic alcohols - one dicarboxylic acid, for example adipic acid at 1,2-cyclohexanediol ozonolysis or 2,3-dicarboxylic acid at glucose (its derivatives) ozonolysis. When the oxidation was carried out at lower pH up to 6.5, a hydrolysis of the protected HO-groups takes place. Such an example is the ozonolysis of 1,2,5,6-di-O-mannitol at pH 6.5 (Figure 2.24).

13

C NMR - 174.7 ppm pH=6.5

5

NaOH 4 GlicAc

[P], mM

3

2

1

PM

0 0

20

40

60

80

Time, min

Figure 2.24 Kinetics of product formation during ozonolysis of 1,2,5,6-di-Omannitol in the presence of 1 wt%. RuCl3.H2O

The rate of glyceric acid formation (174.7 ppm) is almost the same as that of PM consumption but is found to be lower than the rate of NaOH titration. This could be associated only with the partial hydrolysation of PM and the formation of low molecular acids such as formic and acetic acids.

164

Ozonolysis of oxygen-containing organic compounds However, it should be noted that the carbohydrate oxidation by ozone and ruthenium has some shortcomings, such as the removal of ruthenium from the reaction mixture, its volatility, high toxicity and price. It is known that NaIO4, similar to Ru(VIII), can cause the selective C-C bond cleavage at the vicinal hydroxy groups with two carbonyl compound formation. Simultaneously it is reduced to NaIO3. The idea was to use ozone for reoxidation of iodate to periodate. Unfortunately, the rate of this process compared with that of reduction: IO4 + Substrate = IO3 + Products (very fast) IO3 + O3 = IO4 + O2 (slow) Products + O3 = Acids + O2 (fast) is not sufﬁciently high. This requires special conditions for performing ozonolysis: maximum iodate concentration and the absence of organic products in the ozonolysis medium. Such a situation could be achieved if the products of NaIO4 interaction with the substrate are oxidised and removed, and the ozonolysis proceeds further for the NaIO3 oxidation to NaIO4. The oxidation of 1,2-cyclohexanediol (10 ml) by an equimolar amount of NaIO4 under continuous stirring leads to the quantitative formation of adipic aldehyde which is further oxidised by ozone to semialdehyde and adipic acid. No oxidation of iodate to periodate was observed. The application of a number of heterogeneous catalysts Ag2O, Co(AcO)2, FeCl3, Pt/C, Pd/C, Adams catalyst, zeolite catalysts TS-1, microporous titanosilicate (ETAS) and catalysts synthesised by us on the basis of ion exchange: Ce with silicalite - Ce-Silic, Ce and Ru with alumophosphates - Ce-APO and Ru-APO and phthalocyanine (Pc) complexes: PtAu6Pc, PdAu6Pc and (manganese phthalocyanine) MnPc for the selective ozonation of alcohols, model compounds of saccharides and natural carbohydrates, in all cases accelerate the process but did not make the process selective with respect to C-C bond cleavage.

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Ozonolysis of oxygen-containing organic compounds 18. A. Marco, E. Chamarro and S. Esplugas, Aﬁnidad, 1994, 51, 452. 18b. A. Marco, E. Chamarro and S. Esplugas, Aﬁnidad, 1994, 51, 265. 19. J. Starek, A. Zukal and J. Rathousky, Carbon, 1994, 32, 2, 207. 20. S.K. Rakovsky and D.R. Cherneva, International Journal of Chemical Kinetics, 1990, 22, 4, 321. 21. S.K. Rakovsky, D.R. Cherneva, D.M. Shopov and S.D. Razumovskii, Communications of the Department of Chemistry - Bulgarian Academy of Sciences, 1976, 9, 4, 711. 22. S.K. Rakovsky and D.R. Cherneva, Oxidation Communications, 1989, 12, 3, 108. 23. S.K. Rakovsky, L.G. Kulak, E.M. Kuramshin, S.S. Zlotsky and D.L. Rakhmankulov, Communications of the Department of Chemistry Bulgarian Academy of Sciences, 1989, 22, 3, 722. 24. D.L. Rakhmankulov, S.S. Zlotsky, L.Z. Rudnik, G.T. Teregulova and S.K. Rakovsky, Communications of the Department of Chemistry - Bulgarian Academy of Sciences, 1989, 22, 3, 652. 25. M.P. Anachkov, S.K. Rakovsky, A.K. Stoyanov and R.K. Fotty, Thermochemica Acta, 1994, 237, 213. 26. S.K. Rakovsky, D.R. Cherneva and M. Deneva, International Journal of Chemical Kinetics, 1995, 27, 2, 153. 27. S.K. Rakovsky, D.R. Cherneva, M. Deneva and V.V. Ershov, Oxidation Communications, 1997, 20, 2, 169. 28. S.K. Rakovsky, R.A. Sheldon and F.V. Rantwijk, Oxidation Communications, 1996, 19, 4, 482. 29. E.T. Denisov, N.I. Mitskevich and V.E. Agabekov, Mechanism of LiquidPhase Oxidation of Oxygen Containing Compounds, Science and Technique Publishing House, Minsk, 1975. 30. M. Whiting, A. Bolt and J. Parish, Oxidation of Organic Compounds - III, Advances in Chemistry Series, No. 77, ACS, Washington, DC, USA, 1968, p.4.

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168

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Ozonation of Organic and Polymer Compounds 85. E.M. Kuramshin, L.G. Kulak, S.S. Zlotskii and D.L. Rahmankulov, Zhurnal Organicheskoi Khimii, 1983, 22, 1986. 86. V.V. Shereshovetz, L.A. Bakhanova, V.D. Komissarov and G.A. Tolstikov, Zhurnal Organicheskoi Khimii, 1985, 21, 482. 87. V.V. Shereshovetz, V.D. Komissarov, L.A. Bakhanova, RSh, Burkhanova and G.A. Tolstikov, Zhurnal Organicheskoi Khimii, 1983, 19, 892. 88. V.V. Shereshovetz, F.A. Galieva, N.Ya. Shaﬁkov, R.A. Sadikov, A.A. Panasenko, V.D. Komissarov and G.A. Tolstikov, Izvestiya Ackademii Nauk Seriya Khimicheskaya, 1982, 1177. 89. V.V. Shereshovetz, L.A. Bakhanova, V.D. Komissarov, N.S. Vostrikov and G.A. Tolstikov, Izvestiya Ackademii Nauk Seriya Khimicheskaya, 1982, 1922. 90. O. Lorenz, Analytical Chemistry, 1965, 37, 101. 91. R.A. Stein and U. Slawson, Analytical Chemistry, 1963, 35, 1008. 92. G.A. Tolstikov, Reactions of Hydroproxide Oxidation, Nauka Publishers, Moscow, 1976, p.243. 93. L. Fiser and M. Fiser, Reagents for the Organic Syntheses, Volume 3, Mir Publishers, Moscow, 1970, p.66. 94. E.N. Karakulova, Chemistry of Oil Sulﬁdes, Nauka Publishers, Moscow, 1970, p.202. 95. B.M. Brudnik, L.V. Spirikhina, E.M. Kuramshin, U.B. Imasheva, S.S. Zlotskii and D.L. Rahmankulov, Zhurnal Organicheskoi Khimii, 1980, 16, 1281. 96. A. Clerici and O. Porta, Synthetic Communications, 1988, 18, 2281. 97. V. Weber and G. Hokel, Interphase Catalysis in Organic Syntheses, Mir Publishers, Moscow, 1980, p.327. 98. D.L. Rakhmankulov, R.A. Karakhanov, S.S. Zlotzkii, E.A. Kantor and U.B. Imashev, Chemistry and Technology of 1,3-Dioxocyclanes, Itogi Nauki Khimiya Tekhm’ki, 1979, 5, 248. 99. A.C. Baldin in The Chemistry of Peroxides, Ed., S. Patai, John Wiley & Sons Ltd., New York, NY, USA, 1983, p.97.

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Ozonolysis of oxygen-containing organic compounds 100. L.F.R. Cafferata, G.N. Eyler, E.L. Svartman, A.I. Canizo and E.J. Borkowski, Journal of Organic Chemistry, 1990, 55, 1058. 101. C.W. Jefford, A. Jaber, J. Boukouvalas and P. Tissot, Thermochimica Acta, 1991, 188, 337. 102. E. Bernatek and M. Hvatum, Acta Chemica Scandanavica, 1960, 14, 836. 103. O.S. Privett and E.C. Nickel, Journal of the American Oil Chemists’ Society, 1966, 43, 393. 104. N.I. Boldenkov and S.D. Razumovskii, Khimicheskaya Promyshlemnost, 1977, 2, 100. 105. R. Criegee, A. Kerckow and H. Zinke, Chemische Berichte, 1955, 88, 1878. 106a. S.D. Razumovskii and YuN Yur’ev, Zhurnal Organicheskoi Khimii, 1967, 3, 251. 106b. S.D. Razumovskii and YuN Yur’ev, Journal of Organic Chemistry, USSR, 1967, 3, 238. [English translation] 107. P.R. Story, T.K. Hall, W.H. Morrison, III and J.C. Farine, Tetrahedron Letters, 1968, 9, 52, 5397. 108. L.A. Hull, I.C. Hisatsune and J. Heicklen, Journal of Physical Chemistry, 1972, 76, 2659. 109. D.W. Brazier and N.V. Schwartz, Thermochimica Acta, 1980, 39, 7. 110. S.M. Ellerstein in Analytical Calorimetry, Eds., R.S. Porter and J.O. Johnson, Plenum Press, New York, NY, USA, 1968, p.279. 111. L.W. Crane, P.J. Dynes and D.H. Kaelble, Journal of Polymer Science: Polymer Letters, 1973, 11, 533. 112. H. Kissinger, Analytical Chemistry, 1957, 21, 1702. 113. R.N. Rogers and L.C. Smith, Thermochimica Acta, 1970, 1, 1. 114. L. Batt and H.O. McCullough, International Journal of Chemical Kinetics, 1976, 8, 911. 115. L. Batt, K. Christie, R.T. Milne and A.J. Summers, International Journal of Chemical Kinetics, 1974, 6, 877.

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177

Ozonation of Organic and Polymer Compounds

178

3

Ozonolysis of alkenes in liquid phase

3.1. Oleﬁns The reactions of ozone with alkenes are the best known of all reactions of ozone. These reactions take place with high rates, low activation energy and are a basic source for ozonide preparation, i.e., cyclic 1,2,4-trioxalanes [1-7]. The interaction of ozone with the C=C bonds is a powerful method for structural studies of polydienes, functionalising of complex organic compounds at low temperatures and synthesis of valuable oxygen-containing compounds [8-22]. This reaction plays an important role in the atmosphere [23-30] during degradation of rubber materials [31, 32]. All this makes the study of the theoretical and practical aspects of these reactions attractive and relevant for many researchers [1-13]. The ozonation of alkene provoke a great interest both for elucidating the mechanism and reactivity of alkenes and for ozone application in organic synthesis, protection of rubber products against ozone degradation and creation of novel efﬁcient and economical technologies [1, 2, 8, 13, 15, 25-34]. We have carried out detailed and systematic studies on the ozonolysis of oleﬁns and their polymeric analogues [35-51].

3.1.1 Mechanisms The reactions of ozone with alkenes are a subject of interest and study since the discovery of ozone in 1840 [52-84]. The ﬁrst experimental research and conclusions concerning these reactions dated from the beginning of the 20th century [52-55]. Harries observed that certain oleﬁnic compounds reacted with ozone to give peroxide oils to which he ﬁrstly assigned structure (1) and later he changed the structural assignment to structure (2) as shown in Scheme 3.1 [56-63]: C O

C

C

O

O

C O

O

O

1

2

179

Ozonation of Organic and Polymer Compounds H2O C=O + H2O2 + O=C

2

H

2

O C

+ O=C O

O

H

C

C-OH O

O

Scheme 3.1

Harries proposed that the compound with structure (2) decomposed to hydrogen peroxide and carbonyl compounds. However, the possibility for self-decomposition of (2) to 1,2-dioxalane rearranging further to a carboxylic acid and a carbonyl compound could not be excluded. Staudinger [64] suggested a more complicated mechanism of oleﬁn ozonolysis (Scheme 3.2) than Harries: O

O R 2C

R 2C=CR 2+ O 3

CR 2

O

O

R 2C

O

O R 2C

CR 2

;

O

+OOCR2 R 2C=O + O=O=CR 2

O

R=H

O Polymer

R-C-OH

O

CR 2 O

CR 2

R 2C O

O

O

Scheme 3.2

He assumed that ozone reacts with oleﬁn producing an intermediate adduct with a four-member ring, which undergoes rearrangement to 1,2,3-trioxalane, diperoxide,

180

Ozonolysis of alkenes in liquid phase carbonyl compound or an acid or polymerises. Later it was established that this scheme failed to explain a number of experimental observations [66]. In 1949-1951 Criegee proposed another mechanism of ozonolysis which can be considered as classical [66-71]. This mechanism is outlined in Scheme 3.3. R2C=CR2

R2C

O3 R2C

CR2

O

+O

O

O

CR2 O_

O primary ozonide O _ + R2C=O + O-O-CR2 zwitterion

R2C

O CR2 + polymeric ozonides

O ozonide

HG O R2C O

O CR 2 +

OOH polymeric peroxides

;

R2C

G

;

rearrangement products

O

diperoxide _ G=OH, OR, RCOO

Scheme 3.3 According to Scheme 3.3, ozone reacts with C=C bonds forming a primary ozonide (PO) - 1,2,3-trioxalane. The reaction is exothermic producing more than 50 kcal/mol. PO is very unstable and rapidly decomposes to a zwitterion and a carbonyl compound. At least four ways have been suggested by which the zwitterion can stabilise itself: (1) reaction with its own ketone or aldehyde to give ozonide (1,2,4-trioxalane), (2) reaction with the wall of the solvent cage, composed mainly of solvent molecules thus producing hydroperoxides, (3) reaction with other zwitterions outside the cage yielding diperoxide - 1,2,4,5-tetraoxalane and polymeric peroxides, -(-O-O-C-O-O-) n-, reactions with carbonyl compounds resulting from the decomposition of another PO and formation of ‘cross’ ozonides, reactions with preliminary added aldehydes, ketones, alcohols, water, and so on leading to the formation of ‘foreign’ ozonides or peroxide compounds, and (4) undergoes monomolecular rearrangement giving 1,2dioxalane which isomerises further to acid. The proposed mechanism has been conﬁrmed by much experimental data from various researchers [1, 2]. The low temperature reduction of oleﬁn ozonolysis products,

181

Ozonation of Organic and Polymer Compounds obtained at low temperatures, gives 1,2-alcohol with 1,2,3-trioxalane as a precursor [87-89]. The formation of a zwitterion is demonstrated by the formation of: (1) ‘foreign’ ozonide [84], (2) methoxy hydroperoxides in methanol solution [91-94], (3) ‘cross’ ozonides [95-98], and (4) ozonide during the photochemical decomposition of diazo compound solutions in the presence of oxygen and an aldehyde [99]: hN

(C6H5)2CN2

(C6H5)2C:

O2

(C6H5)2C-O-O

radical reactions (C6H5)2C-O-O (C6H5)2C=O+-O-

RCHO (C6H5)2C=O+-O-

O

C6H5

O R

C6H5

H

O

Rieche [73, 74] conﬁrmed the ozonide structure through a counter synthesis, and later it was established by different spectral and theoretical methods [74-83]. The Criegee mechanism is very close to the contemporary concepts for the mechanism of oleﬁn ozonolysis. The additional studies carried out by Criegee accounting for the effect of the number, size, spatial arrangement, electronic properties of the substituents, conformational state and electronic structure of the zwitterion [84-86] on the alkene ozonolysis throw light on the mechanism of the ‘anomalous’ ozonolysis of some oleﬁns with more particular structure and substituents [87-101]. The ozonide stereochemistry can be predicted if the conﬁguration of the zwitterion - anti or syn is known, which is strongly dependent on the nature and size of the substituents and the experimental conditions [100-102]: _

_

O

O +O

+O

C

C R anti

H

H

R syn

The studies on oleﬁn ozonolysis up to 1958 were summarised by Bailey [3]. Many other authors have carried out research on separate problems regarding these reactions [4-16]. These works stimulated a number of new studies on the mechanism and stereochemistry of the reaction, on the ozonolysis of new types of oleﬁns, the

182

Ozonolysis of alkenes in liquid phase preparation of new products and the application of novel methods of analysis [5457, 71-76]. Concurrently with the accumulation of the convincing evidence favouring the Criegee mechanism, other evidence began to appear which could not be explained by the classical Criegee mechanism. For example, upon ozonolysis of cis- and trans-1arylpropenes, the ratios of ozonide to polymer peroxides and/or free aldehydes are quite different for the two isomers [77-79]. It was also reported that the cis:trans ratios of cross ozonides [80-88] obtained from cis and trans unsymmetrical oleﬁns, often differ whereas according to the simple Criegee mechanism cis-oleﬁn should yield cis-ozonide and trans-oleﬁn trans-ozonide. For this reason, Loan and co-workers [87] proposed a scheme which explains the formation of trans- and cis-ozonide from cis- and trans-oleﬁn, respectively (Scheme 3.4).

o + O3

o o

o o

; o

o

o

o

o

o

Scheme 3.4 The mechanism indicated above includes the formation of a seven-member ring intermediate with conformation responsible for the ﬁnal ozonide conformation. Upon ozonolysis of trans-oleﬁns the cis:trans-ozonide ratio should be close to 1, and higher than 1 upon cis-oleﬁn ozonolysis. In case of hindered oleﬁns, the formation of a S-complex rearranging further to ozonide, is considered as most probable. This mechanism lost its importance very quickly as many new experimental results emerged which could not be completely explained by it. Thus the ozonolysis of trans-di-tert-butylethylene yields trans-ozonide [102] while according to the Story mechanism it should give cis-ozonides. Moreover, the Story mechanism could not explain the formation of epoxides which are the major products in ozonolysis of highly hindered oleﬁns. It was found that the foreign aldehyde with deuterated 18O atom is incorporated into the zwitterion in preference to the epoxy orientation of the ozonide cycle [103-112]. This observation is in accord with the classical concept of the Criegee mechanism whereas the Story mechanism supposes the preference of the peroxy-bridge orientation [100, 101]. Bauld, Bailey and co-workers [102] suggested reﬁnements for the Criegee mechanism in order to account for the strong and varied experimental facts on the ozone

183

Ozonation of Organic and Polymer Compounds reaction with oleﬁns. For this purpose, they accepted that: the PO exists in C-C half-chair conformation; the zwitterion (carbonyl oxide, CO) exists in syn and anti conﬁgurations depending on the geometry of the reagent and the experimental conditions; the preferred ozonide conformation is C-O half-chair; the addition of ozone to the C=C bond is 1,3-cis-stereoselective yielding a ﬁve-member ring - 1,2,3trioxolane; the concerted cleavage of PO gives zwitterion and carbonyl compounds, which via 1,3-bipolar cycloaddition results in ozonide formation in the cage, and in the solution volume, the products suggested previously by Criegee; and the reaction of the zwitterion and the carbonyl compound occurs stereoselectively depending on the zwitterion conﬁguration and the preferred conformation of the end ozonide. Thus, Bauld and Bailey formulated the following stereochemical rules: (1) equatorial (trans) substituents in PO are preferentially converted into anti, and axial (cis) substituents into syn carbonyl oxide, (2) the equatorial substituent in the PO is incorporated into a carbonyl oxide zwitterion in preference to an axial substituent, (3) aldehydes interact preferentially with anti-carbonyl oxides so as to orient bulky substituents into a,e-conformation, i.e., cis-ozonide formation, and with syn-carbonyl oxides so as to orient bulky substituents into a,a or e,e-conformation (trans). Thus Bailey succeeded in explaining the cis:trans ratios of normal and cross ozonides, particularly those obtained from oleﬁns with bulky substituents, and trans-oleﬁns yield trans-ozonides and cis-oleﬁns yield cis-oleﬁns, predominantly. These suggestions, however, are true only for oleﬁns with bulky substituents such as, iso-C3H7 or tert-C4H9, but not for oleﬁns with smaller substituents, i.e., CH3, C2H5 [113-116]. On the basis of microwave spectra Lattimer and co-workers [112] showed that ethylene, propylene and trans-2-buteneozonide have an oxygen-oxygen half-chair rather than carbon-oxygen chair conformation as suggested by the Bailey mechanism. This means that the e,e- and a,s-substituents are located in the trans-position and the a,e-substituents in the cis-position [117-123]. The studies on the conformation of PO, ozonide and zwitterions reveal that Bailey’s rule 2 is not always valid and should be reﬁned. It was shown that during the decomposition of cis-2-butene PO the anticarbonyl oxide yields cis-ozonide and the syn-carbonyl oxide trans-ozonide, regardless of the ozonide conformation. The small substituents - CH3, C2H5 and C3H7 in the PO of 1,2-disubstituted ethylenes are oriented in the e,e-position and the bulky ones, iso-C3H7 and tert-C4H9, in the a,a-position [124-129]. According to reference [112] the ozonolysis of oleﬁns takes place as a supra-supracycloaddition of 4n-2n-systems via a ﬁve-member ring formation. The cycloreversible decomposition of PO, depending on the conﬁguration, results in: (1) syn-carbonyl oxide with a-orientation of the R substituent, and (2) anti-carbonyl oxide in the case of e-orientation of the substituent. The stereochemistry of the oleﬁns ozonolysis is shown in Table 3.1.

184

Ozonolysis of alkenes in liquid phase

Table 3.1 Stereochemical course of ozonide formation and decomposition of PO after the mechanism of Bauld-Bailey (before the slash) and Kuczkowski (after the slash) Oleﬁn

PO, C-C half chair/ O-envelope

COX

OZ, O-O-half chair/-

OZ

e/e

anti/anti

e,a/-

cis/cis3

Trans1

a,a/e

syn/syn

a,a/-

trans/trans

2

e,a/e

anti/anti

e,a/-

cis/cis

-

-/trans

1-Alkene

Cis

Cis3

-/a 1

-/syn 2

3

Note: - bulky substituents; - bulky substituents; - small substituents.

Later, Criegee summarised the available data and modiﬁed the mechanism of oleﬁn ozonolysis taking account of the different conformations of the carbonyl oxide [125127]. Murray, Hagen and Bailey established that the cis:trans-ozonide ratios vary at: (1) addition of various complex-forming reagents such as toluene, o-xylene, isodurene, 1-mesithyl-1-phenylmethane, hexamethylbenzene, and (2) change of the heating rate during PO decomposition (Scheme 3.5) [128-130]: _ O

O C H

+

H C

CH3 H - bond

_O

O

+ C

R H

_O

O

+

H

C

R

CH3 anti-complex

syn-complex

Scheme 3.5 The more stable transition state of PO would give predominantly anticarbonyl oxide, being dependent on the size of the substituents. Thus, trans-1,2-di-isopropylethylene in its reaction with ozone in the presence of complexing agents, gives a cis:transozonide ratio >1. It has been found that this ratio remains the same in the absence of a complexation agent and at slow heating from (–155 oC) which, however, is against the stereochemical predictions. This fact could be explained by the occurrence of an equilibrium between the syn- and anti-isomers of CO, which takes place at high rate even at –155 oC passing through a carbonyl oxide formation (Scheme 3.6).

185

Ozonation of Organic and Polymer Compounds

_

O

O

O+

+O

O C+

C R

_

_

O

H

R

C H

R

H syn

anti

Scheme 3.6 This hinders the complexation between the CO and the complexing agent. These results have been observed during the study on the low-temperature ozonolysis of cis-oleﬁns. On the basis of conformational considerations, the anti-isomer of CO appears to be more stable than the syn-isomer although in some cases the latter form can be stabilised on account of H-bond formation (Scheme 3.5). On the other hand, it can be assumed that CO preferentially leads to anti-CO-complex formation. Its stability will increase with the increase of the substituent size and then the equilibrium syn-anti will be shifted to the latter. This will result in rise of the cis-ozonide yield as predicted by the third rule of Bailey [129]. On the basis of experimental and theoretical data [130-134], and applying the rule of least motion whereby the PO decomposition occurs via a minimum change of atom coordinates, Bailey [130] demonstrated that the CO conformation depends rather on the rate of PO decomposition than on the thermodynamic stability of the stereoisomers. On this basis he proposed some additional reﬁnements to the mechanism of oleﬁns ozonolysis. The cis-trans-ozonide ratio depends on: (1) the decomposition of the thermodynamically preferable PO conformer, (2) the decomposition of the kinetically favourable PO conformer, (3) the competition in regard to the second and probably the third rule of Bailey for oleﬁns with small substituents, and (4) the equilibrium of syn-/anti-CO conformers prior to their recombination with the carbonyl moiety. The slow heating of the e,e-conformer, which most likely has a lower activation energy of decomposition than the a,a-conformer, would produce preferentially anti-CO and the cis-ozonide in a smaller amount. In the case of rapid heating the decomposition would occur so quickly that it would not affect the equilibrium of the two conformers as it is shown below: anti-COX k e,e-PO j a,a-PO m syn-COX In references [129, 130] it is reported that rule 3 should be altered so as to conform to the O-O half-chair conformation of the end ozonide [111, 112, 134]:

186

Ozonolysis of alkenes in liquid phase Thus the revised rule 3 should be: upon the interaction of the aldehydes with anti-CO the bulky substituents are oriented equatorially-axially (e,a) producing cis-ozonide, while aldehydes react with syn-CO to orient them diaxially (a,a) producing transozonide. Bulky substituents in PO are oriented diaxially (a,a) resulting in trans-ozonide formation. The mechanism of ozone reaction with oleﬁns has been studied by means of thermochemical and quantum chemical methods in a series of contributions [135145]. Harding and Goddard [137] have found that the exothermicity of the reaction between ozone and ethylene amounts to 92-96 kcal/mol, while it is 101.7 kcal/mol according to reference [11]. Goddard states that the ozonolysis of cis-trans oleﬁns takes place through a biradical pathway (Scheme 3.7): .

R R

. C

R

+ O3

H

O

O

+ RCHO

syn (I)

R

. C

H

H . O

O

R (I)

R

O

C

R

. C

. O

O

trans

B

R

R

O

R

1,5-biradical

1,5-biradical A

R C

C

H

H

O

O

O

C

+ O3

. O

. C

H

O

+ RCHO

R anti R

H

. C

. O

O (II)

H

H

H

(II)

R . O

O O

C

O

C

. C

bulky R

O

1,5-biradical

1,5-biradical

C

D

(II)

C

small R

R C

C

R

R

O

O

R

cis

trans-ozonide

Scheme 3.7 In the transition states A, B, C and D, the orientation of the carbonyl oxygen adjacent to the carbon atom of the peroxymethylene allows the stabilisation of the lone electron pair on the oxygen p-orbital through exchange with the C-O bond. The arrows around the biradical C-O bond denote the most favourable orientations for rotation leading

187

Ozonation of Organic and Polymer Compounds to cyclisation (the bulky substituents should be kept away from their neighbours in their movement). The orbital phase consideration predicts supra-surface addition whereas the rotation around the biradical bond should be clockwise in the transition states B and D, and counter-clockwise in A and C. The pathway of ozonide formation from oleﬁns with bulky substituents (tert-butyl) is denoted by arrows. In the case of oleﬁns with small substituents, i.e., CH3, the transition state C might be assumed to follow the orbital permissible suprasurface addition even when the other transition states (A, B and D) follow the steric directions. In the light of the biradical mechanism the authors interpret the Baileys rules in the following manner: (1) an equilibrium between the syn- and anti-peroxymethylenes (not carbonyloxides) in the absence of a complexing agent, does not occur or take place to a very small extent, because of the high conversion barrier of 29 kcal/mol, (2) the complexing agents accelerate the rate of equilibration which leads to an increase of the anti-peroxymethylene concentration of the cis-ozonide, which is in contrast with the data from reference [131], (3) at higher temperature the cyclisation reaction is less stereoselective which would favour the formation of the more stable trans product, a fact which is experimentally conﬁrmed, and (4) the increase of the solvent polarity increases the lifetime of the 1,5-biradical species and thus reduces the stereoselectivity of the reaction. On the basis of quantum chemical calculations Cremer [126-129] proposed a revised rule for determination of the stereoselective pathways of oleﬁns ozonolysis (Table 3.2).

Table 3.2 Stereochemical courses of ozonide formation depending on the oleﬁn conformation at ozone 1,3-cycloaddition and 1,3-cycloconversion of PO to CO at early (x<0.5) and later (x>0.5) transition state Oleﬁn

Size of R1 & R2

COX x<0.5

COX x>0.5

Ozone x<0.5

Ozone x>0.5

cis-

small

anti-

syn-

cis-

trans-

trans

small

anti-

syn-

cis-

trans-

cis

bulk

anti-

anti-

cis-

cis-

cis

bulk

syn-

syn-

trans-

trans-

It should be noted that the studies discussed above and the conclusions drawn in them are based mainly on the analysis of the products obtained and on the activation energy calculations of the intermediate and stable products. However, the kinetic methods could also provide essential evidence for the elucidation of the mechanism of this reaction.

188

Ozonolysis of alkenes in liquid phase

3.1.2 Kinetics The reaction of ozone with oleﬁns usually proceeds with high rate and low activation energy at temperatures ranging from –100 oC to 300 oC [95, 96, 143, 146].

3.1.2.1 Gas phase Cadle and Schadt were the ﬁrst to determine the values of k for the ozonolysis of ethylene and hexene-2 in the gas phase [147]. Later, the ozonolysis kinetics of various oleﬁns has been studied by many investigators [146-161]. The kinetics have had second-order with k values (20 oC) in the range of 0.8-450 × 103 M-1.s-1 (Table 3.3).

Table 3.3 Arrhenius parameters for the ozone reaction with oleﬁns in the gas phase No.

Oleﬁn

k × 10-3 (20 oC) (M-1.s-1)

log A

Ea (kcal/mol)

1.

Ethene

1.6 p 0.2 1.6; 1.8; 0.8; 1

2.2; 6.2; 6.73; 5.5

4.2 p 0.4; 4.2; 2.6; 4.9

2.

Propene

7.1

5.46

3.9

3.

Butene-1

6.2; 3.9

6.2

1.7

4.

trans-Butene-2

29; 260; 13

5.55; 5.41; 6.93

0.2 p 0.3; 2.3; 1.1

5.

cis-Butene-2

200; 17

6.28

0.96

6.

iso-Butene

14 p 2

6.15; 6.28

2.8 p 0.4; 1.7

7.

2,3-Dimethylbutene

-

6.23

0.82

8.

Pentene-1

3.9; 3.2

-

-

9.

Hexene-1

4.5

-

-

10.

2-Methylbutene

450

-

-

11

Trimethylethene

12

-

-

12.

Cyclopentene

6.1; 4.6

-

-

13.

1,1-Dichloroethene

2.2

-

-

14.

1,2-Diﬂuoroethene

0.24

-

-

15.

Tetraﬂuoroethene

81

-

-

16.

Hexaﬂuoropropene

13

-

-

17.

Octaﬂuorobutene

1.1

-

-

189

Ozonation of Organic and Polymer Compounds The various investigators have reported similar values for k for the reaction of ozone with ethene, but the values of A differ more than four-fold [147, 148]. It appears that the values of A should be similar if one takes into account the structure of trans-butene-2 and iso-butene with a lower Ea for iso-butene. In fact, the values of A and Ea differ essentially and the magnitude of Ea for iso-butene is higher (Table 3.3). It has been found that the yield of ozonides, the major products in the liquid phase, is very low. The reaction mixture contains both lower and higher molecular compounds than the initial oleﬁn, such as hydrocarbons, acids and aldehydes, etc. These products are obtained from the consequent reactions of the C=C bond cleavage products [153, 154]. Compounds with mass numbers up to 5 times order of magnitude higher than the initial compounds have been identiﬁed by means of mass spectrometry (for butenes with a molecular mass >200) [154]. It has been reported that epoxides are the major products of halogenated oleﬁn ozonolysis [155]. The ozonolysis in the gas phase is accompanied by chemiluminescence, a fact which has found practical application for the production of analytical equipment for ozone analysis in the atmosphere. The light emission is due to the electronic transfer of various excited species such as: H2COX (1Aa) and OH (X2Pi; v a b9; A23+). In the reaction schemes outlined below the composition of the various products and the formation of the intermediate excited species are associated with the occurrence of radical steps, due to the breakdown of the O-O bond of the primary ozone [153] (Scheme 3.8) and further isomerisation of the bipolar ion (Scheme 3.9). Scheme 3.8

O O

O O

R C

C

H

R H

O R

R

C H

_ C+ O O

.

O C

H

O R

R-C-CHR H

*

C

. . R + CO2 + H RH + CO2

OH

Scheme 3.9

190

O OOH R

RCOOH

*

Ozonolysis of alkenes in liquid phase The stoichiometry of the oleﬁn-ozone reaction varies in the range from 1:1.4 to 1:2 depending on the reaction conditions. The stoichiometry is exactly 1:1 in the absence of side reactions. The kinetics of the ozonolysis in the gas phase is strongly dependent on the absence or presence of molecular oxygen [153, 156]. Many workers underline that the ozonolysis kinetics is much more complex in its absence, i.e., in an inert atmosphere. Thus the kinetics of the ozone reaction with tetraﬂuoroethylene is described by the following relationship (Equations 3.1-3.6): d[O3]/dt = k1[O3][C2F4] + k2 [O3]2.[C2F4],

(3.1)

logk1 = (8.2 (0.5) – [(9.5 ± 0.7)/2.3.RT]

(3.2)

logk2 = (14.6 (0.5) – [(10.1 ± 0.6)/2.3.RT

(3.3)

and the reaction of ozone with allene [150]: d[O3]/dt = k1[O3][C3H4] + k2 [C3H4].([O3]2/[O3]0)

(3.4)

logk1 = (6.0 (0.7) – [(5.5 ± 1)/2.3.RT]

(3.5)

logk2 = (6.9 (0.7) – [(6.2 ± 0.8)/2.3.RT]

(3.6)

The foregoing discussion shows that the analysis of the kinetic data in the gas phase requires particular attention. Thus one should bear in mind that probably some of the reported constants for the gas phase ozonolysis are actually products, summations or partial quotients of parallel reaction constants.

3.1.2.2 Liquid phase The study of the kinetics of oleﬁn ozonolysis in the liquid phase appears to be a more complicated task than in the gas phase because of the high reaction rate and more complex analysing equipment [145, 146]. The ﬁrst studies in this respect were devoted to determination only of the relative rates in regard to a standard oleﬁn-cyclohexene [162]. The ﬁrst values of k were reported in 1969-1970. It has been found that oleﬁn ozonolysis in the liquid phase follows also second-order kinetics [163-165]. We have established that the outlet ozone concentration is proportional to the change of the oleﬁn concentration during acrylic acid ozonolysis (Figure 3.1).

191

Ozonation of Organic and Polymer Compounds

1.6

[O3]0

CH3COOH

1.2

0.8

[O3]g 0.4

0.0

0

100

200

300

400

500

600

Time, s

Figure 3.1 Kinetics of ozone concentration at reactor outlet upon ozonolysis of 0.02 M acrylic acid in acetic acid medium at 200 oC. The arrows denote the points of stoichiometry and rate constant estimation

The outlet ozone concentration drops abruptly after the start of the reaction and it remains low, close to zero, almost up to the complete oleﬁn depletion (up to 4 minutes). Then it begins to rise rapidly and approaches the ozone concentration at the reactor inlet. The pattern of the kinetic curve depends on k - it becomes rectangular at high values of k and at low values of k the ozone concentration in the stationary region rises, the time necessary for reaching the inlet concentration becomes longer and the curve form is changed dramatically. A 1:1 stoichiometry has been found. After the addition of 1 mol ozone to 1 mol oleﬁn the reaction rate is reduced by a factor of about 6-8. This fact suggests that the bipolar ion and the carbonyl compounds hardly interact with ozone. The major part of the products (>95%) are generated via one single reaction pathway at low and moderate temperatures. The values of A are found to amount to 106 M-1.s-1 which are typical for simple liquid phase reactions. The methods for kinetic studies are based on determination of the consumption rate of one of the reagents. In the absence of intermediate equilibrium steps, the rate of the ﬁrst reaction step can be directly found as follows: The absence of a reverse reaction is testiﬁed by the following two facts: (1) the PO formation is an exothermic process –38 kcal/mol [143], and (2) the accumulation

192

Ozonolysis of alkenes in liquid phase of signiﬁcant amounts of PO at temperatures 110-120 oC. If an inert gas is then blown through the reaction mixture only the dissolved ozone is liberated. If a reverse reaction occurs: O O R H

C

O C

H

R-CH=CH-R + O3

R

at blowing through, the equilibrium will be shifted to the starting compounds and the ozone concentration will be equal to the whole amount absorbed by the system. In this case an oleﬁn rather than PO should be identiﬁed. Upon repeated ozonation the oleﬁn will absorb again an equimolar amount of ozone. The experiments, however, show that only PO is identiﬁed in the reaction mixture, practically no oleﬁn is found and ozone is not consumed which demonstrates the absence of a reverse reaction. The rate constants were measured: (1) through ozonation of a mixture composed of unknown oleﬁn and a reference, for example cylcohexene [163, 164] as discussed above, (2) through ozone bubbling through the oleﬁn solution in an inert solvent (in regard to ozone) and registration of the change of outlet ozone concentration [165], and (3) through fast mixing of ozone and oleﬁn solutions by means of stopped-ﬂow techniques [166]. All these methods provide similar results for the k values but the bubbling methods appears to be more convenient and experimentally available. We have successfully applied this method throughout our studies in all cases when it is possible. The rate constants of ozonolysis of some oleﬁns in liquid phase are presented in Table 3.4. The comparison of the data in Tables 3.3 and 3.4 reveals that, mostly, the rate constants measured in the liquid phase are an order of magnitude several times higher than those found in the gas phase which is in accordance with the collision theory [166]. The study of the rate dependence on the concentration of the reagents shows that in most cases it is ﬁrst order with respect to ozone and slightly greater than one in regard to oleﬁn. However, the kinetics approximate ﬁrst order with respect to oleﬁn at low or sufﬁciently low oleﬁn concentrations. The increase of the concentration results in an increase of the reaction rate more than that predicted by the second-order kinetics. This could be explained by the fact that the PO formation is preceded by the formation of an ozone-oleﬁn complex (Scheme 3.10).

193

Ozonation of Organic and Polymer Compounds

Table 3.4 Rate constants (k × 10-5 M-1.s-1) of oleﬁn ozonolysis in CCl4 at 20 oC No.

Oleﬁn

[153]*

[155]

[156]

1.

Ethene

-

-

0.4

2.

Butene-1

-

-

1.3

3.

trans-Butene-2

0.4-1.6

-

-

4.

cis-Butene-2

0.3-1.0

-

-

5.

Isobutene

0.2

0.97

-

6.

Pentene-1

2.4

-

5.0

7.

2-Methylbutene-2

8.

Cyclopentene

4.5

2.0

4.0

9.

Hexene-1

0.27

0.76

1.4

10.

Hexene-2

-

1.48

5.0

11.

Tert-methylethene

-

2.0

-

12.

Styrene

0.30

1.0

3.0

13.

p-Methylstyrene

0.31

-

-

14.

m-Methylstyrene

Note: *measured in CHCl3 with respect to cyclohexane.

O O R-CH=CH-R + O3

1 Olefin 3

R-CH

CH-R O3

2

R H

C

O C

H R

Scheme 3.10

This complex is thermodynamically stable and at collision with a new oleﬁn molecule is broken down to the initial products. Then the dependence of the observed rate constant (kobs) on the oleﬁn will be described as shown in Equations 3.7 and 3.8:

194

kobs = k1.k2/(k2 + k3 .[Oleﬁn])

(3.7)

1/kobs = 1/k1 + k3/(k1 .k2.[Oleﬁn])

(3.8)

Ozonolysis of alkenes in liquid phase Upon ozonolysis of acrylic acid the values of k1 and k3/k2 amount to 4.2 × 103 M-1.s-1 and 8 × 106 M-1.s-1, respectively. It should be pointed out that these values are higher than those reported in reference [163, 164] as the oleﬁn concentrations are lower at the points of rate constants determination (denoted by arrows). In this case the rate constant was calculated by applying the real oleﬁn concentration which is equal to the difference between the initial and the consumed concentration up to the moment of calculation. Generally, this difference is within the limits of the experimental error and thus the reported constants could be used for kinetic characteristics of the process. However, when possible we have used the values of k1. The temperature dependencies of the rate of hexene-1 and maleic anhydride ozonolysis, obtained by us, are presented in Table 3.5.

Table 3.5 Dependence on temperature of the rate constant k1 of the ozone reaction with some oleﬁns Hexene -1

17 oC

0 oC

–20 oC

–40 oC

–60 oC

–78 oC

E (kcal)

k1 (M-1.s-1)

1.4 × 105

1.5 × 105

1.2 × 105

0.7 × 105

1.0 × 105

0.6 × 105

0.5 p 0.5

Maleic anhydride

43 oC

35 oC

26 oC

17 oC

9 oC

E (kcal)

k1 (M-1.s-1)

68.3

61.5

36.1

25.8

18.4

6.1 p 0.5

As seen, the rate constant of hexene-1 does not change with temperature while the activation energy of maleic anhydride reaches the value of 6.1 kcal/mol. The dependence of the rate constant on the temperature for maleic anhydride, according to our data [32, 33], is described by the following expression (Equation 3.9): k1 = (1.1 ± 0.2).106.exp(-6100 ± 500/RT)

(3.9)

The approximate value of A for hexene-2 is 106-107 M-1.s-1. Generally the oleﬁn ozonolysis is characterised by very low values of Ea = 1-10 kcal/mol. However the inaccurate determination of the rate constants often does not allow the precise deﬁnition of the Arrhenius parameters which in turn could lead to misleading interpretation of the experimental results. The values of Ea are commensurable with a number of other energy factors, inﬂuencing the reactivity, which impedes the evaluation of their contribution [167]. Among them are the reorientation of the surrounding solvent, the thermal energy of the molecules, the temperature

195

Ozonation of Organic and Polymer Compounds dependence of the diffusion coefﬁcients, etc. Consequently, the dependence of the rate constant on temperature cannot be outlined as according to deﬁnition (Equation 3.10): Ea = E0 + 0.5 RT

(3.10)

where Eo is the height of the potential surface at -273 oC. The rate constants for oleﬁns with various structures are given in Table 3.6. The rate constants of the reaction of ozone with oleﬁns with terminal double bonds (hexene-1, octene-1 and decene-1) are high and similar. The isomeric oleﬁns with more internal double bonds (hexene-2 and methyloleate) react much more readily that those mentioned above. The rate of the ozone attack increases as the number of alkyl substituents on the double bond increases (hexene-1 and 2-methylpentene-2, acrylic acid and methacrylic acid, polybutadiene and polyisoprene). Cyclooleﬁns C5-C12 react with relatively similar rate constants but more slowly than those with an open chain (cyclohexene k1 = 4 × 105 and k1 for methyloleate = 1 × 106 M-1.s-1). The presence of phenyl groups results in acceleration of the reaction (styrene) while the introduction of second, third and fourth groups (stilbene, 1,1,4,4-tetraphenylbutadiene) has no further inﬂuence on the reaction rate. The oleﬁn ozonolysis can be regarded as an electrophilic 1.3-addition and similarly to the reactions of Prilezhaev and Diels-Alder which have close electronic multiplicity, one can expect that electron-accepting substituents in the molecule will decrease the reaction rate. Actually, Br, Cl, -COOH and NO2-substituted oleﬁns react much more slowly with ozone [32-35, 168-170]. The comparison of the rate constants allows disclosure of the effects depending on the structure of the oleﬁns under study. Thus, the values of k for high molecular compounds (polybutadiene, polyisoprene and squalene) are lower than those found for their low molecular analogues (methyloleate and tri-trans-cyclododecatriene and 2-methylpentene-2) [171, 172]. It has been also found that the reactivity of cis- and trans-isomers (maleic and fumaric acids, cylcohexene and tri-trans-cyclododecatriene1,5,9, gutta-percha and natural rubber) varies as the trans-isomers are more active in their reaction with ozone [141, 168, 169]. The rate constant of cis-diphenylethene is 8.9 × 104 M-1.s-1 while for trans-diphenylethene it is 1.8 × 105 M-1.s-1. Usually these facts are attributed to the occurrence of the compensation effect. Huisgen [173, 174], however, explained this fact by the repulsion of the cis-substituents in the transition state. The hybridisation of the carbon atoms of the double bond changes from sp2 to sp3 in the activated complex which results in overlapping of the van der Waals radii of the cis-substituents. Thus the rate of the ozone attack is retarded whereas overlapping is impossible in the case of trans-isomers and thus the reaction is faster. Similar differences in the reactivity of cis- and trans-isomers have been also observed in Prilezhaev reactions.

196

Ozonolysis of alkenes in liquid phase

Table 3.6 Rate constants of oleﬁn ozonolysis in CCl4 at 20 oC No.

Oleﬁn

k1 × 10-5 (M-1.s-1)

1.

Ethene

0.4

2.

Butene-1

1.3

3.

cis-Butene-2

1.6

4.

Isobutene

1.0

5.

1,3-Butadiene

0.74

6.

Pentene-1

1.4

7.

2-Me-Butene

5.0

8.

Hexene-1

1.4

9.

Hexene-2

5.0

10.

2-Me-Pentene-1

5.0

11.

Octene-1

1.3

12.

Octadecene-1

1.8

13.

Cyclopentene

4.0

14.

Cyclohexene

4.0

15.

Cyclodecene

4.0

16.

Tri-trans-cyclo-dodecatriene-1,5,9

3.5

17.

Norbornene

5.0

18.

Styrene

3.0

19.

Stilbene

1.8

20.

1,4-Diphenybutadiene

0.9

21.

1,1,4,4,-Tetraphenyl butadiene

0.8

22.

Acenaphthylene

2.2

23.

Acrylic acid

0.042

24.

Methacrylic acid

0.078

25

Methylmetacrylate

0.078

26.

Maleic acid

1.42.10-3

27.

Fumaric acid

0.084

28.

Maleic anhydride

3.6.10-4

29.

2,6-Dimethylhept-2-en-2-carbonic acid

0.026

30.

Oleic acid

10.0

31.

Methyloleate

10.0

32.

Vinylchloride*

0.022

33.

Allylchloride*

0.085

34.

cis-1,2-Dichloethane*

3.57 × 10-4

197

Ozonation of Organic and Polymer Compounds

35.

Vinylidene chloride*

2.11 × 10-4

36.

Trichloroethene* Tetrachloroethene*

3.6 × 10-5 1.0 × 10-5

37.

2-Bromopropene

0.028

38.

Acrylonitrile

1.0 × 10-3

39.

Tetracyanoethene

0.4 × 10-5

40.

1,1,2-Triphenyl-2,2-dinitroethene

3.6 × 10-5

41.

1,1-Diphenyl-2,2-dinitroethene

0.7 × 10-5

42.

Polybutadiene

0.6

43.

Squalene

7.5

44.

Natural rubber

4.4

45.

Gutta percha

1.7

46.

Polychloroprene

0.042

Note: *the values are taken from [154, 158].

The small sizes of the ozone molecule predetermine the insigniﬁcant role of steric effects in the reaction. Only one of the examples in Table 3.6 shows an anomaly in the rate which could be ascribed to steric inﬂuence (k of acrylic, methacrylic and 2,6-dimethyl-hept-2-ene-2-carboxylic acids are 2.3 × 103, 7.8 × 103 and 2.8 × 103 M-1.s-1, respectively). If one takes into account only the inductive effects of the substituents, then the rate constant of 2,6-dimethyl-hept-2-2-carboxylic acid would be 7.8 × 103 M-1.s-1. In fact the latter is 3 times greater than the experimental value which demonstrates the effect of steric factors on the reactivity. The rate of oleﬁn ozonolysis depends signiﬁcantly on the oleﬁn structure (k for oleic acid is six-fold higher than that for tetracyanoethylene) (Table 3.6). The quantitative relationship between the electronic and geometric structure of oleﬁns and their reactivity is a very important stage in studying ozone chemistry. The ﬁrst attempts in this respect were the studies of Cvetanovic and co-workers [163, 164] who found a good linear relationship between log of the rate constant and the ionisation potential of chlorinated ethenes. It has been found that the linear dependence of the reactivity on the free energy change in the system is valid for many organic reactions. The Hammett relationship is well known and widely applied [170] (Equation 3.11):

lgki /k0 = R.S

198

(3.11)

Ozonolysis of alkenes in liquid phase where kj are the rate constants of the compounds studied; k0 is the rate constant of the reference; R is coefﬁcient accounting for the reaction character; S is inductive constant of the substituents. Some other relationships, like that of Taft, have also been used for evaluation of the contributions of inductive, steric and mesomeric effects of various substituents on the rate constant. A dependence of Hammett type has been found upon decomposition of PO and ozonides [171, 172, 175]. It is quite natural that a similar relationship might be observed during ozonolysis of oleﬁns.

Table 3.7 Logarithms of the relative values of the rate constants and the Hammett inductive constants (σpara) of the substituents on the double bonds No.

Oleﬁn

Substituent

3Spara

log ki /k0

1.

Methyloleate

2CnH2n+2

–0.302

1.40

2.

2-Me-pentene-2

2CH3, C2H5

–0.491

1.097

3.

Hexene-2

CH3, C3H7

–0.321

1.097

4.

Styrene

Ph

–0.01

0.875

5.

Styibene

2Ph

–0.02

0.65

6.

Hexene-1

C4H9

–0.151

0.544

7.

Octadecene-1

C16H33

–0.151

0.65

8.

Ethene

H

0

0

9.

Methylmethacrylate

CH3, COOCH3

0.22

–0.7

10.

Methacrylate acid

CH3, COOH

0.28

–0.71

11.

3-Chloropropene-1

CH2Cl

0.18

–0.67

12.

2-Bromopropene

CH3, Br

0.062

–1.15

13.

Acrylic acid

COOH

0.45

–0.98

14.

Acrylonitryle

CN

0.66

–2.6

15.

Maleic acid

2COOH

0.9

–2.45

16.

Fumaric acid

2COOH

0.9

–1.68

17.

1,1,2-Triphenyl-2nitroethene

3Ph, NO2

0.748

–4.04

199

Ozonation of Organic and Polymer Compounds The rate constant of the reaction of ozone with ethene, equal to 4 × 104 M-1.s-1, is accepted as a standard in studying the kinetics of ozonolysis of various oleﬁns. It is calculated on the basis of the k value in the gas phase multiplied by a coefﬁcient of 25 which accounts for the heat of dissolution, 1.9 kcal/mol. The Hammett relationship in Figure 3.2 is plotted on the basis of the data of Table 3.7. As seen the data fall on a straight line (Figure 3.2) thus indicating a linear dependence.

3

= -3.2 R = 0.89883

2 1

2

1 3

7 6

0

4 5 8 10

-1

9 11

13

12 16

-2

-4 -0.8

15

14

-3

17 -0.4

0.0

0.4

0.8

para

Figure 3.2 Hammett relationship of ozone reaction with oleﬁns

The negative value of R = –3.2 indicates the electrophilic nature of the ozone reaction with oleﬁns. The low value of the correlation coefﬁcient R = 0.89883 could attain much higher values if the steric effects of the substituents and the electrostatic interactions are considered. Regardless of this the dependence in Figure 3.2 is quite acceptable for evaluating the rate constants of reactions with new oleﬁns. Another very important question related to the reactivity of oleﬁns, which has not been sufﬁciently studied, is the relationship between the reactivity of C=C bonds and the deformation and steric factors responsible for the oleﬁn conformation. These factors depend on the inter, intra, supra, micro and macro effects. Thus the intramolecular strains arising in the cyclic oleﬁns are due to the deformation of valent, dihedral angles, interatomic distances and transanular interactions [175-183]. The values of the different deformational and steric components responsible for cyclohexene conformation, as calculated by us, are presented in Figure 3.3.

200

Ozonolysis of alkenes in liquid phase

Energy, kcal 60

50

40

30 MMXE 20 SE 10

TOR

0

BND

2

4

6

8

10

12

Cycloolefins

Figure 3.3 The values of C2-C12 cylcooleﬁn energies calculated according to the molecular mechanics theory. MMXE - the Alinger energy; SE - the strain energy; TOR - the torsion energy and BND - the bond energy

It is seen that the dependencies have speciﬁc character with a minimum in case of cyclohexene. Among the oleﬁns, given in Table 3.6, there are a number of oleﬁns with highly strained molecules, such as cyclododecene, cyclododecatriene, norbornene, ethylidenenorbornene, acenaphthylene and 2,6-dimethylhept-2-2-carbonic acid. It has been found that these oleﬁns react with diazomethane in contrast to those with strainfree molecules. The reduction of strain in the sequence of C4, C5 and C6 cyclooleﬁns correlates with the reduction of the corresponding yield of pyrazolines [183]. The reactivity and size relations of cyclo-oleﬁns are demonstrated in Figure 3.4. It is seen that cyclohexene, the oleﬁn with the lowest strain energy of the initial molecule, exerts the lowest reactivity towards ozone. Simultaneously, its activated complex (AC) has the highest conformation energy. Thus Ea - the difference between the energy of the starting oleﬁn and that of the AC - would be the highest which accounts for the lower reactivity of cyclohexene with respect to the other cyclooleﬁns [1]. Oleﬁns may contain several double bonds separated by three simple C-C bonds. In these cases the C=C bonds react as independent kinetic units. The ozone addition to one of them hardly affects the neighbouring ones. The substituent inductive effect is reduced by a factor of about 2.7 through each C-C bond and thus its inﬂuence on the other carbon atoms is signiﬁcantly reduced [183, 184].

201

Ozonation of Organic and Polymer Compounds

10 SEn/SE6

Relat. values

8 6 4 kn/k6 2 0 4

6

8

10

12

Cycloolefin

Figure 3.4 Dependency of the relative rate constants and strain energies of cyclooleﬁns on the cycle size

Upon ozonolysis of compounds with conjugated bonds (1,3-diene, p-divinylbenzenes, etc.), the different reactivity is determined by the structure of the starting diene, the nature of the oleﬁn after the ﬁrst ozone addition and the effect of the ozonide inductive factor on the second C=C bond. This is illustrated by the ozonolysis of divinylbenzene (Scheme 3.11).

Scheme 3.11 The proceeding of the reaction in two steps results in the appearance of two stationary regions on the curve [O3]g = f(T) thus allowing the determination of the two rate constants k and ka (Table 3.8).

202

Ozonolysis of alkenes in liquid phase

Table 3.8 Rate constants of ozone reaction with dienes and trienes in CCl4 at 20 oC k × 10-5 (M-1.s-1)

ka × 10-5 (M-1.s-1)

Divinylbenzene

2.0

0.2

Ethylidenenorbornene

3.6

2.0

Vinylnorbornene

3.3

0.8

Vinylcyclohexene

1.4

0.8

Tetrahydroindene

1.4

1.4

Cyclododecatriene-1,5,9*

3.5

3.5 (3.5)

Compounds

Note: *The third double bond reacts with the same rate as the second.

In the example of divinylbenzene the change in the rate constants after ozone addition could be followed. Its correlation with the general principles of the theory can be also evaluated. The rate constant of the ozone reaction with the ﬁrst C=C bond should be slightly smaller than that of styrene because of the higher conjugation in divinylbenzene. After the ozone attack on the ﬁrst C=C bond the conjugation degree is reduced and ozone interaction with the second C=C bond should take place with a rate similar to that of styrene (3 × 105 M-1.s-1). However, taking into account the electron-acceptor effect of the ozonide cycle, the rate of the second step should be smaller than that of the ﬁrst step, which has been also conﬁrmed by experimental data. The comparison of k for ethylidene- and vinylnorbornene, allows the assumption that ozone attacks ﬁrstly the double bond in the side chain, as the changes in its structure affect the value of k while the values of ka are equal for the two isomers. The C=C bonds in cyclododecatriene-1,5,9 are equivalent due to the long distance between them and the equal conﬁguration of the neighbouring environment. This is demonstrated by the equal rate constants for the three bonds and by the value of the hydrogenation rate of cyclododecatriene-1,5,9 on palladium [185].

3.1.2.2.1 Effect of solvent It is known that the rate of many reactions depends strongly on the solvent polarity [181, 182]. There are a number of examples whereby the reaction mechanism changes with solvent variation and the ozonolysis of oleﬁns is among them. It has been found that alkoxy- and aryloxy-hydroperoxides [186, 187] instead of ozonides, are the major ozonolysis products of oleﬁn ozonolysis in solvents such as alcohols, acids and other compounds.

203

Ozonation of Organic and Polymer Compounds In order to assess the inﬂuence of the solvent on the rate constant we have followed the kinetics of acrylic acid ozonolysis in various solvents. The choice of acrylic acid was mainly for two reasons: (1) its good solubility in the solvents used, and (2) its relatively low rate of interaction with ozone which simpliﬁes the registration of the reaction progress. The following solvents which were used for this study - CCl4, decane, acetic acid and water - differ essentially in regard to their dielectric constants (ε) and their ability to dissolve ozone. Decane and CCl4 are inert towards the intermediate and end products but acetic acid and water react with the dipolar ion. We have found that the reaction obeys ﬁrst-order kinetics with respect to each reagent.

Table 3.9 Rate constants of acrylic acid ozonolysis in various solvents at 20 o C Solvent

Dielectric constant (ε)

Henry’s constant (α)

k × 10-3 (M-1.s-1)

CCl4

2.24

1.8

2.9

n-Decane

2.05

1.5

2.8

99

6.2

1.70

2.6

84

18.2

1.26

2.6

67

30.9

0.95

2.6

50

43.6

0.77

2.6

16

69.0

0.65

2.8

100

80.2

0.42

2.7

Acetic acid/H2O (%)

The data in Table 3.9 show that the value of k does not depend signiﬁcantly on the solvent. Thus two basic conclusions emerge: (1) the solvent does not affect the ﬁrst step of the reaction or, if participates, it is in fact during some of the next reaction steps, as: RHC+OO- + RaCOXOH (RH(HOO)C-O(O)CRa RHC+OC(OO-)CHR + RaC(O)OH (RHC[O(O)CRa]-O-C(OOH)HR

and (2) formation of polar compounds or polar transition states [188-192] is not observed:

204

Ozonolysis of alkenes in liquid phase

C

O

C

O+

C

O

+C

_O

C

_ O O

O C

O +

O_

The polar intermediate compounds should solvate or react with ‘the active solvents’. This should change the k value with the increase of the solvent polarity, and lead to the formation of reaction products with C-C bonds instead of C=C bonds. However, similar compounds have not been experimentally identiﬁed. The constancy of k values in various solvents testiﬁes that the reaction occurs without formation of kinetically active polar compounds due to the high rate of electron delocalisation in the AC. The latter is much slower than the relaxation time of the solvent shell (^10-13 s).

3.1.2.2.2 Effect of conﬁguration We have studied the ozonolysis of structural derivatives of one and the same chemical compound: butane dicarbonic acid and its anhydride, maleic anhydride (I) cyclic form, maleic acid (II) cis-form and fumaric acid (III) trans-form: CH

HC

COOH

HOOC C

O=C

C=O

H

C

II

I

C

H

H

O

H

HOOC

C

COOH III

These compounds, similarly to acrylic acid, react relatively slowly with ozone which makes them suitable objects for investigating the reaction kinetics [32, 33]. The concentrations of the reagents vary from 6 × 10-5 to 0.9 M. Water, glacial acetic acid and chloroform were used as solvent media. The products of the ozone reaction with the double bond in the ethers of maleic and fumaric acids have been identiﬁed using chemical methods and infrared (IR) spectroscopy. The low values of the rate constants suggest that at deﬁnite conditions ozone may also attack the C-H bond (1b) in addition to the basic interaction with the C=C bond (1a): HOOC

HOOC

H

H

+ O3

O3

C HOOC

èëè

C H

HOOC 1a

O3

C

C H

H

HOOC

C

C

HOOC

H 1b

205

Ozonation of Organic and Polymer Compounds In this connection we have determined the reaction stoichiometry and analysed the IR spectra of the solutions during the reaction course. The stoichiometry we found to be 1:1. The iodometrically estimated content of active oxygen in the samples corresponds to the amount of absorbed ozone. The progress of the reaction was accompanied by an increase in the intensity of the C-O bands in the range of 110-1200 cm-1 and narrowness of the C=O bands in the range of 1710-1850 cm-1. However, at high conversion degrees the C=O bands are moved from their initial range due to the inductive effect of the ozonide cycle. With the increase of ozonation duration the bands of the C=C valent vibrations in the 1610-1680 cm-1 range decrease their intensity and an abrupt decrease of the rate of ozone uptake is observed. At complete consumption of C=C bonds the ozone concentration at the reactor outlet becomes equal to that at the reactor inlet. This fact suggests that ozone interacts mainly with the C=C bonds, while its consumption in parallel and consecutive steps is very low. This occurs regardless of the aldehyde formed in ozonolysis, which may also react with ozone. On the other hand the band intensity of the C-H and H-O valent vibrations at 3020 cm-1 and 3550 cm-1, respectively, do not change signiﬁcantly in the course of the ozonation. These experimental observations conﬁrm the assumption that ozone does not attack the C-H bonds at the C=C bonds. This is also shown by the low k value for butandicarbonic acid (1 × 10-3 M-1.s-1) at room temperature. Briener and coworkers [193-195] also conﬁrmed the absence of ozone interaction with the hydrogen of the OH group. Upon ozonation of the aforementioned acids and their esters they obtained a similar product composition. Consequently ozone reacts predominantly with the C=C bonds of I, II and III according to pathway 1a. It has been found that the rate of the reaction and the product compositions do not vary substantially with the solvents used, i.e., water (= 80.1), acetic acid (= 6.2) or chloroform (= 10). However, it should be noted that because of the lower solubility of fumaric acid in chloroform most of the comparative kinetic studies were conducted in acetic acid medium (Table 3.10).

Table 3.10 Kinetic parameters of the ozone reaction with maleic anhydride, maleic and fumaric acids in acetic acid at 2-6 × 10-6 M ozone concentration Substrate

k1 (M-1.s-1)

k3/k2 (M-1)

log A1

E1 p 0.5 (kcal/mol)

I

26 (17.30); 36 (25.80C); 51 (35.00C); 64 (42.50C)

63 (17.30C)

6.30

6.5

II

114 (17.00C); 142 (23.00C); 162 (35.00C); 232 (42.00C)

4560 (17.00C)

5.50

4.6

III

765 (17.00C); 840 (22.00C); 919 (28.50C);1050 (42.50C)

1000 (22.00C)

5.13

3.0

206

Ozonolysis of alkenes in liquid phase We established that at relatively low reagent concentrations the rate of ozone absorption is not proportional to the reagent concentrations although the amount of absorbed ozone is equal to that of the oleﬁn consumer. The reaction exhibits ﬁrst order with respect to ozone and the dependence on the oleﬁn concentration is more complicated. The rate constant decreases with the increase of oleﬁn concentration. The reciprocal magnitude of the observed rate constant depends linearly on the oleﬁn concentration (Figure 3.5).

0.06 0

17.3 C 0.05

0.04

0

25.8 C

0.03

0

42.5 C

0.02

0.01

0.00 0

1

2

3

4

5

3

[I].10 , M

Figure 3.5 Dependence of the reciprocal value of the rate constant on the concentration of maleic anhydride at various temperatures in acetic acid and [O3] = 5 × 10-6 M

These results indicate the occurrence of a side non-efﬁcient reaction, in which the oleﬁn is consumed via a reaction with an intermediate product formed between the ozone and oleﬁn (Scheme 3.12): Oleﬁn + O3 m P1

(1)

P1 m Products

(2)

P1 + Oleﬁn m 2Oleﬁn + O3

(3)

Scheme 3.12

207

Ozonation of Organic and Polymer Compounds On the basis of the kinetic analysis of Scheme 3.12 an expression for the reciprocal value of the observed rate constant, similar to Equation 3.8, is derived: 1/kobs = 1/k1 + (k1.k3/k2) [Oleﬁn]

(3.12)

Actually Figure 3.5 illustrates graphically Equation 3.12. The values of the rate constants as determined on the basis of Scheme 3.12 are given in Table 3.10. The analysis of the data in Table 3.10 reveals two signiﬁcant facts: (1) the high values of k3/k2 ratios, and (2) the increase in the rate constant k1 in the sequence: maleic anhydride (cyclic form)<maleic acid (cis-isomer)
>C=C< + O3 m Products Scheme 3.13 However the literature provides data that the oleﬁns ozonolysis probably takes place via the formation of P-complex according to Scheme 3.14: >C=C< + O3 m P-Complex m Products Scheme 3.14

208

Ozonolysis of alkenes in liquid phase UÊ ««V>ÌÊvÊ , In order to propose the most probable mechanism of the ozone reaction with I, II and II we have applied the method based on comparison of the calculated values of the pre-exponential A according to the activated complex method with different forms of the activated complex in the transition state with those obtained experimentally using two forms of TS: R2C

CR2 O

O

R2C

CR2

O

O O

O LC

CC

The calculated and experimental values for A are presented in Table 3.11. It is seen that the values of A calculated with CC of the activated complex are lower than those with LC. The values of A calculated on the assumption that the energy of free rotation is zero are given in column 4 and those taking account of this energy are placed in column 5. Compared to Aexp, the values of A-CC are lower by about 1-2 orders. The conclusion based on these facts is that the transition state cannot have a cyclic form as these values of A are the highest possible predicted by the theory with this form of the activated complex. The values of A calculated with the linear form of the activated complex are similar to those obtained experimentally. Therefore, it can be assumed that the reaction proceeds via the formation of a P-complex according to Equation 3.13: kobs = k1.k2/(k-1 + k2)

(3.13)

at k2 >>k-1 and kobs = k1 The assumption that k2>>k-1 is supported by the good ﬁt between the calculated value for A of the right reaction with that obtained experimentally for I, II and III ozonolysis. This is also conﬁrmed by the absence of any additional bands in the IR spectra of the ozonised solutions of I, II and II in methylene chloride registered for 48 h at –80 oC.

3.1.2.2.3 Effect of structure For detailed analysis of the effect of the chemical structure of oleﬁns on their reactivity, we have synthesised a number of substituted cyanostilbenes:

209

Ozonation of Organic and Polymer Compounds H C R1

C C

R2

N

Earlier, these compounds were investigated as phosphorescent and colouring agents [196, 197]. Their interaction with ozone, however, has not received considerable attention although their speciﬁc electronic structure provides a unique possibility for assessing the effect of such an important chemical factor on the reaction ability. Some characteristics of the synthesised and investigated cyanostilbenes are presented in Table 3.12 [35, 36].

Table 3.11 Arrhenius parameters for the ozonolysis of I, II and III Compound

kobs (20 oC)

log ACC

(M-1.s-1)

log ALC at Efr= 0

log ALC, at real Efr

log Aexp

Ea (kcal/ mol)

Maleic anhydride

29.1

4.48

7.48

6.30 (1.60)

6.30

6.50

Maleic acid

128

4.42

7.42

5.50 (2.60)

5.50

4.60

Fumaric acid

810

4.33

7.33

5.13 (3.00)

5.13

3.00

Numbers in brackets are the estimated energies of free rotation in kcal/mol.

Table 3.12 Kinetic and physical constants of synthesised and studied cyanostilbenes in CCl4 at 20 oC No.

Cyanostilbene

k × 10-5

mp

(M-1.s-1)

(oC)

log k

3S

1.

2-Phenyl-3-phenyl

1.3

5.114

0

2.

2-Tolyl-3-(4-chlorophenyl)

1.3

5.114

0.687

3.

2-(4-Chlorophenyl)-3-tolyl

1.2

5.079

0.687

4.

2-(3,4-Dimethoxyphenyl)-3nitrophenyl

-

-

0.625

5.

2-(4-Nitrophenyl)-3-(4chlorophenyl)

0.6

4.778

1.005

6.

2-(4-Nitrophenyl)-3-(4nitrophenyl)

-

-

1.556

210

Ozonolysis of alkenes in liquid phase

7.

2-(4-Nitrophenyl)-3-(4dimethylaminophenyl)

1.6

5.204

–0.052

8.

2-(4-Aminophenyl)-3-(4methoxyphenyl)

3.0

5.477

–0.928

9.

2-(4-Nitrophenyl)-3-(4methoxyphenyl)

1.1

5.041

0.510

10.

2-(4-Methoxyphenyl)-3-(4methoxyphenyl)

2.6

5.415

–0.536

11.

2-(4-Aminophenyl)-3-(4methoxyphenyl)

0.5

4.699

0.608

12.

2-(4-Nitrophenyl)-3-(3methylphenyl)

2.4

5.380

–0.091

13.

2-(3,4-Dimethoxyphenyl)-3-(3ﬂuorphenyl)

-

-

0.074

14.

2-(3,4-Dimethoxyphenyl)-3-(4chlorophenyl)

1.7

5.230

–0.170

15.

2-(4-Cyanophenyl)-3-(4dimethylaminophenyl)

5.9

5.771

–0.200

16.

2-Tolyl-3-(4-aminophenyl)

1.4

184

5.146

0.470

17.

2-(3,4-Dimethoxyphenyl)-3-(3,4dibromphenyl)

-

107

-

–0.163

18.

2-(3,4-Dimethoxyphenyl)-3-(1naphthyl)

-

-

-

0.768

19.

2-Phenyl-3-(4dimethylaminophenyl)

-

-

-

–0.840

20.

2-(3,4-Dimethoxyphenyl)-3piperonyl

-

145

-

-

21.

2-(3,4-Dimethoxyphenyl)-3-(3,4dimethoxyphenyl)

-

-

-

–0.306

22.

2-(3,4-Dimethoxyphenyl)-3aminophenyl

-

85

-

–0.813

23.

2-(4-Nitrophenyl)-3-phenyl

-

-

-

0.768

24.

2-(3,4-Dimethoxyphenyl)-3-(2methoxy-5- bromophenyl)

-

175

-

–0.156

25.

2-(3,4-Dimethoxyphenyl)-3-(3ﬂuorophenyl)

-

-

-

0.184

Note: The constants are measured only for tetrachloromethane-soluble compounds.

211

Ozonation of Organic and Polymer Compounds The values of the melting points are given only for the newly synthesised compounds. The concentration of cyanostilbenes in the kinetic experiments amounts to 1.33 mM. For determining the rate constants upon a high reaction rate we have applied the absorption theory which is enhanced by the proceeding of a ﬁrst-order chemical reaction (Equation 3.14) [34]: R0 = [O3]*(k1.D)1/2

(3.14)

where R0 is the absorption rate at bubbling liquid in stationary conditions, mol/(cm3); [O3]* = A.[O3]g is the equilibrium ozone concentration on the liquid-gas surface (mol/ cm3), where A = 2 is Henry’s coefﬁcient at 20 oC; D = 7.8 × 10-8(KMB)1/2.T, (MB.VA0.6)-1 = 3.06 ×10-5 cm2/s - is the coefﬁcient of ozone diffusion (A) in CCl4 (B), calculated by the Wilke and Chang equation [198]; K = 1 is the coefﬁcient of association; MB is the molecular weight of CCl4, g; MB = 1.2 x 10-3 Pa-s which is the viscosity of B; VA = 30 cm3/g, mol the molecular volume and k1 is the rate constant. Equation 3.14 is true at high accelerations of the physical absorption resulting from the chemical reaction at (D × k1)1/2/kL>>1. At known values for D = 3.06 × 10-5 cm2/s, kL z 10-2 cm/s and k1 = 106 × 10-3 = 103 s-1, the condition (D × k1)1/2/kL>>1 is fulﬁlled since the left side of the inequality has a value of 15. The ozone reaction with the cyanostilbenes is a bimolecular process as the rate of the reaction increases linearly with increasing concentrations of both reagents. Consequently ozonolysis is ﬁrst order with respect to each of the reagents. The quantitative analysis of the band at 1152 cm-1, which is responsible for the ozonide formation, reveals that its accumulation rate is almost equal to the rate of cyanostilbene consumption. It has been found that 4,4a-dinitro-cyanostilbene ozonolysis yields 4-nitrobenzenecyanate and 4-nitro-benzaldehyde as the major products, while the ozonide amount is very low. The scheme of the reaction is given in Scheme 3.15. O H O2N

O

+O3

C

NO2

C

O2N

CN

_ O O O2N

+ C H

+

C

C

H

CN

O2N O

C

NO2

CHO

NO2

CN

Scheme 3.15 212

O

O2N

CNCO

Ozonolysis of alkenes in liquid phase In view of the concepts on the PO decomposition mechanism, with electron-donating substituents, a zwitterion at the carbon atom with a CN group could not be formed due to its destabilising effect. The conjugation between the carbonyl compound formed at this carbon atom and the CN and Ph groups prevents ozonide formation via a zwitterion. For this reason the zwitterion leaves the cage and then it interacts with another zwitterion in the solution volume liberating oxygen. This reaction results from the strong electron-accepting action of the nitro groups in the para-position and provides an example of anomalous ozonolysis. Generally, the cyanostilbenes studied obey the Hammett’s relationship and give a straight line with R = –0.4 (Figure 3.6). This value, although substantially lower than that found for substituents directly attached to the double bond, shows clearly the occurrence of conjugation leading to re-distribution of the substituents inductive effects.

5.8

= -0.4 5.6 8

10

12

5.4 14

5.2

16

7

9 5.0 5

4.8 4.6

11

-1.0

-0.5

0.0

0.5

1.0

Figure 3.6 Dependence of the rate constants on the Hammett inductive constants

The small difference in the rates depending on the inductive constants can be attributed to the low A values, most probably, because of the low activation entropy.

213

Ozonation of Organic and Polymer Compounds The cyanostilbenes studied have substituents in the both benzene rings (A and B). This could be regarded as the reason for the neutralisation of the substituents effect, which contributes to the low value of R. The electronic structure of cyanostilbene is shown as:

The values of the charges are determined by quantum-chemical calculations with AM1 Hamiltonian (MOPAC6). Aiming at more accurate assessment of the substituents effect on the reaction ability, we have synthesised 13 new compounds substituted only in the A- and 14 in the B-benzene ring. The rate constants of the ozone reaction with all these compounds have been evaluated and are outlined in Table 3.13. The correlation of these constants with the Hammett constants (Figure 3.7a, b) gives the following values of R = –0.266 and –0.286, for substitution in the A- and B-benzene ring, respectively. The correlation coefﬁcients are not higher than 0.8. It is evident that the two coefﬁcients (are almost equal since the difference of about 7% is within the limits of the experimental error. Thus the reactivity actually does not depend on the substitution position. The similar slopes of the kinetic curves suggest that the role of the CN group in the re-distribution of the electron density and its effect on the reaction ability of these compounds cannot be differentiated in this reaction.

214

Ozonolysis of alkenes in liquid phase

Table 3.13 Rate constants of cyanostilbene ozonolysis in CCl4 at 20 oC No.

A-Substituents/S

k × 10-5 (M-1.s-1)

1.

p-NH2/-0.66

4.13

2.

m,p-CH3O/–0.153

4.13

3.

m-CH3O/0.115

2.82

4.

m-NO2/0.710

4.36

5.

p-F/0.062

2.08

6.

p-CH3/–0.170

3.06

7.

p-Cl/0.227

2.08

8.

p-CN/0.660

2.08

9.

Naphthyl/–0.01

1.49

10.

Pyrene/–0.01

0.56

11.

Biphenyl/–0.01

2.22

12.

Stilbene

1.80

13.

Cyanostilbene

1.30

14.

p-Br/0.232

1.90

15.

p-Cl/0.227

2.09

16.

p-NHCOXCH3/0.00

2.14

17.

p-NO2/0.778

1.39

18.

m-NO2/0.710

1.50

19.

m-Cl/0.373

1.80

20.

m-CH3O/0.115

1.56

21.

m-Br/0.391

1.56

22.

p-CH3O/–0.268

2.12

23.

p-CH3/–0170

2.60

24.

Pyridine

2.01

25.

Naphthyl

3.05

26.

m,p-CH3O/–0.153

3.13

27.

m-OC2H5,p-CH3O/–0.258

3.08

215

Ozonation of Organic and Polymer Compounds

1

2

5.6

= - 0.266 6

lgk

3 5.4

13 5

5.2 -0.8

-0.6

-0.4

-0.2

0.0

8

7

0.2

0.4

0.6

0.8

5.6

27

26

25 = - 0.286

23 5.4 16

lgk

22

15

24 14

19 21

20

5.2

18 17

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Figure 3.7 Dependence of the rate constants on the Hammett inductive constants

3.2 Polydienes The interest in the ozone reaction with polydienes is closely related to the problem of ozone degradation of rubber products. The reaction kinetics is found to be strongly dependent on the polymeric state, a question which has attracted a special interest in recent years [199]. Flory [200] showed that the reactivity of the functional groups in the polymer molecule does not depend on its length. It is also known that some reactions of the polymers proceed much more slowly compared with their low molecular analogues (catalytic hydrogenation). At the same time many fermentative reactions with polymers take place rapidly while similar reactions with low molecular compounds do not occur

216

Ozonolysis of alkenes in liquid phase at all [201]. The folded or unfolded form of the macromolecules provides various conditions for contact of the reagents with the reacting groups [202]. By using the modiﬁed version of this principle [203] it was possible to explain the proceeding of reactions without speciﬁc interactions between the adjacent C=C bonds and the absence of diffusion limitations. The study of the mass-molecular distribution (MMD) is in fact a very sensitive method for establishing the correlation between molecular weight and reactivity. The theory predicts that the properties of the system, polymersolvent can be described by the parameter of globe swelling (G) which deﬁnes the free energy (F) of the system and thus the rate constant of the reaction. For a reversible reaction, i.e., polymerisation-depolymerisation, the dependence of the rate constant of the chain length increase on the molecular weight is expressed by Equation 3.15: ln kpj/kpd= –const.(5G - 3/G).(dG/dM).M0

(3.15)

where M0 is the molecular weight of the studied sample and kpd is the rate constant for inﬁnitely long macromolecules. A good correlation between the theoretical and experimental data for polystyrene solutions in benzene was found in reference [204]. The study of polymer degradation is complicated by the structural peculiarities at the molecular and supramolecular level and by diffusion reasons. It is difﬁcult to ﬁnd simple model reactions for clariﬁcation of the particular properties and for the express examination of the proposed assumptions. An exception in this respect is the ozone reaction with C=C bonds whose mechanism has been intensively studied and could be successfully applied upon ozonolysis of polymeric materials [178]. This question will be discussed in detail at the end of this Chapter and in Chapter 4. Table 3.14 summarises the rate constants of the ozone reaction with some conventional elastomers and polymers and their low molecular analogues, synthesised by us. It is seen that the reactivity of elastomers and polymers and their corresponding low molecular analogues, as demonstrated by their rate constants, are quite similar, thus suggesting similar mechanisms of their reaction with ozone. This statement is also conﬁrmed by: (1) the dependence of k on the inductive properties of substituents such as k of polychloroprene is higher than that of vinylchloride due to the presence of two donor substituents, and (2) the dependence of k on the conﬁguration of the C=C bond in trans-isomer (gutta percha) and cis-isomer (natural rubber). It has been found that the effects related to the change of the macromolecule length or the folding degree do not affect the ozonolysis in solution. Probably this is due to the fact that the reaction is carried out with thermodynamic elastomer solutions in which the macromolecules can perform free intramolecular movements and do

217

Ozonation of Organic and Polymer Compounds not react with adjacent macromolecules. Moreover, the rate of macromolecule reorganisation is probably higher than the rate of their reaction with ozone as the experiment does not provide evidence for the effects of the change of the parameters mentioned above [205-207].

Table 3.14 Rate constants of ozone with polymers and low molecular analogues in CCl4, 20 oC Compound Polyvinylchloride Vinylchloride 2-Bromopropene Polybutadiene Cyclododecatriene-1,5,9 Polybutadiene-styrene

Mw

k × 10-4 (M-1.s-1)

8 × 105

0.42 p 0.1

62.45

0.18

121

0.28 p 0.05 5

3.3 × 10

6.0 p 1

162

35 p 10

8 × 104

6p1

3 × 10

4

27 p 5

Natural rubber

1 × 10

6

44 p 10

2-Me-pentene-2

85

Squalene

410

Gutta percha

Polystyrene Cumene Polyisobutylene Cyclohexane

5 × 10

35 p 10 74 p 15 5

0.3 × 10-4

120

0.6 × 10-4

1.7 × 105

0.02 × 10-4

84

0.01 × 10-4

However, it should be noted that k of the elastomers are about 2-6 times lower that those of the low molecular analogues. The accuracy of Ea determination does not allow estimation as to which of the two parameters A or Ea one should relate the decrease in k. If we assume that the mechanism of ozone reaction with monomers and elastomers is similar, i.e., the reactions are isokinetic, then Amon = Apol. At kmon/kpol = 2 - 6 the difference in Ea at 20 oC will be 0.5-1.0 kcal/mol. At the low experimental

218

Ozonolysis of alkenes in liquid phase values of Ea, these differences will become commensurable with them and thus the determination of Ea is not accurate enough. In this case two assumptions could be made which can give a reasonable explanation for the lower values of kpol: (1) reorientation of the macromolecules is a slower process that that of oleﬁns which would results in Apol lower than Amon, and (2) the addition of ozone to C=C bonds is accompanied by the rehybridisation of the C atoms from sp2 to sp3 and the movement of the polymer substituents during AC formation will be more restricted than those in oleﬁns mainly because of their greater molecular mass and sizes. This will ultimately result in a decrease of the rate constant. Table 3.14 shows some examples of ozonolysis of saturated polymers - polystyrene and polyisobutylene. These reactions, as mentioned earlier in Chapter 1, take place not via the mechanism of ozone reaction with the double bonds but through a hidden radical mechanism with rate constants of 4-5 orders lower.

3.2.1 Polybutadiene Some industrial samples of rubbers have been the object of investigation (Table 3.15). The polymers were puriﬁed by triple extraction with methanol in a Soxhlet extractor. The experiments were conducted in a bubbling reactor containing a polymer solution (0.06-1%) in tetrachloromethane. The progress of the ozonation was followed by viscosimetry, gel chromatography, IR and nuclear magnetic resonance (NMR) spectrometry, iodometry and through triphenylphosphine reduction. Because of the high viscosity and large rate constants the reaction takes place in the diffusion and mixed region. In order to obtain correct kinetic data we have used the theory of boundary surface (Equation 3.16) [34]: [O3] = A[O3]0.exp[– D(k.c.D)1/2]

(3.16)

where [O3] is the ozone concentration at the boundary surface; A is Henry’s coefﬁcient; [O3]0 is the ozone concentration in the gas phase at the reactor inlet; D is the boundary surface depth; k is the rate constant of the ozone reaction with diene; c is the concentration of the monomeric units; D is the diffusion coefﬁcient of ozone in the liquid phase.

219

220 94-98

5

94-97

87-93

1,4-cis (%)

94

2-4

3-8

1,4-trans (%)

-

-

3-5

1,2(%)

-

1-2

-

3,4(%)

180

380

454

Mv × 10-3

1.8

2.0

2.1

n

Note: Mv is the average molecular weight, determined viscosimetrically after equation [H] = k × MvAa, where [H] = (H1/C)(1 + 0.333H1), H1 = Hrel–1; Hrel is the intrinsic viscosity; C is the solution concentration; k = 1.4 × 10-4 is the Staudinger constant and Aa = 0.5-1.5 is the constant depending on the rubber type, being one for natural rubber; Mv z Mw; n = Mw/Mn, where Mw and Mn are the average weight and number average molecular mass, respectively.

-C(Cl)=C-

-C(CH3)=CH-

SKI-3S

Denka M40

95-98

-CH=CH-

SKD 94-98

Unsaturation degree (%)

Monomeric unit

Elastomer

Table 3.15 Some characteristics of polydiene samples

Ozonation of Organic and Polymer Compounds

Ozonolysis of alkenes in liquid phase It was found that the relative viscosity decreases exponentially upon ozonation of SKD and SKI-3S solutions (Figure 3.8).

4.0

3.5

3.0

2.5

3

2

1

2.0

0

5

10

15

20

25

Ozonation time, min

Figure 3.8 Dependence of the relative viscosity (Hrel) of SKD (Russian butadiene rubber) solutions (0.6 g in 100 ml CCl4) on reaction time at ozone concentrations of: 1: 1 × 10-5 M; 2: 4.5 × 10-5 M; 3: 8.25 × 10-5 M

The dotted lines denote the theoretical exponential functions. As the viscosity is proportional to the molecular weight it follows that the polydiene consumption is described by ﬁrst or pseudo ﬁrst-order kinetics. The value of F, standing for the number of degraded polymeric molecules per one absorbed ozone molecule can be used to calculate the degradation intensity. The value of this parameter (F) may be estimated using Equation 3.17:

F = 0.5 [(Mvt)-1 – (Mv0)-1].P/G

(3.17)

where Mvt is the molecular weight at time t; Mvo is the initial molecular weight; P is the amount of polymer; G is amount of consumed ozone.

221

Ozonation of Organic and Polymer Compounds The dependence between F and G is a straight line for a given reactor and depends on the hydrodynamic conditions in the reactor.

1.6

1

1.4

2

.102

1.2

3

1.0 0.8

0.6 0.4 0.0

0.5

1.0

1.5

2.0

2.5

G.105, mols

Figure 3.9 Dependence of F on G for SKD (0.6/100) at various ozone concentrations: 1: 1 × 10-5 M; 2: 4.5 × 10-5 M; 3: 8.25 × 10-5 M

It is seen from Figure 3.9 that the F values increase linearly with the reaction time and decrease with increase in ozone concentration. The corresponding F values for SKI-3S (Russian synthetic cis-polyisoprene rubber) and Denka M40 ozonolysis are similar. The F values for Gm0 were used to avoid the effect of hydrodynamic factors on them. The values of F found for SKD, SKI-3S and Denka M40 at [O3] = 1 × 10-5 M amount to 0.7 × 10-2, 0.78 × 10-2 and 0.14, respectively, and the slopes are: –40, –70 and 200 M-1, respectively. Substituting with the known values for the parameters in Equation 3.16 we have obtained D within the range of 1 × 10-3 to 2 × 10-4 cm, which indicates that the reaction takes place in the volume around the bubbles, and hence in the diffusion region. The ozonolysis of polydienes in solutions is described by the Criegee mechanism. The C=C bonds in the macromolecules are isolated as they are separated by three simple C-C bonds. According to the classical concepts, the C=C bond conﬁguration and the electronic properties of the groups bound to them also affect the polymer reactivity, similarly with the low molecular oleﬁns. The only difference is that the polymer substituents at the C=C bonds are less mobile which inﬂuences the sp2-sp3 transition

222

Ozonolysis of alkenes in liquid phase and the ozonide formation. In the ﬁrst stage, when PO are formed, the lower mobility of the polymer substituents requires a higher transition energy, the rate being lower compared with that with low molecular oleﬁns and the arising strain accelerates the PO decomposition to a zwitterion and carbonyl compound. The lower mobility of the polymer parts impedes further ozonide formation and brings about the zwitterion leaving the cage and going into the volume which in its turn accelerates the degradation process. The latter is associated with either its monomolecular decomposition or its interaction with low molecular components in the reaction mixture. The efﬁciency of degradation is determined by the C=C bond location in the macromolecule, for example, the C=C bond location from the macromolecule centre to its end, is in the range from 2 to 1 (Equation 3.18): M1 = (1/G).M0,

(3.18)

where M2 = M0 – M1; 1≤G≤2 is the coefﬁcient indicating the C=C bond location; M0, M1 and M2 are the molecular weights of the initial macromolecule and the two degraded polymer parts, respectively.

1.6

1.6

1.4

1.4 3

1.2

1.0

F.102

1.0 1

0.8

0.8

2

0.6

0.6

0.4

0.4

0.2

0.2 0

2

F.10

1.2

4

6

8

[O3].105, M

Figure 3.10 Dependence of F on ozone concentration for elastomer solutions: 1: SKD (0.6/100); 2: SKI-3S (0.6/100); 3: Denka M40 (1/100)

At G = 2, i.e., when the broken C=C bond is located in the macromolecule centre, the values of M1 and M2 will be exactly equal to M0/2, at Gm1, i.e., at the terminal C=C bond in the polymer chain, the value of M1 will approximate to M0 and thus

223

Ozonation of Organic and Polymer Compounds the value of M2 will be practically insigniﬁcant. For example, M2 may be 50-1,000 which is 3-4 times less than that of the macromolecule and in fact the degradation process will not occur. The viscosimetric determination of the molecular weight which we have applied in our experiments has an accuracy of p 5% and does not allow the differentiation of molecular weights of 22,700, 19,000 and 9,000 for the corresponding rubbers. This suggests that the cleavage of C=C bonds located at a distance of 420, 280 and 100 units from the macromolecule end would not affect the measured molecular weight. As the reaction of elastomer ozonolysis proceeds in the diffusion and diffusion-kinetic region, at low conversions each new gas bubble in the reactor would react with a new volume of the solution. On the other hand, the reaction volume is a sum of the liquid layers surrounding each bubble. It is known that the depth of penetration from the gas into the liquid phase is not proportional to the gas concentration and thus the rise of ozone concentration would increase the reaction volume to a considerably lower extent than the ozone concentration. This leads to the occurrence of the following process: intensive degradation processes take place in the microvolume around the bubble and one macromolecule can be degraded to many fragments while the macromolecules out of this volume, which is much greater, may not be changed at all. Consequently, with an increase in ozone concentration, one may expect a reduction of MMD and an increase of the oligomeric phase content. This will result in an apparent decrease of F at the viscosimetric measurements. The previous discussion allows the correct interpretation of the data in Figure 3.10. On ozonation of the SKD solution (1/100) at 10% conversion an intensive band of C-O in ozonides [207, 208] was observed at 1115 cm-1 in the IR spectrum. A less intensive band at 1735 cm-1, characteristic of C=C vibrations in aldehydes and esters (Figure 3.11, 2), was also registered. As the ozonides did not react quantitatively with HI their reaction with triphenylphosphine (Ph3P) was used for the determination [209]: Ozonide + Ph3P = Ph3PO + 2Aldehyde The amount of ozonide as estimated from the band intensity at 1735 cm-1 (Figure 3.11, 3) was 70 p 5% with respect to the amount of the absorbed ozone. It has been found that the amount of C=O groups as estimated from the band intensity at 1735 cm-1 that appears prior the Ph3P treatment, is 11% of the absorbed ozone (the extinction coefﬁcient is taken from the calibration to >C=O in butyric aldehyde). These functional groups are formed as a result of the proceeding of reactions parallel with those of ozonide formation. Control experiments have demonstrated the relatively high thermal stability of SKD ozonide [210, 211]. Usually it is assumed that the zwitterion isomerisation into acid is responsible for the degradation of macromolecules. However,

224

Ozonolysis of alkenes in liquid phase absorption in the 1710-1716 cm-1 band was not detected, although the extinction coefﬁcient for the >C=O bonds in acids is two to three times higher than that of >C=O in aldehydes and ketones [208, 209]. The acid groups may react with the zwitterions, so that their concentration in solution remains below the threshold sensitivity of the spectroscopic method. This assumption is supported by the fact that butyric acid added to the SKD solution is consumed during the reaction (Figure 3.12).

Figure 3.11 Infrared spectra of SKD solutions (1/100): 1 - non-ozonised; 2 - ozonised to 10% conversion; 3 - solution after Ph3P treatment

The higher band intensity at 1735 cm-1, obtained on ozonolysis in the presence of acid, compared with that of the same band in its absence, reveals overlapping of the >C=O absorption in carbonyl and ester groups [209]. The interaction between the zwitterion and carboxylic group is also conﬁrmed by the appearance of a band at 3480 cm-1 at 50% conversion (Figure 3.12). The latter may be attributed to vibrations of the hydroperoxide OH groups. Thus, it can be concluded that the decay of the excited acid molecule resulting from the zwitterion isomerisation to free radicals [210] is not the only route for their consumption. Degradation may also result from the interactions between zwitterions, leading to di- and polyperoxides which are very unstable and readily decompose into polymer carbonyl compounds with lower molecular weight

225

Ozonation of Organic and Polymer Compounds and an oxygen molecule. It has been found that the most efﬁcient way for reduction of the molecular weight is provided by the interaction of zwitterions formed in the chain centre with low molecular compounds and oligomers.

Figure 3.12 Infrared spectra of SKD solutions (1/100): 1 - nonozonised + butyric acid; 2 - ozonised to 20% conversion; 3 - ozonised to 50% conversion without butyric acid The thermal decomposition data differential scanning calorimetry ((DSC) spectra) of ozonised rubber (SKD) in an inert atmosphere (argon) in the temperature range of 50-180 oC were processed using the equation suggested by Gorbachev and co-workers (Equation 3.19) [211-213], Equations 2.2-24 from Chapter 2 and the expression of Julai and Grinhau (Equation 3.20) [213]: ln h/h1/[ln(1–A)/(1–As)] = {(Ea/RTs) . [(T–Ts)/T] . 1/[ln(1–A)/(1–As)]} + n

(3.19)

where A is the conversion degree; h is the peak height; T is the temperature; s is an index standing for the values of the DSC peak maximum; n is the reaction order:

As = 1 - 1.062n1/(1-n)

where As accounts for the transformation degree in the DSC maximum.

226

(3.20)

Ozonolysis of alkenes in liquid phase The results from the study on the thermal degradation of SKD and Diene 35 NFA rubbers are summarised in Table 3.16.

Table 3.16 DSC analysis of products obtained from ozonolysis of polybutadiene rubbers SKD and Diene 35 NFA Heating rates (oC/min)

1

2

5

274/216

338/260

355/276

-/-

370/-

370/-

T (oC)

129/125

136/133

147/144

As

0.65/0.70

0.67/0.70

0.68/0.75

E (kJ/mol)

99.9/91.2

105.5/93.7

121/109.6

n

0.7/0.73

0.73/0.73

0.79/0.73

$H1 (kJ/mol ozonide) $H2 (kJ/mol O3)

3.2.2 Cis-1,4-polyisoprene The positive inductive effect of the methyl group in polyisoprene enhances the rate of ozone addition to the double bonds from 6 × 104 for SKD to 4 × 105 M-1.s-1 for SKI-3S. The asymmetry double bond allows the formation of two zwitterions: one as in butadiene (A) and the corresponding ketone and the other, in which one of the hydrogen atoms is substituted by a methyl group (B) and the corresponding aldehyde. Since the stabilisation of the zwitterion depends on the electron donor properties of the substituents, the ratio of zwitterions A/B< 1 and is equal to –0.56. The interaction between zwitterions and aldehydes leads predominantly to ozonide formation while that with ketone is more difﬁcult although there are conﬂicting opinions in the literature on the latter [214-219]. The band at 1725 cm-1 observed in the spectrum of the ozonised SKI-3S solutions (Figure 3.12) may be attributed to the absorption of the keto group in the ketone formed together with the zwitterion (A) [220]. By means of a calibration using methylethyl ketone, it was calculated that the amount of ketone is approximately 40% of that of the absorbed ozone. Consequently, the higher degree of SKI-3S degradation compared with that of polybutadiene is due to impeded zwitterion A interaction with the corresponding ketone resulting in its insigniﬁcant conversion according to reactions leading to a decrease of the molecular weight. The ozonide formation has been demonstrated by the occurrence of IR bands at 1110 an 1070 cm-1 (Figure 3.13, 2).

227

Ozonation of Organic and Polymer Compounds

Figure 3.13 IR spectra of SKI-3S solutions: 1 - nonozonised; 2 - ozonised to 50% conversion

3.2.3 Polychloroprene The electron-accepting properties of the chlorine atom at the polychloroprene double bond reduces the reactivity of Denka M40 as demonstrated by its relatively low rate constant, i.e., k = 4 × 103 M-1.s-1. In this reaction the ratio between the zwitterion A and B, according to theoretical calculations, is in favour of A, the ratio being A/B = 4.55. The A formation is accompanied by choroanhydryde group formation and that of B with aldehyde. In both cases the ozonide formation is insigniﬁcant and the zwitterions react predominantly in the solution volume resulting in enhancement of the degradation process. The intensive band detected at 1795 cm-1 in the IR spectrum of ozonised Denka M40 solutions (Figure 3.14) is characteristic of a chloroanhydride group [220]. This fact correlated well with the conclusion about the direction of the primary ozonide decomposition. It also reveals insigniﬁcant conversion of chloroanhydride to monomer and polymer ozonides. It is difﬁcult to ascribe the absorption maximum at 1735 cm-1 to a deﬁnite functional group because besides aldehydes and esters, chloroanhydrides also absorb in this region. However, the band at 955 cm-1 is typical of chloroanhydrides. Two other bands - at 1044 and 905 cm-1 - may be attributed to the C-O vibrations. The valent vibrations characteristic of HO-groups are observed in the 3050-3500 cm-1 band. The iodometrical analysis of active oxygen in the ozonised Denka M40 solutions shows that the amount of ozonide is approximately. 43%. It is of interest to note

228

Ozonolysis of alkenes in liquid phase that the HI reaction with ozonised polyisoprene solutions occurs quantitatively for 3-4 hours, while in SKD the same reaction proceeds only to 20% after 24 hours. The above data, however, provide insufﬁcient information for the preferable route of the ozonide formation (via dimerisation, polymerisation of zwitterions or secondary processes).

Figure 3.14 IR spectra of Denka M40 solutions: 1 - nonozonised; 2 - ozonised to 40% conversion

The DSC analysis of the products of Denka M40 ozonolysis together with those of SKD and cumene peroxide as reference, are presented in Table 3.17. The data in Table 3.17 reveal that the chloroprene rubber ozonolysis yields polyperoxide, as the enthalpy of its decomposition is found to be very close to that of dicumylperoxide. The higher value of Ea (approximately two times) testiﬁes to the possible formation of polymer peroxides. Moreover, the SKD ozonolysis leads predominantly to the formation of ozonide (89% yield), its enthalpy being signiﬁcantly higher than that of Denka M40.

229

Ozonation of Organic and Polymer Compounds

Table 3.17 DSC analysis of Denka M40 ozonolysis Parameter

Denka M40

SKD

Cumene peroxide

Conversion (%)

28

18

-

Weight loss (%)

7-13

6-11

-

$H1 (kJ/mol O3)

239

337

-

$H2 (kJ/mol)

-

-

215

Ea (kJ/mol)

70

128

140

Ti (oC)

51

86

-

Tm ( C)

96

147

-

o

132

175

-

o

Te ( C)

Note: $H1 is the enthalpy related to 1 mol ozone; $H2 is the enthalpy of cumene peroxide, Ea is the decomposition activation energy; Ti, Tm and Te are the temperatures at the beginning, maximum and end of the DSC curves.

3.2.4 Butadienenitrile rubbers The most important characteristics of the nitrile rubbers under study are shown in Table 3.18.

Table 3.18 Characteristics of the butadiene-nitrile rubbers investigated Elastomer

Acrylonitrile (%)

1,4-trans (%)

1,4-cis (%)

1,2(%)

Mv × 10-3

Mw/ Mn

SKN-18

20

64

21

15

380

3.0

SKN-28

28

65

24

11

300

3.0

SKN-40

39

72

22

6

230

2.7

Mv = The molecular mass determined by a viscosimetric method

The results from the NMR study of the nitrile rubbers structure are listed in Table 3.19.

230

Ozonolysis of alkenes in liquid phase

Table 3.19 Distribution of the triad sequence of 1,4-butadiene (B) and acrylonitrile (A) using 1H-NMR data Triads

SKN-18

SKN-26

SKN-40

BBB

73

56

33

BBA

9

14

18

ABA

18

30

49

It was shown that the viscosimetric measurements provide lower values for of the ozone degradation level compared with those obtained by the IR data. This observation may be attributed to the fact that the reaction between ozone and the rubber is localised in a very thin layer around the gas bubbles, while the major part of the macromolecules do not undergo degradation in the solution volume. The intensive progress of the reaction in a very small volume provides conditions for the formation of signiﬁcant amounts of low molecular products without signiﬁcant change in the molecular weight. Nevertheless, the viscosimetric determinations of a series of samples conﬁrm some tendencies of the degradation process. The results from the viscosimetric studies of three nitrile rubbers are shown in Table 3.20. From Table 3.20 one can see that F depends on three factors: (1) the ozone concentration, (2) the nature of the solvent, and (3) the rubber structure. Under the experimental conditions of the bubbling method, the inﬂuence of ozone concentration is associated with the insigniﬁcant increase of the reaction volume and thus to the number of interactions of one macromolecule. The role of the solvent is associated with: UÊ *ÀÛ`}Ê V`ÌÃÊ vÀÊ }iÀÊ iµÕLÀÕÊ âiÊ VViÌÀ>ÌÃ]Ê `ÕiÊ ÌÊ ÌiÊ different values of Henry’s coefﬁcients for different solvents (CCl4 = 2 and CHCl3 = 2.8), UÊ Ài>ÌÊvÊiÜÊVvÀ>ÌÊV`ÌÃÊvÀÊÌiÊ>VÀiVÕiÃÆÊivviVÌÊÊÌiÊ electronic properties of the reaction centre and the free access of ozone to it and thus on the degradation degree, and UÊ /iÊ«>ÀÌV«>ÌÊvÊÌÀiÊÀÕLLiÀÊÊÌiÊâÞÃÃÊ«ÀViÃÃÊÜVÊ>ÃÊÌÊLiiÊ experimentally registered).

231

Ozonation of Organic and Polymer Compounds

Table 3.20 Number of chain scissions per molecule of reacted ozone (φ) for ozone degradation of nitrile rubbers at various ozone concentrations in CCl4/CHCl3 solvents Elastomer

Concentration (g/100 ml)

F × 102, 0.0025 mM [O3]0

F × 102, 0.005 mM [O3]0

F × 102, 0.01 mM [O3]0

SKN-18

0.45

2.5/4.8

0.9/2.1

0.6/1.6

SKN-28

0.49

-/7.0

-/5.4

-/1.7

SKN-40

0.40

-/7.5

/6.1

-/2.5

It is seen that the increase in the acrylonitrile content in the elastomers studied reduces the probability of the formation of low molecular fragments and consequently the degradation degree should increase. Indeed, such a dependence on has been experimentally observed (columns 3-5). It should be noted that during the ozonolysis of SKN-18 (for CHCl3 solutions up to 25%) gel formation is hardly observed to 30% conversion. Thus, it can be assumed that under the experimental conditions used the contribution of crosslinking reactions (interaction between the zwitterions and aldehydes with high molecular fragments or other structural reactions between macromolecules resulting in a rise of the molecular weight) to the overall balance of reacted ozone is very small. It was shown that the degradation of macromolecules is a result of the zwitterions reaction with low molecular compounds in the bulk solution, monomolecular isomerisation of the former to acids and di- and polyperoxides decomposition with oxygen evolution [221-223]. Several signiﬁcant features were found in the 1H-NMR spectra of the ozonised CCl4 solutions of SKN-18: a signal due to the aldehyde proton at 9.75 ppm; a few signals in the 4.97-5.20 ppm region [223-225] ascribed to methine protons and a multiplet at 2.75 ppm with an integral intensity of about 40% of that of the aldehyde peak. The ratio of the aldehyde to the ozonide groups calculated from the integral intensities is about 40:60. As SKN-26 and SKN-40 are not dissolved in CCl4, the ozonolysis and the 1H-NMR spectra were recorded in CHCl3 and CDCl 3 solutions. The insoluble gel after ozonation was separated by ﬁltration and CDCl3 was added. The following peaks were detected in the 1H-NMR spectra: 9.76 ppm (s); 4.97-5.20 ppm (nd); 2.73 ppm (m) and a series of very weak peaks in the 3.12-4.20 ppm range.

232

Ozonolysis of alkenes in liquid phase

Figure 3.15 1H-NMR spectra of SKN solutions (0.75/100); 1 - nonozonised; 2 ozonised to 20% conversion Simultaneously the IR spectra (Figure 3.15) of nitrile rubbers before and after ozonation were registered.

Figure 3.16 IR spectra of SKN-18 solutions (0.75/100): 1 - nonozonised solution; 2 - ozonised to 20% conversion

233

Ozonation of Organic and Polymer Compounds The IR spectra of SKN-26 and SKN-40 are very similar to that of SKN-18 shown in Figure 3.16. The kinetics of ozonide and aldehyde group formation for the three rubbers was studied by monitoring the changes in the optical density at 1110 and 1731 cm-1, respectively. The effect of the reacted ozone and the type of solvent, i.e., CCl4 or CHCl3 was investigated. It may be seen that in all cases the kinetic curves are linear up to 20% conversion. The slopes of the dependence for the solution of SKN-18 depend on the solvent nature and it amounts to 2.4 × 10-2 M aldehyde/2 × 10-4 mol absorbed ozone in CHCl3 and is 1.2 times higher than that in CCl4. The registered optical density of the ozonides does not depend on the solvent but the slope of the curve is found to be 2 times lower than that of the aldehydes in CHCl3. The slopes of the kinetic curve for aldehyde formation for the solutions of SKN-26 and SKN-40 (CHCl3) are identical. However the accumulation of aldehydes takes place at a 2.25 times higher rate. The corresponding slopes are about 8% greater than those of SKN-18. Using literature values for the extinction coefﬁcients for ozonides (ε = 115 [223]) and aldehydes (ε = 230 [223, 226]) it was possible to evaluate the ratio aldehyde/ozonide for the three rubbers studied (SKN-18; SKN-26 and SKN-40) in CHCl3 which are 1.03, 1.12 and 1.12, respectively, at ambient temperature and 20% conversion. For SKN-18 in CCl4 this ratio amounts to 0.85. The difference in the values of this ratio obtained in the two solvents is probably due to the stabilisation of the zwitterion in the more polar solvent, i.e., chloroform, and the differences between the various rubbers is associated with the various amounts and size of the polymer fragments containing 1,2-double bonds. The role of nitrile groups is demonstrated by the electronic effects of the strong electron-accepting CN group and its inductive effect transferred via two S-bonds. The quantitative interpretation of the NMR spectra of SKN-18 ozonised in CHCl3 gives the value of 0.82 for the aldehyde/ozonide ratio which is within the accuracy limits of the extinction coefﬁcient determination, and correlates with the IR data. It has been found that from 1 mol of absorbed ozone, 0.52 moles of aldehyde and 0.72 moles of ozonide are formed in SKN-18 ozonolysis in CCl4. Thus the ratio amounts to 0.72. The results from the two spectral studies support our suggestions for the important role of zwitterion reactions in the ozone degradation of nitrile rubbers. Their reactions out of the cage lead to the formation of di- and polyperoxides which are converted to aldehydes with evolution of oxygen. The effect of temperature on aldehyde/ozonide ratio is illustrated by the kinetic parameters given in Table 3.21. In the ozonolysis of nitrile rubbers the amount of aldehyde groups formed varies within the range of 50-70%, while on ozonolysis of 1,4-cis-polyisoprene it is approximately 11%. The signiﬁcant difference could be explained by the presence of nitrile groups

234

Ozonolysis of alkenes in liquid phase in the polymer molecules and the difference in the conﬁguration of the C=C bonds: 64-72% trans- for the nitrile rubbers and 94-97% cis- for the isoprene rubber. Data in the literature show that the ozonide yield from trans-oleﬁns is considerably lower than that from cis-oleﬁns [223, 224].

Table 3.21 Activation energy for the formation of aldehydes and ozonides in SKN-18 ozonolysis Solvent

Ea (kJ/mol, ozonides)

Ea (kJ/mol, aldehydes)

CCl4

–0.8

4

CHCl3

–0.4

0.8

3.2.5 Ethylenepropylene rubbers Ethylenepropylene rubbers (EPDM) are copolymers of 65-80% ethylene with 3520% propylene and a third copolymer - diene 1-2% (for example, cyclopentadiene, 1,4-hexadiene, 2-ethylene-norbornene-5). They are characterised by high resistance towards the action of temperature, oxygen and ozone, mainly because of the extremely low content of C=C bonds. The vulcanising properties are due to the presence of the diene copolymer. In most cases the EPDM rubbers are used alone, and only in particular cases, for the preparation of efﬁcient antiozonant and antioxidant materials, are they used in combination with diene elastomers due to their low compatibility with the elastomers. Their application as antiozonant additives will be discussed further in detail. The investigations were carried out on Keltan 778 samples (ethylene:propylene: ethylenenorbornene = 75.5:23:1.5 with Mv = 193,000 and Mv/Mn = 1.84) in CCl4 (0.6/100): H3C

CH

(CH2CH2)n(CH2CH)m CH3 1

CH

(CH2CH2)n(CH2CH)m CH3

CH3 2

The diene copolymer undergoes polymerisation both via the vinyl and norbornene double bond (1 and 2) in 1:4.66 ratio. These structures have been identiﬁed by means

235

Ozonation of Organic and Polymer Compounds of 1H-NMR - 250 MHz. The spectra analysis suggests the existence of two preferable conformations for the methyl group, during the polymerisation via the norbornene C=C bond. The ﬁrst is when the methyl group is oriented towards the polymer chain and the second when it is oriented in the opposite direction. Upon ozonolysis of EPDM the reaction centres are the C=C bonds of the diene copolymer although their concentration is almost 70 times lower compared with that of the C-H bonds, which is due to the substantially higher rate constants (up to 4-6 orders) [227]. Because of the relatively high viscosity of the solutions the rate constants of ozonolysis could not be determined accurately by using Equation 1.4. The value of the activation energy estimated from the dependence of the viscosity change of the ozonised EPDM-rubber solutions on the temperature is found to be Ea = 33 kJ/mol. Ozone attacks these double bonds resulting in the formation of two zwitterions in the volume. The interaction between the latter leads rather to structurisation of the macromolecules and growth of the polymer molecular mass than to its reduction. At low temperatures this process is more efﬁcient and thus the activation energy has a negative value. The presence of peroxide bridges is easily established. The treatment of the ozonised solution with PPh3 gives PPh3O and the molecular weight is restored. The thermal decomposition of ozonised Keltan 778 solutions in an inert atmosphere makes possible the estimation of the Arrhenius parameters for the monomolecular decomposition of the crosslinked molecules. The activation energy was estimated to be 19 kcal/mol. That is in a very good agreement with other data on the decomposition of compounds containing peroxide bridges [228, 229]. In order to determine the type of the bridges the 1H-NMR spectra of the initial rubber have been recorded. It reveals four doublets assigned to the four methine protons: (1) cis-vinylene - 5.08 ppm, (2) trans-vinylene - 5.30 ppm, (3) methine proton in norbornene, located in the direction of the polymer matrix - 5.58 ppm, and (4) methine proton orientated in the opposite direction to the polymer chain - 5.70 ppm. The quantitative analysis shows that trans-vinyl radicals (2) constitute the major portion of the free radicals while the norbornene double bond is found to be the most reactive in the polymerisation process due mainly to the highly strained hydrocarbon structure. The latter could not form ozonide at the rupture of the double bond but easily leads to polyperoxide formation.

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241

Ozonation of Organic and Polymer Compounds 88. O. Lorenz and C. Parke, Journal of Organic Chemistry, 1965, 30, 1976. 89. R. Murray, P. Story and L. Loan, Journal of the American Chemical Society, 1965, 87, 3025. 90. E. Briner, E. Dallwigk and M. Ricca, Helvetica Chimica Acta, 1958, 41, 1390. 91. E. Briner and M. Ricca, Helvetica Chimica Acta, 1958, 41, 2578. 92. E. Briner and S. Fliszar, Helvetica Chimica Acta, 1959, 42, 1310, 2063. 93. R. Criegee, S. Bath and B.V. Bornhauph, Chemische Bericht, 1960, 93, 2891. 94. P. Kolsaker, Acta Chemica Scandanavica, 1965, 19, 223. 95. F.L. Greenwood and B.J. Haske, Tetrahedron Letters, 1965, 6, 11, 631. 96. F.L. Greenwood and B.J. Haske, Journal of Organic Chemistry, 1965, 30, 1276. 97. R. Murray, R. Youssefyeh and P. Story, Journal of the American Chemical Society, 1966, 88, 3143. 98. F. Greenwood, Journal of the American Chemical Society, 1966, 88, 3146. 99. R. Murray and R. Youssefyeh, Journal of the American Chemical Society, 1967, 89, 2429. 100. P. Story, R. Murray and R. Youssefyeh, Journal of the American Chemical Society, 1966, 88, 3144; 101. P. Story, R. Murray and R. Youssefyeh, Journal of the American Chemical Society, 1967, 89, 4473. 102. N.L. Bauld, J.A. Thompson, C.E. Hudson and P.S. Bailey, Journal of the American Chemical Society, 1968, 90, 1822. 103. P. Story, C. Bishop, J.R. Burgess, R.W. Murray and R.D. Youssefyeh, Journal of the American Chemical Society, 1968, 90, 1907. 104. R.W. Muray and R. Hagen, Journal of Organic Chemistry, 1971, 39, 1103. 105. P.R. Story, J. Alford, J. Burges and W. Ray, Journal of the American Chemical Society, 1971, 93, 3042.

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Ozonolysis of alkenes in liquid phase 106. D.P. Higley and R.W. Murray, Journal of the American Chemical Society, 1976, 98, 4526. 107. K.L. Gallaher and R.L. Kuczkowski, Journal of Organic Chemistry, 1976, 41, 892. 108. S. Fliszar and J. Carles, Journal of the American Chemical Society, 1969, 91, 2637. 109. J. Castonguay, M. Bertrand, J. Carles, S. Fliszar and J. Rousseau, Canadian Journal of Chemistry, 1969, 47, 919. 110. C. Gillies and R. Kuczkowski, Journal of the American Chemical Society, 1972, 94, 7609. 111. C. Gillies and R. Kuczkowski, Journal of the American Chemical Society, 1972, 94, 6337. 112. R. Lattimer, R. Kuczkowski and C. Gillies, Journal of the American Chemical Society, 1974, 96, 348. 113. J. Renard and S. Fliszar, Journal of the American Chemical Society, 1970, 92, 2628. 114. S. Fliszar, J. Renard and D. Simon, Journal of the American Chemical Society, 1951, 93, 6953. 115. S. Fliszar, Journal of the American Chemical Society, 1972, 94, 7386. 116. R. Lattimer, R. Kuczkowski and C. Gillies, Journal of the American Chemical Society, 1973, 95, 1348. 117. P. Hiberty, Journal of the American Chemical Society, 1976, 98, 6088. 118. R. Rouse, Journal of the American Chemical Society, 1973, 95, 3460. 119. S. Fliszar and J. Renard, Canadian Journal of Chemistry, 1970, 48, 3002. 120. S. Fliszar and M. Gaunger, Journal of the American Chemical Society, 1970, 92, 3361. 121. S. Fliszar and M. Gaunger, Journal of the American Chemical Society, 1969, 91, 3330. 122. S. Fliszar and J. Renard, Canadian Journal of Chemistry, 1967, 45, 533.

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Ozonation of Organic and Polymer Compounds 123. S. Fliszar and J. Carles, Canadian Journal of Chemistry, 1969, 47, 3921. 124. P. Bailey, Chemica e l’Industria, 1957, 1148. 125. R. Criegee, Chemische Bericht, 1975, 108, 743. 126. R. Criegee and A. Rustaiyan, Chemische Bericht, 1975, 108, 749. 127. R. Criegee, A. Bancin and H. Reul, Chemische Bericht, 1975, 108, 1642. 128. R.W. Murray and R. Hagen, Journal of Organic Chemistry, 1971, 36, 1098. 129. P.S. Bailey, T.M. Ferrell, A. Rustaiyan, S. Seyhan and L.E. Unruh, Journal of the American Chemical Society, 1978, 100, 894. 130. P.S. Bailey and T.M. Ferrell, Journal of the American Chemical Society, 1978, 100, 899. 131. R.A. Rouse, Journal of the American Chemical Society, 1973, 95, 34600. 132. R.A. Rouse, Journal of the American Chemical Society, 1974, 96, 5095. 133. P.C. Hiberty, Journal of the American Chemical Society, 1976, 98, 6088. 134. R.A. Rouse, International Journal of Quantum Chemistry Symposium, 1973, 7, 289. 135. S.W. Benson, F.R. Cruickshauk, D.M. Golden, G.R. Haugen, H.E. O’Neal, A.S. Budgers, R. Shaw and R. Walsh, Chemical Reviews, 1968, 68, 279. 136. W.A. Goddard, III, T.H. Dunning, Jr., W.J. Hunt and P.J. Hay, Account of Chemical Research, 1973, 6, 368. 137. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1975, 97, 6293. 137b. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1975, 97, 6300. 137c. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1976, 98, 6093. 138. F.M. Bodrowicz and W.A. Goddard, III, in Methods of Electronic Structure Theory, Ed., H.F. Schaefer, III, Modern Theoretical Chemistry, Volume 3, Plenum Press, New York, NY, USA, 1977, 3, p.79.

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Ozonolysis of alkenes in liquid phase 139. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1978, 100, 7180. 140. P.S. Bailey, T.M. Ferrell and A. Rustaiyan, Journal of the American Chemical Society, 1976, 98, 638. 141. D. Cremer, Journal of the American Chemical Society, 1981, 103, 13, 3619. 142. M Olzmann, E. Kraka, D. Cremer, R. Gutbrod and S. Andersson, Journal of Physical Chemistry A, 1997, 101, 49. 143. S.D. Razumovskii, Ozone and its Reactions with Organic Compounds, Institute of Chemical Physics – Russian Academy of Sciences, 1973. [DSc Thesis] 144. C.E. Thorpe and A.J. Gaynor, inventors; Cudahy Packing Company, assignee; US2857410, 1958. 145. C. Noller, J. Carson, H. Martin and K.S. Hawkins, Journal of the American Chemical Society, 1963, 36, 1, 24. 146. J. Treacy, M. Curley, J. Wenger and H. Sidebottom, Journal of the Chemical Society - Faraday Transactions, 1997, 93, 16, 2877. 147. R.D. Cadle and C. Schadt, Journal of the American Chemical Society, 1952, 74, 6002. 148. J.J. Buffalini and A.P. Altschuller, Canadian Journal of Chemistry, 1965, 43, 2243. 149. P.L. Hanst, E.R. Stephens, W.E. Scott and R.C. Doerr in Ozone Chemistry and Technology, Eds., J.S. Murphy and J.R. Orr, The Franklin Institute Press, Philadelphia, PA, USA, 1975, p.958. 150. T. Vrbaski and R.J. Cvetanovic, Canadian Journal of Chemistry, 1960, 38, 1053. 151. J. Heickelen, Journal of Physical Chemistry, 1966, 70, 477. 152. J. Herron and R. Huie, Journal of Physical Chemistry, 1974, 78, 2085. 153. H. O’Neil and C. Blumstein, International Journal of Chemical Kinetics, 1973, 5, 397. 154. S.M. Japan, C. Wu and H. Niki, Journal of Physical Chemistry, 1974, 78, 8.

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Ozonation of Organic and Polymer Compounds 155. S. Toby, F.S. Toby and B.A. Kaduk, International Journal of Chemical Kinetics, 1976, 8, 25. 156. B.J. Finlayson, J.N. Piits Jr. and R. Atkinson, Journal of the American Chemical Society, 1974, 96, 5356. 157. L. Hull, I. Hisatsune and J. Heicklen, Canadian Journal of Chemistry, 1973, 51, 1504. 158. S.M. Japan, C. Wu and H. Niki, Journal of Physical Chemistry, 1976, 80, 2057. 159. F.S. Toby and S.Toby, Journal of Physical Chemistry, 1976, 80, 2313. 160. D.A. Hansen and J.N. Pitts, Chemical Physics Letters, 1975, 35, 569. 161. F.S. Toby and S. Toby, International Journal of Chemical Kinetics, 1974, 6, 417. 162. W. Pritzkow and G. Schappe, Journal für Praktische Chemie, 1969, 311, 689. 163. D.G. Williamson and R.J. Cvetanovic, Journal of the American Chemical Society, 1968, 90, 14, 3668. 164. D.G. Williamson and R.J. Cvetanovic, Journal of the American Chemical Society, 1968, 90, 16, 4248. 165. S.D. Razumovskii, Izvestitya Akademii Nauk SSSR Seriya Khimicheskaya, 1970, 335. 166. S.W. Benson, The Foundations of Chemical Kinetics, Mir Publishers, Moscow, 1964. 167. E.F. Caldin, Journal of the Chemical Society, 1959, 3345. 168. S.D. Razumovskii and G.E. Zaikov, Zhurnal Organicheskoi Khimii, 1972, 8, 464. 169. S.D. Razumovskii, L.M. Reutowa, G.A. Niazashvili, I.A. Tutorskii and G.E. Zaikov, Doklady Academii Nauk SSSR, 1970, 194, 1127. 170. L.P. Hammett, Physical Organic Chemistry: Reaction Rates, Equilibria and Mechanisms, McGraw-Hill Book Company, New York, NY, USA, 1940, p.188.

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Ozonolysis of alkenes in liquid phase 171. S.D. Razumovsky and Y.N. Yuriev, Zhurnal Organicheskoi Khimii, 1968, 4, 1716. 172. S.D. Razumovsky and Y.N. Yuriev, Tetrahedron Letters, 1967, 8, 40, 3939. 173. R. Huisgen, Journal of Organic Chemistry, 1976, 41, 403. 174. R. Huisgen, Angewandte Chemie, 1963, 75, 604, 742. 175. E. Bernatek, P. Kolsaker and T. Ledaal, Tetrahedron Letters, 1963, 4, 20, 1317. 176. P.D. Bartlett and M.R. Rice, Journal of Organic Chemistry, 1963, 28, 3351. 177. W. Woods, R. Carboni and J. Roberis, Journal of the American Chemical Society, 1956, 78, 5653. 178. V.T. Aleksanyan, H.E. Sterlin and A.A. Mel’nikov, Izvestitya Akademii Nauk SSSR, Seriya Physica, 1958, 22, 1073. 179. E. Wiberg and B. Nist, Journal of the American Chemical Society, 1961, 83, 1226. 180. N. Allinger, Journal of the American Chemical Society, 1958, 80, 1953. 181. H. Krieger, Suomen, Khem, 1962, 38, 136. 182. L.M. Sverdlov and E.P. Krasnov, Optika i Spektroskopiya, 1959, 6, 334. 183. H. Paub, J. Sange and A. Kausmann, Chemische Bericht, 1965, 98, 1789. 184. A.A. Popov, S.D. Razumovskii, V.M. Parfenov and G.E. Zaikov, Izvestitya Akademii Nauk SSSR Seriya Khimicheskaya, 1975, 282. 185. N.D. Gil’chenok, S.D. Razumovskii, V.K. Tziskovskii, Catakytic Reaction in Liquid Phase, Nauka Publishers, Alma-Ata, Khazakhstan, 1967, p.189. 186. E. Amis, Inﬂuence of the Solvent on the Mechanisms of Chemical Reactions, Mir Publishers, Moscow, 1968. 187. M.G. Buligin and G.E. Zaikov, Izvestitya Akademii Nauk SSSR Seriya Khimicheskaya, 1968, 491. 188. S.D. Razumovskii and Yu.N. Yur’rev, Neftekhimiya, 1966, 6, 737.

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Ozonation of Organic and Polymer Compounds 189. D.R. Kerur and D.L.M. Diaper, Canadian Journal of Chemistry, 1973, 51, 3110. 190. D.L.M. Diaper, Oxidation and Combustion Reviews, 1973, 6, 145. 191. P.S. Bailey, C. Abshire and S. Manthia, Journal of the American Chemical Society, 1960, 82, 6136. 192. P.S. Bailey, J.A. Thompson and B.A. Shoulders, Journal of the American Chemical Society, 1966, 88, 4098. 193. E. Briner and D. Frank, Helvetica Chimica Acta, 1938, 21, 1297. 194. E. Dallwigk and E. Briner, Helvetica Chimica Acta, 1958, 41, 1030. 195. E. Briner, S. Fliszar and G.P. Rossetti, Helvetica Chimica Acta, 1964, 47, 2041. 196. R.H. Bauer and M. Senfarth, Berichte, 1930, 63B, 2691. 197. I.S. Plotnikov, Izvestitya Akademii Nauk SSSR Seriya Khimicheskaya, 1916, 1563. 198. R. Wilke and P. Chang, American Institute of Chemical Engineering and Science, 1966, 21, 999. 199. T.V. Alfrai, Chemical Reactions of Polymers, Mir Publishers, Moscow, 1967, p.9. 200. P. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, USA, 1953, p.41. 201. A.M. Kotliar and N.J. Moravetz, Journal of the American Chemical Society, 1955, 77, 3692. 202. A.N. Giazer and E.L. Smith, Journal of Biological Chemistry, 1961, 236, 2948. 203. A.A. Berlin, A.A. Sayadyan and N.S. Enikolopyan, Vysokomolekulyarnye Soedineniya Seriya A, 1969, 11, 1893. 204. S.D. Razumovskii, G.A. Niazashvili, Yu.N. Yur’ev and I.A. Tutorskii, Vysokomolekulyarnye Soedineniya Seriya A, 1971, 13, 195. 205. Yu.S. Zuev and V.F. Molofeevska, Degradation and Stabilisation of Rubbers, Gostkhimizdat, Moscow, 1960, p.27. 248

Ozonolysis of alkenes in liquid phase 206. S.D. Razumovskii, O.N. Karpuhin, A.A. Kefely, T.V. Pokholok and G.E. Zaikov, Vysokomolekulyarnye Soedineniya Seriya A, 1971, 13, 782. 207. A.A. Kefely, S.D. Razumovskii and G.E. Zaikov, Vysokomolekulyarnye Soedineniya Seriya A, 1971, 13, 803. 208. K. Nakanisi, Infrared Spectra and the Structure of Organic Compounds, Mir Publishers, Moscow, 1965. 209. CRC Atlas of Spectral Data and Physical Constants for Organic Compounds, Ed., J.G. Grassell, CRC Press Inc., Boca Ratan, FL, USA, 1973. 210. R. Murray and P. Story, Ozonation in Chemical Reactions of Polymers, Mir Publishers, Moscow, 1967. [In Russian] 211. L.W. Crane, P.J. Dynes and D.H. Kaelble, Journal of Polymer Science: Polymer Letters, 1973, 11, 533. 212. H. Kissinger, Analytical Chemistry, 1957, 21, 1702. 213. G. Gyulai and E.J. Greenhow, Thermochimica Acta, 1973, 6, 254. 214. K.W. Ho, Journal of Polymer Science, Part A, 1986, 24, 2467. 215. Yu.S. Zuev, Elastomer Cracking under Peculiar Operating Conditions, Khimia Publishers, Moscow, 1980. [In Russian] 216. G.G. Egorova, V.S. Shagov in Synthesis and Chemical Transformation of Polymers, Leningrad State University, Leningrad, 1986. [In Russian] 217. K.W. Ho and J.E. Gutman, Journal of Polymer Science: Part A Polymer Chemistry, 1989, 27, 2435. 218. B.I. Tikhomirov, O.P. Baraban and A.Y. Yakubchik, Vysokomolekulyarnye Soedineniya Seriya A, 1969, 11, 306. 219. P. Dole Robbe, Bulletin de la Societe Chimique de France, 1980, 3160. 220. K.J. McCullough and M. Nojima in Organic Peroxides, Ed., W. Ando, John Wiley & Sons Ltd, Chichester, UK, 1992. 221. D. Brazier, Rubber Chemistry and Technology, 1980, 53, 437. 222. L.F.R. Cafferata, G.N. Eyler, E.L. Svartman, A.I. Canizo, and E.J. Borkowski, Journal of Organic Chemistry, 1990, 55, 3, 1058.

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Ozonation of Organic and Polymer Compounds 223. C.W. Jefford, A. Jaber, J. Boukouvalas and P. Tissot, Thermo-chimica Acta, 1991, 188, 337. 224. A.C.Baldin in The Chemistry of Functional Groups: Peroxides, Ed., S. Patai, Interscience Publishers, London, UK, 1983. 225. M.J. Hackathorn and M.J. Brock, Rubber Chemistry and Technology, 1972, 45, 1295. 226. M.J. Hackathorn and M.J. Brock, Journal of Polymer Science, Part A-1, 1975, 13, 4, 945. 227. D.W. Brazier and N.V. Schwartz, Thermochimica Acta, 1980, 39, 7. 228. The Sadtler Handbook of Proton NMR Spectra, Sadtler, Philadelphia, PA, USA, 1978. 229. S.M. Ellerstein in Analytical Calorimetry, Eds., R.S. Porter and J.O. Johnson, Plenum Press, New York, NY, USA, 1968.

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4

Degradation and Stabilisation of Rubber

In the light of the contemporary search for highly efﬁcient technologies, the studies in the ﬁeld of degradation and stabilisation of elastomers, vulcanisates and rubber products receive considerable attention. The demand for producing new articles with new properties, the growing production of elastomers and the expanding of the ﬁelds of their application, together with the higher technical requirements and the worsening of the exploitation conditions [1-3] make the research in this ﬁeld important and necessary. The study of the kinetics and mechanism of polydiene degradation and stabilisation in real conditions makes possible: 1) Establishment of the deﬁnite steps responsible for the degradation processes that result in deterioration of the physicochemical and mechanical properties of elastomers and vulcanisates and the products from them. 2) Development and application of efﬁcient methods for stabilisation that prevent or retard the degradation processes. 3) More complete utilisation of raw materials and prolongation of the ‘life’ of rubber goods. 4) Creation of scientiﬁc-quantitative bases for ensuring and prediction of the storage and exploitation terms of rubber products [4-6]. As ozone is one of the basic factors resulting in polydiene degradation we focus our attention on investigating its effect on rubber goods both alone and in combination with oxygen, temperature and mechanical stresses. We have studied the degradation processes in pure systems such as nonﬁlled, radiation vulcanised elastomers, in sulfur vulcanised model mixtures and real mixtures and products. The term degradation stands for the loss of physicochemical, mechanical and exploitation properties of elastomers and rubber products associated with molecular mass reduction under the action of various degradation agents. The contributions by various researchers and companies on this problem so far have served as a base for our studies. In regard to the processes of stabilisation we have directed our studies to the synthesis of antiozonants, elaboration of their protective mechanisms, development of highly

251

Ozonation of Organic and Polymer Compounds efﬁcient antiozonants and antioxidative compositions and highly stabilised elastomers systems. We have also followed the role of the various components of rubber mixtures such as photostabilisers, antifatigue agents, and so on. [7]. Stabilisation is a general term used to describe the process of preventing the various materials from the harmful effect of degradation agents. Aiming at the development of an appropriate scientiﬁc basis for prediction of the behaviour of elastomers and rubber products during storage and exploitation we have successfully applied the Arrhenius equation.

4.1 Ozonolysis of Elastomers in Elastic State As we have already shown, the ozonolysis of oleﬁns and elastomers in the liquid phase is an electrophilic process in regard to ozone. The rate of ozonolysis depends slightly on the temperature and decrease if substituents at the double bonds have electronwithdrawing properties. It is also dependent on the oleﬁn conﬁguration, the transisomers being more reactive by about 10-fold than the cis-isomers. The ozonolysis of elastomers in solution is governed by molecule mobility and macromolecule conformation. In general, the values of the rate constant for the elastomers are lower as compared with those of oleﬁns but the yield of the C=C cleavage products is higher for polymers. The main reasons for these observations are: 1) The low mobility of polymer chains hinders the ozone access to the C=C bonds in the macromolecule. 2) The formation of primary ozonides (PO) is also hindered since the sp2-sp3 rehybridisation in the transition state is accompanied by reorganisation of large polymer units. 3) The latter makes the PO decomposition slower than the oleﬁns PO. 4) After the PO decomposition to zwitterion (carbonyl oxide - CO) and carbonyl compound (CC), the low mobility of the polymer fragments prevents the appropriate orientation of CO and carbonyl compound that leads to ozonide formation which although being unstable does not change the molecular weight of the elastomer, and 5) The low mobility of the polymer residual fragments increases the possibility for the CO interaction outside of the kinetic cage which in its turn enhances the degradation efﬁciency. The physicochemical and mechanical properties of rubbers in solid-elastic state are determined mainly by their chemical structure, macromolecule conﬁguration and

252

Degradation and Stabilisation of Rubber conformation, the segmental mobility, the type and the properties of the supramolecular structures, defects in the mass and on the surface, and so on [8-17]. The segmental mobility, supramolecular structure and the ratio between the amorphous and crystalline phase play an essential role in the degradation process of elastomers in solid state. It is known that the diffusion of various gases including ozone in the amorphous phase is more facilitated than in the crystalline one. The greater access of the C=C bonds to ozone attack in this phase together with the higher ozone solubility provide suitable concentration conditions for acceleration of the process [18]. The degradation reaction that takes place on the boundary layer is facilitated by the concentration of stresses in the linking phase units. There should be mentioned the reactions of monomolecular decomposition of the various intermediate polymer compounds, including polymer zwitterions. The surface defects as well as those in the solution are actually the reaction centres from which the degradation process starts. They are also the entry for the ozone degradation In the course of their exploitation the rubber materials are subjected to considerable mechanical stresses resulting from elongation, load, tearing, shrinking in static and dynamic conditions. These stresses play an important role in accelerating the degradation process acting as mechanical and thermal promoters of polymer molecules decomposition. Usually the mechanical stresses are converted into heat without deteriorating the rubber articles. They are also accumulated and concentrated, to a smaller extent, as stresses in net defects, interphase surfaces, in surface defects, thus changing the supramolecular structure, conﬁguration and conformation of the macromolecule through retardation of the relaxation processes. We have made an attempt to evaluate the contribution of the main factors responsible for the degradation of elastomers and their vulcanisates. In this connection we have compared the rate of oxidative and ozone degradation. It has been established that the ﬁrst one is proportional to the rate of the chain oxidation process as described by: WO2 (kp.(Wi/kt)1/2.[RH]

(4.1)

where; kp is chain propagation rate constant = 1 s10-1 M-1.s-1; kt is the rate constant of chain termination = 1 s107 M-1.s-1; Wi - initiation rate; [RH] = 14 M - rubber concentration. Substituting the known values for the constants and concentration in Equation 4.1 the following expression for the rate of oxidative degradation is obtained: Wox (0.014.(Wi)1/2

(4.2)

253

Ozonation of Organic and Polymer Compounds The second rate is proportional to the rate of ozone reaction with the elastomer: Woz = koz.[O3].[RH]

(4.3)

where: koz = is the rate constant of ozone interaction with C=C bonds 105M-1.s-1; [O3] the real ozone concentration in the atmosphere = 1 s10-8 M. Again by using the known values we get: WO3 = 0.014

(4.4)

The comparison of Equations 4.2 and 4.4 reveals that the two rates differ by the multiplier (Wi1/2) which is always smaller than unity. This fact can be regarded as the main reason that makes the study and ﬁght against ozone degradation so important. It is true that ozone degradation takes place in the presence of other degradation agents like oxygen, heat, light, mechanical stresses, etc., which complicates further the research in this ﬁeld. Our experiments show that this problem can be satisfactorily solved as linear combination of its one-dimensional components:

n

J = c1Jc 2Jc x Jx =

3 c xJx

x=1

where: c - constant; and J- parameter related to the degradation process. What is the main result arising from the ozone effect on rubber samples? Rubber samples pre-stretched (E) by more than 10% and then exposed for a deﬁnite time in an ozone atmosphere are covered by a network of cracks that grow with time and ﬁnally break down [20-23]. The longitudinal axis of the cracks depending on the deformation force (up to elongations b200%) and the rubber type is rectangular (orthogonal) to the extension direction, while at higher deformations the crack orientation begins to change and may reach a parallel orientation. Another interesting phenomenon which has been observed by many authors [25-27] is the occurrence of critical deformation. At E = 10-50% depending on the type of the elastomer a maximum in the degradation rate is observed with a simultaneous increase of the total area and cracks depth (Figures 4.1 and 4.2) [24].

254

Degradation and Stabilisation of Rubber

12

10

1

8

6

3 4

2

2

0 0

20

40

60

80

100

120

, %

Figure 4.1 Dependence of the rate of crack growth (v) on deformation during ozonation: 1 - Natural rubber (NR), [O3] = 1.5 x 10-3 mol/m3; 2 - Butadiene styrene rubber - SKS-30 with dominant trans-conﬁguration of the butadiene units, [O3] = 0.9 x 10-3 mol/m3; 3 - Polychloroprene rubber (PCR) - with predominant trans-conﬁguration, [O3] = 4.5 x 10-3 mol/m3

35

1

30 25 20 15

2

10

3

5 0 0

200

400

600

800

, %

Figure 4.2 Dependence of the time breakdown on the deformation: 1 - NR, 3 SKS-30, [O3] = 4.5 x 10-4 mol/m3; 2 - PCR, [O3] = 2.2 x 10-3 mol/m3

255

Ozonation of Organic and Polymer Compounds From the Figures 4.1 and 4.2 can be seen that the maximum rate of ozone degradation is registered in the range of critical deformations. This effect being more pronounced in the case of NR. Different points of view are put forward by different authors to explain the nature of this phenomenon. For that reason we have made an attempt to summarise and analyse the basic and widespread opinions related to the mechanism of this observation. Newton [28] tried to explain this anomalous phenomenon by the mutual effect of cracks and relates the extreme character to the cracks number and sizes. He assumed that at small deformations a few cracks are formed in energetically rich surface defects, and an increase in the deformation energy begins to concentrate on energetically poorer defects thus transforming them in to reaction cores for crack formation. At an E equal to the critical deformation, the number of the reaction centres reaches its maximum and, the degradation rate reaches its maximum values too. At higher E values, the cracks begin to inﬂuence each other, thereby slowing their mutual growth and retarding the degradation. The further increase in E results in accomulation of the deformation energy in the lips of the cracks - they increase their own depth and size leading to a rise of the degradation rate. Bradden and Gent [29, 30] considered that the crack formation and growth is closely related to the appearance of critical deformation whereby a sufﬁcient amount of energy is accumulated for the formation of new surfaces. Solomon and Tellamon [31, 32] put forward the opinion that the ozone cracking is an unique phenomenon that is characteristic only for elastomers with double bonds. Based on the similarity between static wear and corrosion decay, Zuev [33, 34] assumed that elastomer’s degradation is related to the static wear of the vulcanisates, accelerated by the strong corrosive action of ozone. The basic argument in favour of this explanation is the observed similarity in the change of the rubber durability on exposure to ozone with the change of hardness and resistance to sheare at deformation. He considered, however, that the nature of the corrosive agent is of no importance since the speciﬁcity of the chemical reaction is of minor importance. He attributed the observed changes only to the extent of change of the polymer orientation and its acceleration or relaxation resulting from the applied deformation. The analysis of the mechanisms discussed previously reveals principal differences between them with respect to the nature of the critical deformation. Some of the mechanisms explain the crack formation only by physical or only by chemical reactions. The simultaneous action of these two factors has not been considered in explaining the existence of critical deformations. In our opinion the mechanism of ozone effect on rubber specimen could be much better understood if the study is concentrated on the initial steps of the reaction prior to the appearance of the ﬁrst cracks [35-37]. Figures 4.3 and 4.4 show the changes in the the quantity of the C=C bonds, and the accumulation of C=O-groups during

256

Degradation and Stabilisation of Rubber ozonation of the SKI-3 sample, vulcanised by gamma-radiation, depends on the elongation. 1 - D1720/D1640 -C=O-groups; 2 - D1640/D1375 - C=C-groups; ATR - KRS-5 - 45o; [O3] = 7 x 10-5 mol/m3, at room temperature. [ATR - attenuated total reﬂection; KRS-5 - (TlBr-TlI) is a gorgeous red crystal commonly used for attenuated total reﬂection prisms for IR spectroscopy.

1.8

>C=O 1.6 1.4 1.2

1 1.0 0.8 0.6

2 0.4

>C=C<

0.2 0

10

20

30

40

50

60

, %

Figure 4.3 Dependence of the optical densities (D) ratio on the deformation E

1.5

1

1.0

2

0.5

0.0 20

40

60

80

100

120

140

160

180

200

, %

Figure 4.4 Dependence of the chemical relaxation time on radiation vulcanised samples of SKI-3 in an ozone atmosphere at 20 ºC , 1- [O3] = 2 x 10-3 mol/m3 and 2 - [O3] = 1 x 10-2 mol/m3

257

Ozonation of Organic and Polymer Compounds The maximum rate of oxidation corresponds to E = 20% after which it goes down markedly. This testiﬁes to the effect of strain on the rate of the chemical process. Comparing these data with the results from the chemical relaxation (Figure 4.4) it becomes obvious that the maximum rate is achieved at E = 20-30% after which it begins to decrease with the further rise of E. This effect is most pronounced at small ozone concentrations. The results in Figures 4.3 and 4.4 are in a good correlation with the data for the cracks formation and breakdown (Figures 4.1 and 4.1). That observation leads us to conclude that the initial rate of the ozone degradation predetermines the course of the degradation process. On the other hand we should note that the amorphous/crystalline phase ratios in crystallising elastomers like NR do not change substantially up to the critical values of E.

(a)

(b)

Figure 4.5 Roentgen images of cis-1,4-polyisoprene at: (a) - E = 100% and (b) - E = 400%

It is clearly seen that at deformations of up to 100%, crystallisation processes do not occur, while at E = 400% well shaped crystallites are formed. The pattern of sample cracking is primary dependent on rubber type (Table 4.1) [38, 39].

258

Degradation and Stabilisation of Rubber

Table 4.1 View and shape of rubber specimen cracks in ozone atmosphere - 6 ppm, 20 oC and E = 20% up to their breakdown Rubber

Form of cracks

(a/b)**

Picture of cracks

NR*

Oval

2-4

A4T1

SKI-3

Oval with sharp ends

>4

A4T1

Rectangular, with furrow tight side

>2

A1T3

Oval, with some cross ﬁbers

>3

A3T2

Cyrillic letter (ZH)

>1.7

A2T3

Poor deﬁnite rectangular

3

A2T2

SKD, (1,4-cis-polybutadiene Russia) SKI-3/SKD Bulex 1500, (Polybutadiene styrene 70:30 - product of Bulgaria) Bulex M27, (oil-27% ﬁlling polybutadiene-styrene rubber - Bulgaria)

Note: * - also for ﬁlled sample; ** at oval - the ratio of ellipse axes, at rectangular - the ratio of faces; A - number of cracks: A1 = 1-5, A2 = 5-40, A3 = 40-120, and A4>120; T - depth of cracks; T1 - surface cracks; T2 - cracks b1 mm, T3 - cracks >1 mm, and T4 - cracks through the total depth of the specimen studied.

The process of crack formation was determined by means of reﬂected-light optical spectroscopy methods using metalographic-MMR-4 LOMO microscope (Russia) with a magniﬁcation of x600 [39]. The shape of the cracks depends on the rubbers type. They are shown in Figure 4.6. In this case cracks are formed when the molecular interactions result in visible supramolecular changes. The parameters of cracking are time of ﬁrst appearance (to), shape and sizes of cracks and time of break down (tc) of the samples depend on: 1) the elongation extent; 2) the extent of static and/or dynamic deformations and their frequencies; 3) rubber type; 4) ozone concentration; 5) temperature; 6) for vulcanisates - ingredients and the preparation conditions [40]. Scheme 4.1 shows the inﬂuence of external deformation on ozone interaction with rubbers.

259

Ozonation of Organic and Polymer Compounds

1

2

3

4

Figure 4.6 Shape of the cracks depends on the type of rubber at E = 20%, 20 oC, 10 ppm ozone. 1 - NR; 2 - SKI-3; 3 - SKD; 4 - Bulex 1500

In this case, F the force acts along the longitudinal axis of the macromolecules. The ﬁrst step which is not shown in the Scheme includes the PO formation which decomposes further to zwitterion and aldehyde. The applied force facilitates the O-O and C-C bond cleavage as part of the stretching mechanical energy is converted into bond energy thus forming zwitterion and aldehyde. The next step, as suggested by the Criegee mechanism, includes the reaction of the zwitterion in or outside the cage. The force application impedes the ozonides formation since it hinders the orientation of the broken ends of the macromolecules. Thus the macromolecule is cleaved at the place of the double bonds and a crack is formed. This Scheme exempliﬁes the relation between the molecular changes and their inﬂuence at the macro level [41-45]. The picture of crack formation depends dramatically on the extent of elongation. When natural rubber samples elongated to different extents, were ozonated, it was found out that the number of cracks linearly increases with increasing elongation up to 82% and the crack depth goes through the maximum at 32%. According to our analysis, the degradation during the time of processing is dominated by one or another reason, but is totally the result of the joint action of different factors and the main one among them is the chemical reaction of ozone with the double bonds of the polymeric chains straining under the stress.

260

Degradation and Stabilisation of Rubber

Scheme 4.1

4.2 Antiozonants The diene elastomers: natural and synthetic butadiene, isoprene, butadiene-styrene rubbers which compose the major part (over 80%) of all rubber products are generally exposed to ozone degradation. Synthetic rubbers such as polychloroprene, butadienenitrile, polyisobutylene, ethylene-propylene, ﬂuoro, silicon, polyurethane, and so on are relatively more stable when exposed to ozone. In our studies we have focussed our attention on the ozone degradation of conventional rubbers. As we have already pointed out, the ozone concentration in the atmosphere varies in the range of 0.2-0.6 ppm. However, it can reach values up to 60-100 ppm in some areas, arising from the photochemical oxidation of organic compounds in the presence of nitrogen oxides in the polluted city atmosphere (smog), photochemical decay of algae and sea organisms on the seashore, the ultraviolet (UV) and microwave radiation, electric charges and other radiation in atmospheric oxygen. As the degradation rate is proportional to the ozone concentration (Equation 4.3) it is obvious that in these areas the degradation will increase almost two-fold.

261

Ozonation of Organic and Polymer Compounds The basic protection against ozone involves the application of small amounts up to 10 phr (parts per 100 rubber) of organic, inorganic, polymeric and other additives, called antiozonants as components of the rubber compositions. The creation and selection of antiozonants is a complex problem and irrespective of the numerous empirical approaches there is no, so far generally accepted, quantitative theory on this problem. There are, however, several basic requirements and rules which direct the synthesis and selection of the compounds suitable for application as antiozonants: They should: UÊ ÃÕÀiÊ «À}i`Ê âiÊ ÀiÃÃÌ>ViÊ vÊ ÌiÊ ÀÕLLiÀÊ >ÌiÀ>ÃÊ Õ`iÀÊ ÃÌ>ÌVÊ >`Ê dynamic conditions. UÊ iÊÌiÀ>ÞÊÃÌ>Li]Ê>ÛiÊ>Ê>ÌÃVÀV}ÊivviVÌÊ>`ÊLiÊwÀiÊÃ>vi° UÊ iÊV«>ÌLiÊÜÌÊÌiÊÀÕLLiÀÊÝÌÕÀiÃ]ÊÌÊ`ÃVÕÀ]Ê`vvÕÃiÊi>ÃÞÊÊÌiÊ«ÞiÀÊ matrix and not be easily washed away. UÊ iÊVÕÀiÃÃÊ>`ÊÃÌ>LiÊ`ÕÀ}ÊÃÌÀ>}i° UÊ iÊÃ>vi° UÊ /iÊ À>ÜÊ >ÌiÀ>ÃÊ >`Ê ÌiV}iÃÊ vÀÊ ÌiÀÊ «Ài«>À>ÌÊ ÃÕ`Ê LiÊ i>ÃÞÊ available. In addition, according to the assumed mechanism of ozone interaction with rubber solutions, discussed in the previous chapter, the following speciﬁc requirements can be formulated: 1. Antiozonants (AO) can preserve the elastomers if: Ê

UÊ

Ê "Ê>ÀiÊÀi>VÌi`ÊÜÌÊâiÊv>ÃÌiÀÊÌiÊâiÊÜÌÊi>ÃÌiÀÊ>`ÊvÊ>ÌÊÌiÊÃ>iÊ time the AO diffusion rate from the bulk to the surface of the elastomers is correspondingly.

Ê

UÊ

Ê"Ê>ÀiÊvÀi`ÊV«iÝiÃÊÜÌÊâiÊÀÊ`iV«Ãi`ÊÌÊV>Ì>ÞÌV>Þ°

Ê

UÊ

Ê"Ê>ÀiÊ`i>VÌÛ>Ìi`Ê r ÊL`ÃÊLÞÊvÀ}ÊV«iÝiÃÊÜÌÊÌi°

Ê

UÊ

Ê iÊ"Ê>ÀiÊ«ÞvÕVÌ>ÊV«Õ`ÃÊÌiÞÊV>Êii«ÊÌiÊiVÕ>ÀÊÜ}ÌÊvÊ / the elastomers by networking of the broken parts of the macromolecules.

2. Antiozonants and their ozonation products should physically prevent the ozone having access to the C = C bonds by: Ê

UÊ

262

i}ÊiÀÌÊV«Õ`ÃÊÌÜ>À`ÃÊÌiÊâiÊ>ÌÌ>V°

Degradation and Stabilisation of Rubber Ê

UÊ

Ê >VÌ>Ì}ÊÌiÊÀi>Ý>ÌÊvÊÌiÊÃÌÀ>ÊvÀi`]Ê°i°]ÊÌiÞÊ>VÌÊ>ÃÊ>Ìv>Ì}ÕiÊ additives.

It is obvious that practically all these requirements cannot be satisﬁed by a single compound. That is why the stabilisation of elastomers is carried out by using stabilising systems - mixtures of various antiozonants which could ensure a complex protection, both chemical and physical one. The rate constants of ozone reaction with amines, hydroquinolines, thiosemicarbazides, some aminophosphorus compounds, thiophenols, aminophenols, and so on are up to 3-4 orders higher than with elastomers [48]. Among them the derivatives of N,Nasubstituted-p-phenylenediamines (PPHDA), the salts of dialkyldithiocarbamates, dihydroquinolines, and so on are the most powerful antiozonants. Some inorganic compounds such as dithiocarbamate and dithiophospate complexes of some alkaliearth and transition metals also exhibit good antiozonant properties. It has been also found that the oxides of the latter are good catalysts of ozone decomposition. However, the rate constants of ozone reaction with saturated organic compounds such as parafﬁns and their oxygen-, halogeno-, nitrile, nitro-, thio-, sulfo-, phosphocontaining derivatives, waxes, i.e., parafﬁn mixtures with deﬁnite molecular mass distribution, polyoleﬁns, saturated ﬂuoro-, chloro- and silicon containing elastomers [48] appear to be much lower (up to 4-6 orders) than with elastomers. These compounds prevent the degradation process by reducing the ozone access to the elastomer, form surface layers, and at the simultaneous application with low molecular antiozonants guarantee favourable conditions for their diffusion to the reaction zone. Saturated polymer additives also operate through a kinetic factor associated with the decrease of the surface C=C bonds. Many organic and inorganic metal-containing compounds and complexes, the active and nonactive ﬁllers in the rubbers compositions act as deactivators of the double bond. Such compounds are those formed during the vulcanisation of rubbers with ions of copper, magnesium, nickel, zinc and other metals present. The formation of complex and surface compounds with participation of the C=C bonds substantially decrease their reactivity towards the ozone action. The derivatives of PPHDA and dihydroquinolines also exert crosslinking functions and the latter are the best known as antifatigue additives.

4.2.1 Mechanism of Action The use of stabilising additives as antiozonants depends primarily on the type of rubbers and the ﬁeld of their application for producing rubber goods and car and trunk pneumatic tyres [50].

263

Ozonation of Organic and Polymer Compounds The selection of a particular antiozonants is based on considering the probability of its interaction with the various ingredients of the rubber composition and its compatibility with the vulcanising agents [51]. PPHDA being one of the most efﬁcient and widely applied antiozonants in the rubber industry has been intensively studied with a view of elaborating its mechanism of action. The most widespread mechanism for the antiozonants action is the kinetic approach based on the concurrent interaction of ozone with antiozonants and polydiene [52-56]. A characteristic peculiarity of these antiozonants is that they alone or their ozonation products can form a protective surface ﬁlm [57] thus preventing the ozone having access to rubber [58]. Lorenz and Parks [35] considered that the antiozonants react mainly via three reaction routes: 1) Directly with ozone; 2) With the peroxide compounds reducing them; 3) With two polymer molecules containing aldehyde groups. Rakovsky and co-workers [59, 60] related the antiozonants efﬁciency to the inhibition of free radical reactions initiated by ozone. Braden and Gent [62] distinguished two basic classes of antiozonants: 1) antiozonants that prevent the crack growth, and, 2) antiozonants that increase the critical energy of crack growth. Storey and co-workers, [62] attributed the high antiozonant activity of PPHDA to the presence of two conjugated nitrogen atoms, one of which can act as a nucleophile due to the presence of the second one. The different efﬁciencies of antiozonants is due to the various nucleophility of the nitrogen atoms, which in its turn affects the decomposition rate of the peroxide compounds on the rubber surface. The stronger the nucleophilic effect is, the higher the rate of nitroxyl radical formation. The latter initiate reactions with short kinetic chains and form cross bonds thus reducing the number of adjacent double bonds. Alkyl substituents that increase the nucleophility of the nitrogen atoms increase the antiozonants activity in comparison with those without aryl substituents. This action of PPHDA is conﬁrmed by the fact that ozone penetration in rubber is at depth of about 1.3 s 10-5 m. Other authors stated that the bifunctional antiozonants prevent the growth and formation of cracks in elastomers through combining the two zwitterions, obtained

264

Degradation and Stabilisation of Rubber from the decomposition of two PO, thus forming a new bond between the broken ends of the macromolecules [63, 64]. The protective action of antiozonants is also associated with the relative stability of the free radicals generated from them during the ozone degradation of elastomers [65]. Michaelis [66] has shown that the more effective PPHDA lead to the formation of stable semiquinone structures while the less active ones form unstable ones. On the basis of these results Barnhard and Newby [67] assumed that the stable semiquinone structure prevents the macromolecule degradation through destroying the ozonides and peroxides. In our studies on SKI-3 degradation in the presence of various PPHDA it has been demonstrated that some of them exhibit initiating properties in dependence on the applied deformations [68, 69]. The foregoing survey shows that the antiozonant action of PPHDA is a rather complex and ambiguous process. We believe that the study on the kinetics and mechanism of antiozonants action and the establishment of relationships for prediction of their efﬁciency or for the synthesis of new antiozonants with deﬁnite properties is of great practical importance [70-73]. Bailey suggested the following pathways for ozone interaction with di-tert-butylamine as a low molecular model of PPHDA- antiozonants: H (C 4H 9) 2NH + O

(C 4H 9) 2NOOO

3

H (C 4H 9) 2NOOO

(C 4H 9) 2NO + O

(C 4H 9) 2NO + HO

2

(C 4H 9) 2NOOO

3

O C 4H 9NO 2 + C 4H 9O + O

(C 4H 9) 2NOOO

2

O

(C 4H 9) 2NOOO

+

(C 4H 9) 2NOC 4H 9 +

(C 4H 9) 2NO

C 4H 9NO 2 + O

2

O OOO (C 4H 9) 2NOC 4H 9 + O

C 4H 9O

C 4H 9O + RH

(C 4H 9) 2NOC 4H 9

3

CH 3COCH

3

+ CH

(C 4H 9) 2NO + C

4H 9O

+ O

3

C 4H 9OH + R

265

Ozonation of Organic and Polymer Compounds The electron spin resonance (ESR)-spectra characteristic for the nitroxyl radical, as well as the 2-methyl-dinitromethane, tert-butyl alcohol, acetone and oxygen which were obtained conﬁrm this mechanism. One molecule of amine decomposes up to four ozone molecules. If one takes into account the possibility of ozone interaction with radicals, without nitrogen, then the number of decomposed ozone molecules should be greater which additionally enhances the amine efﬁciency. The inﬂuence of ozone on pure rubber and antiozonants solutions in inert solvents has been the subject of numerous investigations. These studies have contributed to the discovery of some of the basic steps of antiozonants transformations and rubber decomposition products, which can undergo mutual interactions and affect the elastomers degradation process [74-76]. Upon the ozone reaction with elastomers with low C=C bonds content or with electronaccepting substituents at the C=C bonds such as butylrubber, polychloroprene, and so on, the basic reactions of degradation are related either to the chain-radical route or molecular reactions of ozone addition to the C-H bonds and isomerisation of RO2 and RO radicals. We have evaluated the role of ozone as an initiator of chain-radical oxidation processes in Chapter 1. The protection of such rubbers is accomplished via the antiozonants reaction with ozone thus converting the active oxy- and peroxyradicals into non active molecular products. Thiosemicarbamates and thiosemicarbazides [77, 78] have been used as good, noncolouring antiozonants instead of PPHDA. These compounds easily donate electrons and the values of k for their reaction with ozone amount to 106-107 M-1.s-1 (by about 2-3 orders higher than k found for the ozone reaction with C=C bonds in elastomers) and Ea = 1.3 kcal/mol [79, 80]. It is known that the value of the Ea of diffusion is of the same order while that for solid state may be higher. This suggests that the degradation reaction will be limited by the mass transfer rather than by the change of temperature. For this reason the antiozonants should be compatible with the rubber mixture and should easily diffuse in the polymer matrix. As we have mentioned previously the stabilising compositions are composed of various antiozonants or of antiozonant blends with polyoleﬁns and saturated elastomers such as butyl rubber or triple copolymers of ethylene with propylene and diene, with ozone resistant elastomers like chorine, ﬂuorine or nitrile rubber, and so on. The antiozonants action can be enhanced by the use of synergetic mixtures and the ozone resistance of the rubber products can be increased by using antiozonants mixtures spread on the surface. The latter are antiozonants solutions in gasoline, chloroform, tetrachloromethane, turpentine, and so on.

266

Degradation and Stabilisation of Rubber

4.2.2 Synthesis The organic compounds used as antiozonants and antioxidants usually contain various numbers of heteroatoms: N1, N2, N3, N4, N1O2, O2S1, O1, P2S4, and so on. The synthetic methods for their preparation depend on the type of the antiozonants and the initial products which are mostly obtained from petroleum.

4.2.2.1 Paraphenylenediamine In our [81-83] and other studies [84-84] it has been convincingly established that among the great variety of chemical compounds used as antiozonants the substituted N,N-dialkyl, diaryl- or alkylaryl-PPHDA are the most powerful antiozonants exhibiting a broad spectrum of useful properties. The modern tendency in PPHDA antiozonant preparation is closely associated with the creation of new high-end technologies. The latter require that the whole process of additives preparation including the separate reactions of amination, alkylation (arylation), reduction and oligomerisation up to the commercial state of the antiozonants should be carried out in one reactor by using a highly selective and efﬁcient catalytic system. The semi-products for PPHDA preparation: p-chloronitrobenzene, p-dinitrobenzene, p-chlorophenol, p-dichlorobenzene, phenylp-phenylenediamine, and naphthyl-p-phenyldiamine, in fact become semi-products for producing the commercial antiozonants. The search is basically directed to expanding the range of raw materials by including low-cost and available products from petroleum processing, coke preparation, and so on.

4.2.2.1.1 Alkylation in the Presence of Hydrogen Catalytic alkylation in the presence of hydrogen is widely applied to synthesis and industrial preparation of PPHDA-antiozonants. Catalysts based on cerium, colbalt, nickel, palladium, platinum and zirconium are most suitable for reducing the alkylation process. This is a process of introducing an alkyl group into the molecule by the interaction of amino, nitro or nitroso compounds with ketones, aldehydes or alcohols in the presence of hydrogen. The most efﬁcient antiozonants are obtained upon alkylation with ketones, mainly, unsymmetrical one like methylethylketones with alkyl substituent varying from C3 to C5-C7. The most used ketones are methyl ethylketone (MEK), methyl isobutylketone, methyl isoamylketone, methyl hexylketone, and so on. Alcohols should be preliminarily dehydrogenated, which makes the process more expensive.

267

Ozonation of Organic and Polymer Compounds N,N´-dialkyl-PPHDA is obtained from a mixture of p-nitroaniline or p-phenylenediamine with at least 2 moles of aldehydes or ketones, hydrogenated at a temperature of 100200 oC and a hydrogen pressure of 1-20 MPa in the presence of CuO, Cr2O3 and BaO or their mixtures [87]. Other catalysts used for the same purpose are: CaO, Ce2O3 and BaO, Ba, CuCrO2 or Cu/ZnO [88], with a ratio between the ketone and the p-nitroaniline in the range from 2:1 to 10:1 under hydrogen pressure of 140 at a temperature over 100 oC. The interaction of paranitroaniline with MEK in the presence of copper chromite at PH2 – 5 MPa and temperature of 110-160 oC leads to N,Na-di-sec-butyl-PPHDA. The reaction proceeds according to the scheme: H 2N

NO2 + 3H2

H 2N

NH2 + 2H2O

H 2N

NH2 + 2 CH3COC2H5 + 2H2

C2H5CH(CH3)HN

NH(CH3)HCH5C2H5

Patents [91, 92] concerning the conversion of paranitroaniline into N,Na-di-sec-butyl PPHDA, the most suitable conditions for this reaction are a temperature of 180-200 o C, PH2 = 3-6 MPa and a molar MEK: PPHDA ratio = 10. Other conditions for the alkylation of p-nitroaniline with MEK are presented in [91-99]. Copper chromite has been used as a catalyst not only in the above reaction but also in the preparation of N-(1-Methyl-hexyl)-N´-phenyl-benzene-1,4-diamine(4C6H5NCH6H4NHCH(CH)3C5H11) from 4-aminodiphenyl amine and methyl hexyl ketone in a ratio of 1:2 [100]. Substitution of 4-CH3O-4-CH3-pentene-2-one for methyl hexyl ketone leads to the formation of 4(4-CH3O-4-CH3-pentyl)-amino diphenyl amine [101, 102]. Alkylation with a sec-butyl alcohol proceeds in the presence of 15% Ni, 5% Cu and 1% Mn on SiO2 and PH2 = 1.4 MPa [103]. The yield of the ﬁnal product increases after re-distillation of aniline or p-nitroaniline [104, 105]. When PPHDA or N-aryl-N-(4-aminophenyl) amine is alkylated with furfurol or tetrahydrofurfuryl alcohol, N, N´-difurfuryl-PPHDA is obtained on the Pd/C LiAlH4 or Raney-nickel catalyst [106, 107]:

H2N

NH2

+

2

CH2OH O

CH2 HN O

268

NH CH2 O

5% Pd/C

Degradation and Stabilisation of Rubber If a mixture of diphenylamine and furfuryl alcohol is allowed to stand for 80 days at room temperature, 4,5-di(diphenylamino) cyclopentene-2-one with antiozonant properties is obtained [107]. Diatomite-supported nickel (63%) is used for the preparation of N(1,3-dimethylbutyl)Na-phenyl-PPHDA from 4-amino-diphenylamine and methyl isobutyl ketone in a ratio of 1:2 [108, 109]:

C6H5-C6H4-NH2 + CH3COCH2CH(CH3)2 o

atm, 165 C }50-60 }}}} }m

(CH3)CH2CH(CH3)CH2NH-C6H4-NH-C6H5

Modiﬁcation of nickel catalysts with cobalt leads to an increase of its selectivity [110]. This catalyst is very active and can cause hydrogenation of the phenol ring. That is why addition of negligible amounts of organic sulfur compounds such as sulfates, mercaptans, thio acids and sulﬁdes to the reaction mixture is recommended [111, 112]. Selenium and tellerium as catalysts preserve their activity for a long time. They are not poisoned by sulfur compounds (no puriﬁcation of hydrogen is needed) and lead practically to no side reactions [113, 114]. Nickel catalysts are also used during the conversion of N-alkyl-4-nitrosoaniline into N-alkyl-N-phenylenediamine, while iron-copper-chromium catalysts are utilised in the reaction of the N-alkyl-N-phenylenediamine with ketones. The yield (74%) can be enhanced up to 96.3% by the addition of 0.34% triphenyl methane [115, 116]. One of the most effective antiozonants, N-(1,3-di-methyl-butyl)- N´-phenyl-PPHDA is obtained from nitro-, nitroso- or p-aminophenylenediamine and iso-butylmethyl ketone in the presence of Pd/C catalyst under hydrogen atmosphere [117-121]. The reaction proceeds according to the following general scheme:

269

Ozonation of Organic and Polymer Compounds

NH

NO2

H2 NH

NO

+

C H 3C O C H 2C H ( C H 3) 2 -H 2O

NH 2

NH

NH

N H C H ( C H 3) C H 2C H ( C H 3) 2

If alkylation is conducted with acetone at a hydrogen pressure ranging from 0.5 to 30 MPa and a temperature of 100-250 oC, the ﬁnal products will be N-isopropylNa-phenyl-PPHDA [122, 123]: C6H5NHC6H4NO + CH3COCH3 = C6H5NHC6H4NHCH(CH3)2 The yield can be enhanced by modiﬁcation of the catalyst with thiocarbamide [125]. Another catalyst that is used is platinum (0.2-2%) on an appropriate support. This catalyst is reduced at a temperature of 95 oC and its activity is enhanced by addition of (CH3)2S or hexyl mercaptan [126, 127]. When platinium catalysts are used, the initial p-nitrosodiphenylamine should contain neither sulfur nor chlorine since these elements deactivate the catalyst [128]. N,N´-di-(1-ethyl-3-methylphenyl)-PPHDA, N,N´-di-(1-methyl-heptyl)-PPHDA and N,N´-(1,3-dimethyl-pentyl)-PPHDA obtained on 1% Pt/C belong to the widely applied antiozonants [129, 130]. N,N´-di-(1,3,3-trimethyl)-PPHDA with a mp of 98 oC is more appropriate from a technological point of view than are liquid PPHDA. It is obtained from p-nitroaniline and methyl-neopentylketone according to the scheme:

270

Degradation and Stabilisation of Rubber

O 2N C 6H 4N H 2

+

2C H 3C O C H 2C ( C H 3) 3

C H C O O H , Pt/ C 3

P

H

2

( C H 3) 3C C H 2C H ( C H 3) N H C 6H 4N H C H ( C H 3) C H 2C ( C H 3) 3

The preparation of lower-toxicity antiozonants such as N-(1,3-dimethylbutyl)- N´(1,4-dimethylpentyl)-PPHDA and N-isopropyl- N´-methylpentyl-PPHDA is described in [131-133]. The presence of halogens in the aliphatic part of the PPHDA increase their activity as is shown for the example of N-(2-chloroallyl)-N-phenyl-PPHDA. It has a 75% higher activity than N-alkyl-N´-phenyl-PPHDA [134]. A mixture of differently substituted PPHDA is prepared by reducing alkylation of 4-nitrodiphenyl or (N-isopropyl-p-nitroaniline, p-nitroaniline or PPHDA) with two or more ketones at a temperature of 25-120 oC, a hydrogen pressure of 2.8 MPa and 1% Pd/C catalyst [135, 136]. Hydrogenation of a mixture of O2NC 6H 4NH 2 and the ketones CH3COCH 2 CH(CH 3) 2 and CH 3COCH 2CH 2CH(CH 3) 2 in a ratio of 1:2 leads to 4.2% p-(CH 3) 2CHCH 2CHCH 3NH 2C 6H 4 and 41.8% (CH 3) 2CHNC 6H 4NHCH(CH 3) CH2CH(CH3)2 [137]. When the ratio between the ketones is 1:1, about 20% of symmetric di-sec-alkyl-PPHDA is also obtained [137]. The products of the reaction between p-nitroaniline and a mixture of methyl-iso-butylketone and methyl-isoamylketone (1:1) are 18.6% N,N´-di(1,3-dimethylbutyl)-PPHDA, 30.6% N,N´-di(1,4dimethylpentyl)-PPHDA and 50.2% N-1,3-dimethylbutyl- N´-(1,4-dimethylpentyl)PPHDA. We have synthesised a number of complex catalysts based on Pt(II) and Pd(II) with phenyl-azo-2-naphthol: (1), 1,2,4-trianthraquinone (purpurin) (2), 8-oxyquinoline (3), anthranilic acid (4), chloroanilic acid (5) and dimethylglyoxime (6) on G-Al2O3 [138]. The complexes are obtained by the treatment of G-Al2O3 supported ligands (mentioned previously) with aqueous solutions of K2PtCl4, K2PdCl4, Pd(OAc)2 at a metal ion:ligand ratio of 1:2.5:

271

Ozonation of Organic and Polymer Compounds

N 2

N

A l2O 3

+

N

K 2M eC l 4

-K C l -H C l

O

N

Me

OH

N

O

N

A l2O 3

1 A l2O 3 O O

O

HO

O

O

O

A l2O 3

Me

;

O

O

N

OH

O Me

O

N 3

2

A l2O 3

O C

H 2N

O 4

CH 3 O O

Me C

A l2O 3

OH

Cl

O

Cl

O

O

O

5

CH 3

N

N Me

H

Cl

O

Cl

HO

Me A l2O 3

O

O

NH2

O

N

O O

H

N

O A l2O 3

H 3C

CH 3

O

O Me

HO

O

N

O 7

6 A l2O 3

The supported metal content is 0.5-3 wt%. In a separate test with the homogeneous and immobilised complex (1) we have established that the immobilisation does not affect the catalyst activity. This parameter has been evaluated in a model reaction of nitrobenzene hydrogenation in various solvents (Table 4.2).

272

Degradation and Stabilisation of Rubber

Table 4.2 Catalytic activity of Pd(II)-complexes in nitrobenzene hydrogenation to aniline. Catalyst weight: 0.1 g, Pd = 0.62%, nitrobenzene = 1 mmol, 67 ºC, PH2 = 10,325 Pa. Catalysts

Solvent

Rate of Hydrogenation [mol H2/(min.g.atom Pd)]

Pd(purpurin)2

Octane

62.3

Pd(purpurin)2

Heptane

47.2

Pd(purpurin)2

Decane

32.0

Pd(purpurin)2

Cycloheptane

22.0

Pd(purpurin)2

DMFA

21.2

Pd(purpurin)2

Ethanole

22.4

Pd(purpurin)2 + 8-oxyquinoline

Octane/DMFA

59.5/27.1

Pd(purpurin)2 + phenylazonaphthol

Octane/DMFA

59.5/27.1

Pd(purpurin)2 + dimethylglyoxime

Octane/DMFA

47.7/30.9

Pd(purpurin)2 + chloroanile

Octane/DMFA

18.5/16.9

DMFA = N,N-dimethylformamide

The catalysts preserve their activity for a long time, even after 14-fold application. These catalysts hydrogenate nitrobenzene to phenylhydroxylamine in DMFA and to aniline in alcohols. The reaction kinetics was followed by determination of the ozone uptake, chromatographically (high-performance liquid chromatography, gas and thinlayer chromatography), the nitrobenzene consumption and phenylhydroxylamine and aniline formation. The strong complexing functions of DMFA stabilise phenylhydroxylamine and the reaction stops kinetically at the absorption of 2 moles hydrogen, while in alcohols (ethanol, iso-propanol) the reaction goes on up to 3 mol ozone uptake forming aniline. The hydrogenation of p-nitroaniline in DMFA in the presence of Pd(purpurin) gives a quanitative yield of p-phenyldiamine.

4.2.2.1.2 Alkylation in the Absence of Hydrogen Substituted p-phenyldiamines can be obtained without using hydrogen and expensive noble metals as catalysts [139, 140]. When N-alkyl-4-nitroaniline is mixed with

273

Ozonation of Organic and Polymer Compounds n-hexanol and 59% NaOH, and heated for 1 hour at a temperature of 100 oC, it yields monosubsituted PPHDA: RHNC6H3(R1)NO2 +2R2CH2OH m RHNC6H3(R1)NH2 where: R is a C1-C12 alkyl or naphthyl radical, and R1 is a C1-C12 alkyl radical. Monosubstituted PPHDA can also be obtained by interaction of N-substituted aminophenols with ammonium in the presence of an acid catalyst (Lewis acid): RHNC6H4OH + HNH2 m RHNC6H4NH2 + H2O Other catalysts used in this reaction are FeCl3 or a mixture of free iodine and powdery iron in a ratio of 6:1 [141, 142]: RNH2 + HOC6H4NH2 (RNHC6H4NH2 + H2O where: R = C6H5-, C6H4CH3 or C6H11-. Heating at 215-220 oC in an nitrogen atmosphere of a mixture of aniline, cyclohexylamine and N-(1,3-dimethylbutyl)-p-aminophenol in the presence of iodine, powdery iron and NH 4Cl leads to the formation of 10% N,N´-(1,3dimethylbutyl)-PPHDA, 5.5% N-cyclohexyl-N´-(1,3-dimethylbutyl)PPHDA, 30% N-(1,3-dimethylbutyl)-N´-phenyl-PPHDA, 8% N-cyclohexyl-Na-phenyl-PPHDA and 9% N,N´-diphenyl-PPHDA as ﬁnal products [143]. Substituted N,N´-diphenyl-PPHDA with low melting points appear after a reaction of hydroquinone with a mixture of amines (35% aniline, 25% toluidine and 40% xylidines) during heating in the presence of a catalyst (FeCl3) of condensation [144]. Reaction of substituted PPHDA with thiols or formaldehyde in an acid medium results in their conversion into active stabilisers having a complex behaviour against the action of oxygen, ozone, light and heat [145-147]. Their general formula is: R1-S-CH2-C6H2-(R2)(R3)-NH-C6H4-NH-R4 where: R1 is alkyl C1 - C20, aryl C6 - C14, alkylaryl; R2 and R3 are H, alkyl C1-C7, aryl C6-C14 and R4 is [-C6H4-CH2-S-R1].

274

Degradation and Stabilisation of Rubber Alkylated 2,4-toluenediamines are good antiozonants and antioxidants, which interrupt the chain-radical process taking place during the oxidation of polymeric materials [148]. A mixture of 2,4-toluenediamine, dicyclopentadiene and pentene is stirred in the presence of a catalyst (12% Li and 87% Si) under elevated pressure in a nitrogen atmosphere. A mixture of 3-(cyclo-pent-2-enyl)-2,4-toluenediamine and 5-(cyclopent-2-enyl)-2,4-toluene is obtained. The conversion of 2,4-toluenediamine amounts to 64%.

4.2.2.2 Hydroquinolines Hydroquinolines are prepared by the classical method of Scraup using a reaction of aromatic amines with aldehydes or ketones in the presence of oxidants and acid catalysts according to the following scheme:

OHC OHC

NH2 +

CH

CH 2

CH 2

CH 2 N H

HOHC CH CH 2 N H

-H O 2

N H

A mixture of aniline and C8-C15-alkylated amines (45% C12, 15% C11 and 12% C10) in an acid medium is heated up to a temperature of 140 oC for 12 hours, after which acetone is added. The ﬁnal product is an oil containing 58% 6-dodecyl-1,2dixydro-2,2,4-trimethylquinoline, 6% dodecylaniline and oligomer of 1,2-dihydro2,2,4-trimethylquinoline [149, 150]. Addition of acetone to 4,4a-(ethylenedioxyl)-dianiline for 12 hours in the presence of benzenesulfonic acid at 150-160 oC yields 6,6a(ethylenedioxyl)-di-1,2-dihydro-2,2,4trimethylquinoline [151]. When 2,2,4-trimethyl-1,2-dihydroquinoline is allowed to polymerise in the presence of hydrochloric acid, an active polymeric antiozonant is obtained. The monomer is formed during reaction of aniline with acetone at a temperature of 120 oC in the presence of p-toluenesulfonic acid as a catalyst [152]. The polymer is isolated by dissolution of the reaction mixture in benzene, followed by neutralisation and evaporation of the solvent. The reaction mixture can be dispersed in water and then granulated. The antiozonants obtained are usually applied in a combination with N-phenyl- N´-isopropyl-PPHDA [153, 154].

275

Ozonation of Organic and Polymer Compounds

4.2.2.3 N,N´-Disubstituted Hexahydropyrimidines These are nonstaining antioxidants [155], which are prepared by condensation of cyclohexanone or its derivatives with ammonia. Tetrahydropyramidine is formed ﬁrst and then reduced to hexahydropyrimidine whose pyramidine ring is open to form diamine. The two amino groups form bonds with aldehydes and yield N,N´substituted hexahydropyrimidine. The reaction proceeds as follows:

3

+ 2N H 3 O

N

-H 2O

NH

H2

HN

NH

H2

( C H 3) 2C H C H O , H 2

H 2N

NH

C H 2O

( C H 3) 2C H C H 2H N ( C H 3) 2C H C H 2H N

NH

NH

4.2.2.4 N-Substituted Dimethylpyrols These compounds are also nonstaining antiozonants [156]. They are obtained from 1,4-dicarbonyl compounds passing through an enol intermediate which reacts with amines giving:

CH 3 Z -alkylene-N CH 3 where Z is substituted NH2; RNH-; R2N-; R-NH-C(S)-N=N- or -(R)N-C(S)-NH2

4.2.2.5 Enamines The enamines are nonstaining antiozonants with the general formula: (R1)(R)N-HC=C(R2)(R3)

276

Degradation and Stabilisation of Rubber where: R1, R2 and R3 are alkyl groups with C1 - C6. These compounds are prepared by condensation of amines with ketones. The amines used are pyrolidine, pyperidine, substituted morpholines, and so on: RR1NH + (R2)(R3)CHC(R4)O m (R)(R1)N-(R4)C=C(R2)(R3) + H2O If the ketones are unsymmetric, a mixture of products is obtained. Condensation of 2,6-dimethylmorpholine with 2-ethylhexanal yields 1-(2,6-dimethylmorpholino)2-ethyl-hexene-1 which is usually applied for natural rubber and butadienestyrene rubber [157]: RCH2CHO +HN(R')2 m RCH2CH(OH)N(R')2 k (RCH=CHN(R')2 + H2O

CH 3 O

CH 3

N H + O H C -C H ( C 2 H 5 ) ( C H 3 ) C H 3

N -C H =C ( C 2H 5) ( C H 3) C H 3

O

CH 3

CH 3

4.2.2.6 Nitrone Compounds The C-substituted or unsubstituted aryl-N-substituted nitrones are appropriate not only as antiozonants but also as antifatigue inhibitors [158]. Particularly effective in this respect are aldonitrones of the type:

R2

R4

R1

C H =N -R 6 R3

R5

O

where: R1 is a mono- or disubstituted aminogroup; R2 and R3 - alkyl or alkoxy groups; R4, R5 = R2 and R6 - iso-propyl, sec-butyl or tert-butyl group.

277

Ozonation of Organic and Polymer Compounds The nitrone compounds may be prepared by a variety of ways, for example: (a) by oxidation of the corresponding aryl-N-substituted or unsubstituted (branched alkyl or cycloalkyl) hydroxylamines; (b) by reaction of aryl ketones with primary hydroxylamine; (c) by N-alkylation of the corresponding oxime; (d) by reaction of the corresponding ketimine with primary hydroxylamine; (e) by oxidation of the corresponding N-substituted imine. The most used nitrone is 4-hydroxy-3,5-dimethyl-N-tert-butylnitrone obtained when N-tert-butyl-hydroxylamine and 3,5-dimethyl-4-hydroxybenzaldehyde in a ratio of 1:1 are dissolved in absolute alcohol and the mixture is allowed to stand for ﬁve days at room temperature: CH 3

CH 3 CH O

HO

+ H O N H -C ( C H 3 ) 3

CH 3

C H =N -C ( C H 3) 3

HO CH 3

O

4.2.2.7 Derivatives of 3(5)-Methylpyrazone The general formula of these compounds is:

where: R is -N

;

N

The ozone resistance of rubber mixtures based on natural rubber with ozonine-16 (pyperidinomethyl-3,5-methylpyrazole) and ozonine-51 (hexamethyleneimino-3,5methylpyrazole) exceeds 1.5 to 2 times that of mixtures containing Plastanox 2246 (2,2a-methylene-bis-4-methyl-6-tert-butylphenol) and is close to that reached by tributylcarbamide. A mixture of ozonine-16 or ozonine-51 with 2246 in a ratio of 1:1 displays the highest activity [159].

278

Degradation and Stabilisation of Rubber

4.2.2.8 Enolethers These compounds are used as antiozonants for natural and synthetic rubber. They are slightly staining. The following enolethers are synthesised: 2H-pyran-2-methyl(3,4-dihydro-2H-pyran-2-carboxylate); 3,4-dihydro-2,5(dimethyl or diisobutyl, didecyl)-2H-pyran-2-methyl-(3,4-dihydro-2,5-dimethyl-2H-pyran-2-carboxylate) with the general formula:

R1

R1

O C H 2 -O -C O

O

where: R is alkyl, aryl or alkylaryl radical. By condensation of 3,4-dihydro-2H-pyran-2-carboxyaldehyde and aluminium isopropylate with stirring for 4 hours at 40 oC, 75% of 3,4-dihydro-2H-pyran-2methyl-(3,4-dihydro-2H-pyran-2-carboxylate) is obtained [160]. The mixture of different enoethers results in better antiozonant activity. For example, a mixture of hexanediol-1, 6-di-(2-methyl-prop-1-ethyl)ether and 1,1a-bis-(2-methyl-prop-1epoxy)-dietheylether is prepared by aldol condensation of iso-butyl aldehyde and 1,2-hexanediol in the presence of toluene sulfonic acid, quinoline and cyclohexane as a solvent [161].

4.2.2.9 Ethers Ethers with the general formula:

are odourless antiozonants, where R1 is a hydrocarbon group which may contain a heteroatom; X is S or O; R2 and R3 are H, -CH3. These ethers are used in combination with R-CO-NH-NH2 (where: R is an alkyl group with C3-C10) and exert high antiozonant ability [162].

279

Ozonation of Organic and Polymer Compounds

4.2.2.10 Cyclic and Acyclic Acetals and Ketals Cyclic and acyclic acetals and ketals are nonstaining antiozonants with general formula [163-166]:

where: n and m range from 0 to 12; R1, R2=H, alkyl group with C1-C3, alkylene group containing halogens, cyclo or bicycloalkenes with C5-C7, aryl groups with C6 -C10 or alkylaryl groups. They are prepared by the reaction of alcohols with aldehydes or ketones in the presence of catalysts: RCHO + 2RCH2OH m RCH(OCH2R)2 (R)2CO + 2RCH2OH m (R)2C(OCH2R)2 The catalysts used are: H2SO4, H3PO4, p-toluene sulfonic acid, boron triﬂuoride, and so on. These cyclic acetals are usually applied with microcrystalline waxes [167]. Acetals and ketals with unsaturated cyclic groups are less volatile, have no odour and are compatible with rubbers [168].

4.2.2.11 Other Classes of Compounds

4.2.2.11.1 Bis-Alkylaminophenoxy Alkanes Bis-alkylaminophenoxy alkanes with the general formula:

(where: X=halogen, R= sec-alkyl radical with C3-C8, R' is -CH2-, -CH2-CH2-) are obtained from nitrophenol by reducing alkylation in the presence of Pt/C as a catalyst [169].

280

Degradation and Stabilisation of Rubber

4.2.2.11.2 Derivatives of 2,2,7,7,-Tetramethyl-1,4-Diazocyclopentane The derivatives of 2,2,7,7-tetramethyl-1,4-diazocyclopentane with the formula:

where: n is 1, Y is H, CH3, O and -NH, R denotes alkyl groups with C1 = C20; where n is 2, R is alkylene groups with C1-C20, cycloalkene groups with C7 -C18 or alkylaryl groups with C5 - C12, are also applied as antiozonants for polymers. Their preparation is accomplished in a reaction of 1,4-diazocyclopentane-5-one with isocyanate [170].

4.2.2.11.3 Bis-Cyclopentadienyl Compounds Bis-cyclopentadienyl compounds with the general formula:

where: X is A,A´-p-xylilene or 4,4´-oxydiphenyl-dimethylene, which are good antiozonants [171].

4.2.2.11.4 Alkylnaphthenes Alkylnaphthenes are applied as stabilisers of cables and insulating materials. They are obtained by alkylation of naphthaline with dodecane chloride in the presence of AlCl3 at 80 oC or 1-hexadecanol in the presence of BF3 at 130 oC: C10H8 + C12H25Cl j B-C12H25-C10H7 + HCl C10H8 + C16H33OH m B-C15H30-C10H7 + H2O The yield is 60% [172].

281

Ozonation of Organic and Polymer Compounds

4.2.2.11.5 Aminomethylene Derivatives of Furane Aminomethylene derivatives of furane are cheaper and nonstaining antiozonants [173].

4.2.2.11.6 Lactams Strong antiozonants are the lactams with the general formula:

where n = 3-20; m = 1-2; R - alkyl or phenyl radical. Comparison based on antiozonants activities indicates E-caprolactam and caprylactam to rank ﬁrst [174].

4.2.2.12 Sulfur-Containing Compounds The derivatives of isothiocarbamate used as antiozonants for natural rubber, butadiene rubber, isoprene rubber and butadienestyrene rubber are noncolouring and nonstaining, and have the general formula: R4-N=(SR1)-N-R2R3 where: R1, R2, R3 and R4 are alkyl C1-C30, phenylalkyl C1-C6 or cycloalkyl radicals [175]. A high antiozonant activity is exhibited by sulfides R-S-R [176], where R is [C6H5NHC6H4NHCO]nX, n is 1 or 2, X denotes methylene, ethylene or 1,2-propylene radicals. When R = H, the sulﬁdes are obtained by a reaction of 4-aminodiphenylamine with acids having the general formula: H-S-X-(-SO2H)n

282

Degradation and Stabilisation of Rubber If R' = -S-R, the acids should have the formula (HOOC)n-X-S-S-X-(COOH)n, (dithioacetic, B-dithiopropionic). The sulﬁdes obtained are bis-(4-anilinophenyl aminocarbonyl methyl)disulﬁde and bis-(4-aminophenyl aminocarbonyl ethyl) disulﬁde. These antiozonants are easily soluble in rubber compositions. Sulfonate antiozonants with the general formula R1R2SO3N(R3OH) (R is alkyl radical with C1-C13, R2 - aryl radical with C1-C6, R3 -alkylene with C4-C6) are nonstaining, nontoxic and, above all, cheap to prepare [177]. The initial substances used are alkylated products, resulting from the preparation of dodecyl benzene (hydrocarbon radicals with C24-C30) which is sulfonated with oleum and then reacted with alkanolamines at a pH of 7.5. The sulﬁdes of 3,4-dehydropyperidine are efﬁcient antiozonants. They have the general formula:

where: X = S, SO, SO2; Y, Y* = methyl, isopropyl, sec-butyl radicals. Di-4-(3,4-dihydro-2,2,6,6-tetramethyl-piperidine)-sulﬁde, sulfoxide or sulfone are prepared by interaction of:

with a gaseous mixture of hydrogen chloride and hydrogen sulﬁde [178, 179]. Ozone resistance is observed with the products of mercaptoaldehyde condensation with phenols [180-183]:

283

Ozonation of Organic and Polymer Compounds

where: R, R1 are H, CH3, tert-butyl radicals. Triazinedisulﬁde derivatives to be used as antiozonants are relatively easily prepared by a reaction of benzthiazoldisulﬁde with 2(4-morpholinyldithio)benzthiazole and 1,3,5-triazine dithiol in the presence of a neutralising agent (NaOH) [184, 185]. In comparison with zinc-dialkyldithiophosphates, dithiol derivatives exhibit a higher antiozonant activity. They have the general formula:

where: R1 and R2 are alkyl radicals with C1-C12; T is H or SR2; R2 denote: s alkyl radicals with C1-C20. The reaction of alkylated phenols with sulfur in the presence of a basic catalyst yields 4-(3,5-diisopropyl-4-hydroxyphenyl)-1,2-dithiole-3-thione [186, 187]:

HO

S S

2-Vinylphenothiazine enhances the stability of rubber products towards ozone, temperature and repeated deformation [188]. Butoxyethyl N,N´-dimethyldithiocarbamate is used as antiozonant of light rubbers [189]. 284

Degradation and Stabilisation of Rubber Nitrile antiozonants with the general formula:

R1

S

(CH)n

C

N

R2 where: R1 - alkyl radicals with C1-C18 or (CH2)n-CN and R2 - H, CH3 or C2H5 are nonstaining antiozonants and they are characterised with low toxicity (LD50 = 5300 mg/kg) and low production expenses [190].

4.2.2.13 Si-Containing Compounds Aminoorganosilanes with the general formula: RnSi(CH2NXY)4-n where: R - alkyl radical; X - aryl, alkoxyaryl, aminoaryl, phenyl, and naphthyl radical; Y - alkylaryl, aminoaryl radicals show a good antiozonant activity. Dimethyl-bis-tert butylhydroxysilane used for the stabilising of butyl rubber is prepared by interaction of 3,5-di-tert-butyl-4-hydroxy-benzyl alcohol with dimethylchlorosilane [191, 192]. The oligodiphenylsiloxyquinone, which is a good thermostabiliser, is isolated by heterophase condensation of diphenylchlorosilane with aromatic diols in the presence of suitable catalysts [193].

4.2.2.14 P-Containing Compounds Phosphites and phosphates of 5-norbornene-2-methanol are used as antiozonants of polychloroprene rubber and have the following general formula: HP(O)(OR)2, (RO)3P, (RO3)PO where: R is 5-norbornene-2-yl methyl. The interaction of PCl3 with 5-norbornene-2-methanol in a pyridine medium results in a mixture of bis-(5-norbornene-2-methyl) phosphite (17%), tris-(5-norbornene-2methyl) phosphate (22%) and tris-(5-norbornene-2-methyl) phosphate (15%). The

285

Ozonation of Organic and Polymer Compounds reaction with POCl3 leads to the formation of phosphates only [194]. Aryl phosphites are applied as stabilisers of synthetic rubber [195, 196]. Organic polymers are stabilised by amide esters of phosphonic acid having the general formula: P(ZR)(Z'R')(NR2R3), where: Z, Z' - O, NH; R2, R3, R and R' - linear or cyclic, substituted or unsubstituted aliphatic or aromatic radicals [197].

4.2.3 Application The stabilising systems on the basis of well-known inhibitors and their mixtures developed by various authors are presented in Table 4.3.

Table 4.3 Antiozonant (AO) systems for rubbers and speciﬁc protective properties No.

Stabilising system

Elastomer

Protecting ability

Reference

EPDM

Increasing the O3 resistance at elongation

[198]

1.

a) Para-phenylene-diamine (PPHDA) type AO; b) Imidazole

2.

a) N-Phenyl N´-isopropyl-PPHDA – (4010 NA) b) N-Pheny-B-naphthylamine (Neozone D)

Diene rubbers

O3

[199]

3.

a) Reaction products of acetone with diphenylamine b) Zn-2-mercapto-toluyl imidazole

EPDM

O2, O3, t0

[200]

4.

a) 2,6-di-tert-butyl-cresol b) 4,4a-thio-bis (6-tert-butyl)-mcresol (SC)

Styrenebutadiene rubbers (SBR)

O3, t0

[201]

286

Degradation and Stabilisation of Rubber

Nitrile rubbers

O3

[202]

a) N,N´-bis(2-hydroxy-4,6dimethylbenzyl)-piperazine b) 2,2a-methylene-bis (4-methyl, 6-tert-butylphenol) - (2246)

NR

t0

[203]

7.

a) Na-aluminium-methylsilicate

Carboxylate synthetic latexes

O3, nonstaining, nontoxic

[204]

8.

a) PPHDA type antiozonants – 1-4 php b) Imidazole – 0.2-4 php

Chlorosulfonated polyethylene

t0, O2

[205]

9.

a) PPHDA type AO – 0.1-5 php b) p-Di-nitrosoarene – 0.1 php

NR

O3, O2

[206]

10.

a) PPHDA type AO b) Bis-maleimide a:b=80-20:20-80

Vulcanisates

O3

[207]

11.

a) N,N´-Diphenyl-PPHDA, 0.1-5 php b) Petroleum type waxes, 0.1-10 php

Isoprene rubber

O3, nonstaining

[208]

12.

a) Polymerised 2,2,4-trimethyl1,2-dihydro-quinoline (Flectol H) b) 4010 NA c) N-Phenyl N´- (1,3dimethylbutyl) - PPHDA (S13) a:b:c = 50:25:25

NR

O3, inhibitor of ﬂex cracking, antifatiguing

[209]

13.

a) N,Na-Diphenyl-PPHDA b) N,Na-Ditoluyl - PPHDA a:b = 10-50:90-50

NR

O3, non-staining

[210]

14.

a) 4010 NA b) Montmorillonite

NR

O3

[211]

5.

Cd salts of I. ArNRArNR7-R1-COOH II. ArNHAr-R4-COOH III. OH(R5)(R6)ArR2 -COOH R - H, halogen atom, C1-C8 alkyl; R1 - C1-C20 alkylene, -CO-R2; R2 - C1-C20 alkylene; R3 - C1-C8 alkylene; R4 - -OCO-R2, R5 and R6 - H or C1-C8 alkyl, R7 - H or C1-C4 alkyl

6.

287

Ozonation of Organic and Polymer Compounds

15.

a) Phenothiazine b) N-Alkyl-N´-phenyl - PPHDA a:b = 20-60:80-40

NR

16.

a) Diisopropyl-di-thiophosphate b) Di-hydro-di-phenylpropane c) 4010 NA a:b:c = 1:0.5:0.5

SBR

O3, light protection

[213]

17.

a) S13 b) Flectol H c) 6-Anilino-2,2,4trimethylquinoline

Diene rubbers

O3, non-staining

[214]

18.

a) 4010 NA b) S13

Diene rubbers

O2, O3, t0

[215]

19.

a) Neozone D – 0.5-2 php b) 9-Phenyl-10-p-vinyl-phenylanthracene – 2-7 php

Vulcanisates

O3, t0

[216]

20.

a) Deca-cis-(nonylphenyl)hepta-cis-(di-propylenglycoloctaphosphite) b) N,N´-di-(2-octyl) – p-phenylenediamine a: 0.75-1.75 php b: 1.25-0.25 php

NR

O2

[217]

21.

a) N-iso-propyl-amino-di-phenylamine b) 2,6-Di-tert-butyl-phenol (Ionol)

Diene rubbers

t0

[218]

22.

a) Phenothiazine – 20-80% b) Flectol H - 80-20%

SBR

thermal, anti-fatiguing

[219]

23.

a) 2246 – 0.3-0.75% b) 2-Mercapto-benzimidazole (MBI) - 0.15-0.5%

NR

O2, O3

[220]

24.

a) 1,1,3-Tris-(2-methyl-4-hydroxy5-tert-butyl-phenyl) butane (Topanol CA) b) 4010NA a: 5-50; b: 95-50 wt%

Synthetic rubbers

O2

[221]

25.

a) 1,3-Bis-pyperidinomethyleneimidazoline-2-thion b) 2246

Diene rubbers

O3

[222]

288

O3

[212]

non-staining

Degradation and Stabilisation of Rubber

26.

a) S13 b) Flectol H

NR

O3

[223]

27.

a) N-(2-aminophenyl)phosphoramidate b) Zn-dihydrocarbyldihydrophosphate

NR and Synthetic rubbers

O3, non-staining

[224]

28.

a) S-Containing sulfur ester b) Amine or phenolic antiozonants

NR and Synthetic rubbers

t0, O2

[225]

29.

a) Topanol CA – 5-50 wt% b) Tributoxyethyl-phosphate - 5550 wt%

Synthetic rubber

O2, increase dispersity and penetration ability

[226]

30.

a) Diethyldithio-carbamate b) Co-diethyldithio-carbamate c) 4010NA

Diene rubbers

O3

227

31.

a) Phenylhydrazone b) Amine or S-containing AO

NR

O2

[228]

32.

a) MBI b) Flectol H

EPDM and diene rubbers

O3, O2

[229]

33.

a) 4010 NA b) Flectol H

Diene rubbers

O3, O2, anti-fatiguing

[230]

34.

a) 4010 NA - 1.7 php b) Flectol H - 1.3 php or c) 4010NA - 1.9 php d) SC - 1.1 php

Isoprene rubber, SBR

t0, O3

[231]

35.

a) 4010 NA b) Flectol H a:b = 1-2.5:0.5-2 php

NR, Isoprene rubber, and SBR

t 0, O 3

[232]

36.

a) 4010 NA b) SC a:b = 1-2.5:0.5-2 php

NR, IR, BSR

O3

[233]

37.

a) N,N´-bis(1,5-dimethylpentan)PPHDA (S77) b) SC a:b = 1-2.5:0.5-1 php

NR, Isoprene, SBR

O3

[234]

38.

a) S77 b) Flectol H a:b = 1:0.5 php

NR, Isoprene rubber, SBR

O3

[235]

289

Ozonation of Organic and Polymer Compounds

39.

a) S77 b) MBI a:b = 1-2.5:0.5:2 php

Isoprene rubber and SBR

O3

[236]

40.

a) 4010 NA b) Flectol H a:b = 1.7:1.3 php

SBR

O3

[237]

41.

a) MBI b) N-Phenyl Na-octyl-PPHDA

NR

anti-fatiguing, O3, t0

[238]

42.

a) 4010 NA b) Neozone D

Vulcanisates

t0

[239]

43.

a) Unsaturated cyclo-aliphatic carboxylated ester, 1 php b) Parafﬁn - 0.25 php

All rubbers

O3, non-odouring

[240]

44.

a) MBI b) Castor oil

Chloroprene rubbers

O3

[241]

45.

a) 2246 - 0.5 php b) Tetrakis (2,4-di-tertbutylphenyl-4,4a-diphenyl) phosphite – 0.25 php

NR, SBR

O 3, t 0

[242]

46.

a) PPHDA type AO– 1-3 php b) Parafﬁn – 1-5 php

Vulcanisates

O3, t0

[243]

47.

a) S13 - 1 php b) Naphthenes - 0.5-10 php

Diene rubbers

O3, t0

[244]

48.

a) Di-tert-butyl phenols - 1-5 php b) Zinc salt of mercaptoimidazole - 2-8 php

EPDM

t0, O3

[245]

Note: EPDM: Ethylene propylene diene terpolymer; t0: temperature resistance

We have developed methods for evaluating the antizonant’s efﬁciency, a series of synergetic stabilising systems for rubber mixtures, highly stabilising elastomers compositions for sidewalls and protectors of light and heavy pneumatic tyres (PT) and highly effective stabilising compositions for hose faces and chambers [247-257]. By applying Equation 1.4 the rate constants of six industrial antiozonants of PPHDA type, were determined and their values are listed in Table 4.4 [258].

290

Degradation and Stabilisation of Rubber

Table 4.4 Rate constants of PPHDA-type antiozonants at 20 ºC in CCl4 No.

Antiozonant

k s 10-7 (M-1.s1)

1.

N,N´-Dinitroso-di-(C7-C9)-alkyl-PPHDA

0.5

2.

N,N´-Di-(2-ethylhexyl)-PPHDA

0.7

3.

N-Phenyl-Na-iso-propyl-PPHDA

1.2

4.

N,Na-Di-(1,4-dimethylpentyl)-PPHDA

1.3

5.

N-iso-propyl-PHDA

1.4

5.

Polymerised 2,2,4-trimethyl-1,2-dihydroquinoline*

2.2

6.

N-Phenyl-N´-cyclohexyl-PPHDA

2.4

* quinoline type antiozonant

An express method for determination of the antiozonant’s efﬁciency depending on the rubber type is developed. The antiozonants activity is evaluated by comparison the values of the rate constant. This method includes the separate determination of the molecular masses of rubbers in solution with and without antiozonants and of the corresponding amounts of absorbed ozone. The antiozonant efﬁciency is deﬁned as a ratio between the amount of consumed ozone in an elastomer solution containing antiozonant and the amount of reacted ozone in the antiozonant-free solution (Figure 4.7) through which an equal change of the molecular mass of both the solutions is attained. The registration of the changes begins at initially equal molecular masses, further goes on with maintaining the ratio between the amount of absorbed ozone in solution containing antiozonant and the antiozonant amount in the range from 0.1 to 1 and ends at values of the antiozonants activity vary from 1 to 3. The quantitative determination of the antiozonants efﬁciency was estimated from the relationship [259]: Ef = $G2/$G1

(4.5)

where: $G1 - the amount of consumed ozone changing the molecular mass of sample 1 from M1 to M2; $G2 - the amount of absorbed ozone changing the molecular mass of sample 2 from M1 to M2, $G2 values were selected in the region whereby the ozone uptake varying from 0.1 to 1 mol corresponds to 1 mol antiozonants.

291

Ozonation of Organic and Polymer Compounds

4.0

3.5

3.0

2.5

2.0

2

1 1.5

G1

G2

1.0 0

5

10

15

20

25

-5

G.10 , mol

Figure 4.7 Variation of the relative viscosity of SKD (0.6 g/100 ml solvent) – (1) and the same solution containing 3.7 mM 4010 NA - (2) at 20 oC

On the basis of the synergistic stabilising system including N,N´-bis-1,4-dimethylpentylp-phenylenediamine (S77) and polymerised 2,2,4-trimethyl-1,2-dihydroquinoline (Flectol H) in a ratio from 0.5 to 2 and in amounts ranging from 1.5 to 4.5 ppm we have prepared a highly stabilising rubber mixture (Table 4.5). The behaviour of model samples was evaluated under conditions of ozone ageing (30 oC, ozone - 6 ppm and elongation deformation of 20%) and atmospheric ageing (20% elongation deformation on a roof in Soﬁa during the summer in 1982) and the results obtained are shown in Table 4.6. Among the investigated combinations, only system No.3 exhibits synergistic effects. Applying the same proportions and conditions we have also evaluated the synergistic properties of the following systems: No.4 - S77/Sc [4,4-thio-bis-(2-methyl-6-tertbutylphenol)] - 1.8/1.1 php; No.5 - 4010NA/SC = 1.9/1.1 php and No.6 - 4010NA/ Flectol H = 1.7/1.3 php (Table 4.7).

292

Degradation and Stabilisation of Rubber

Table 4.5 Rubber compositions under vulcanisation conditions: 153 oC, 25 min No.

Ingredients

Base sample No.1 (php)

Base sample No.2 (php)

New system No.3 (php)

1.

Natural Rubber (NR)

10.0

10.0

10.0

2.

Bulex M-27 (SBR)

20.0

20.0

20.0

3.

Bulex 1500 (SBR)

30.0

30.0

30.0

4.

Isoprene Rubber (SKI-3)

40.0

40.0

40.0

5.

Pyrolen - highly aromatic resin

7.0

7.0

7.0

6.

Carbon black - PM-50

25.0

25.0

25.0

7.

Carbon black - PM-75

25.0

25.0

25.0

8.

ZnO

3.0

3.0

3.0

9.

Stearic acid

2.0

2.0

2.0

10.

PN-6SH - highly aromatic oil

10.0

10.0

10.0

11.

S77

3.0

-

1.7

12.

Flectol H

-

3.0

1.3

13.

2–Benzthiazolylsulfenmorpholid - Santocure MOR

1.2

1.2

1.2

14.

Sulfur

1.9

1.9

1.9

Table 4.6 Results from ozone and atmospheric ageing tests Sample

Ozone degradation, ﬁrst cracks (min)

Atmospheric degradation, ﬁrst cracks (months)

Atmospheric degradation, breaking down (months)

No.1

80

9

11

No.2

20

5

6

No.3

>120

>12

>12

293

Ozonation of Organic and Polymer Compounds

Table 4.7 Results from ozone and atmospheric ageing tests Sample

Ozone degradation, ﬁrst cracks (min)

Atmospheric degradation, ﬁrst cracks (months)

Atmospheric degradation, breaking down (months)

No.4

280

-

>12

No.5

120

9

>12

No.6

230

9

11

The stabilising system S77/Vulcanox MB-2A (2-methyl-mercaptobenzimidazole) = 2.9/1.0 php was studied in two compositions: 1) SKI-3 - 50.9; Bulex M-27 - 50.0; carbon black - PM-50 - 45.0; Highly aromatic oil - PN-6SH 5.0; ZnO - 5.0; stearic acid - 2.0; vulkazit CZ (2-cyclohexylmercaptobenzothiazole)-1,2 and sulfur -2.2; 2) SKI-3 -100.0; PM-50 - 45.0; PN-6SH - 5.0; ZnO - 5.0; stearic acid - 2.0; Vulcazit CZ - 1.2 and sulfur - 2.3 (php), vulcanised at 150 oC for 15 and 10 minutes, respectively. These samples with new rubber compositions have shown higher stability than standard ones. That statement is conﬁrmed by the fact that the values of physicomechanical parameters, such as relative elongation and tensile strength are kept unchanged longer during artiﬁcial ageing. The samples elongated to 20% at two different temperatures (70 oC and 95 oC) for 21 days. The proposed synergetic stabilising systems were applied for the development of universal reception for PT sidewalls and protectors with high antiozonant and antioxidant stability, containing complex elastomer compositions with reduced content of NR and high PMP: 1) Sidewalls: NR - 10.0; SKI-3 - 40.0; Bulex M-27 - 20; PM-50 - 25.0; Carbon black PM-75 - 25.0; Highly aromatic resin - Pyrolen - 7.0; PN-6SH - 10.0; ZnO - 3.0; stearic acid - 2.0; microcrystalline wax (Ozonshuzvax 111 - BASF) - 2.0, Santocure MOR - 0.9 and sulfur - 1.9 (php) vulcanised at 153 oC for 25 minutes. The rubber specimen resistance towards ozone and atmospheric ageing is found to be superior by 29-40% and 50% during exploitation and storage conditions, respectively, as compared with the usual rubber compositions. 2) Protectors: SKI-3 - 25.0; Bulex 1500 - 50.0; SKD-25 - 25.0; carbon black PM100 - 65.0.; Pyrolen - 2.0; ZnO - 3.0; stearic acid - 2.0; petroleum bitumen

294

Degradation and Stabilisation of Rubber (Rubrax) - 5.0; Ozonshuzvax 111 - 2.0, Santocure MOR - 1.2 and sulfur - 1.9 (php) vulcanised at 153 oC for 25 minutes. The resistance of new rubber samples against artiﬁcial ozone and atmospheric impact, is dependent on the stabilising system. Applying a similar approach we have proposed some highly stabilised elastomer compositions for hose sidewalls and chambers possessing high resistance to water and air action. On the basis of the synergetic stabilising systems developed we put forward the following rubber compositions: 1) Hose sidewalls: Bulex 1500 - 100.0; PM-6SH - 9.0; ZnO - 3.0; stearic acid - 1.5; microcrystalline wax - 2.0; Vulcazit CZ- 1.0 and Sulfur - 1.9, vulcanised at 160 o C for 25 minutes. 2) Hose chamber: Oil ﬁlled butadiene-styrene rubber (SKMK-30ARKM-15-made in Russia) - 100.0; PM-50 - 90.0; PM-75 - 5.0; PN-6SH - 8.0; ZnO - 3.0; Asphalt 7.0; Stearic acid - 2.0; Vulkazit CZ - 1.3 and Sulfur - 2.0, vulcanised at 160 oC for 15 minutes. 3) Hose chamber: SKMK-30ARKM-15 - 92.0; high molecular styrene resin (KER 1904) - 8.0; PM-94 - 94.0; PN-6SH - 1.4; asphalt - 11.0; ZnO - 2.5; stearic acid - 2.8; Vulkazit CZ - 2.1; N-cyclohexyl-thio-phthalimide - 0.45 and sulfur - 1.8, vulcanisation conditions - 160 oC and 15 minutes. The resistance of the proposed mixtures for sidewalls and chambers during accelerated thermal (70 oC) and ozone ageing is improved if compared to the commonly used stabilising compositions. Thus, depending on the stabilising system they are superior by 1.7 times for the faces mixtures and from 2.0 - to 4.0 times for the chamber ones. The degradation process in real conditions is closely related to two main reactions: oxidation and ozonolysis. The ﬁrst one is initiated by temperature, static and dynamic deformations, light, radiation, electrons action, defects in the solid body, initiators, catalysts, oxy-reduction agents, bases, acids, and so on. The monomolecular conversions of Rv, ROv and RO2v - radicals or in some cases of the ion intermediate are among the degradation factors which should be taken into account. The ozonolysis degradation is influenced, mainly, by the ozone concentration, deformations and defects and to a smaller extent, by temperature due to the relatively low values of Ea of the ozone reaction of the C=C bonds and the zwitterion reactions. In this case the mono- and bimolecular reactions with zwitterions participation and the peroxide bonds decomposition which replace the strong C=C bonds formed in ozonolysis are responsible for degradation occurrence.

295

Ozonation of Organic and Polymer Compounds

4.2.4 High Molecular Antiozonants A number of highly efﬁcient low molecular antiozonants because of their low solubility in polymers, volatility, blossom ability, and ability to be washed and hydrolysed cannot be used in many cases. In view of this, creation, proposing and application of highly effective high molecular antioxidants (HMAO) are quite modern investigations and provoke an intensive research. This is because the HMAO are able to overcome the forgoing disadvantages. The elastomer’s stabilisation by HMAO can be accomplished via two basic routes: by addition of preliminary prepared polymer stabiliser to the parent polymer or by chemical bonding of the active additive in the polymer matrix. The ﬁrst approach for stability improvement is more and more often recommended. The ozone resistant polymer component may be polyvinylchloride polyvinyl chloriode (PVC) [260], ethylene-vinylacetate-copolymer [261], polychloroprene rubber copolymer of butadiene and butene n-butene, without C=C bonds [263], chlorosulfopolyethylene [264], and so on. However, recently the EPDM are attracting much attention, particularly, their application as protective polymers. This is associated with two main reasons: 1) highly protective action; 2) negligible effect on important vulcanisate parameters [265, 266]. Among them EPDM with ethylidenenorbornene or cyclopentadiene [267] as a third copolymer are the most affective antiozonants additives. The ratio between the EPDM concentration and the basic elastomer group varies from 1:9 to 30:70, mostly from 10:90 to 25:75 and the addition of up to 5 php of PPHDA to the rubber composition is highly recommended [268-271]. Andrews proposed that the HMAO particles mechanically prevent the growth of microcracks which are formed under ozone action [272]. This suggests that HMAO should form a continuous disperse medium which can block the microcracks formation and development in the phase of the second polymer. However, this factor is necessarily but not sufﬁcient to ensure high ozone protective properties of HMAO. Another mechanism for protection includes the blocking of the C=C bonds by the HMAO molecular segments thus reducing the ozone assess to them. This can be realised only in case of high degree of mutual polymers dispersion, i.e., at the formation of monophase systems. A mixture of butadienenitrile rubber (SKN 40) and PVC [273] is such an example. In the general case of a two phase system the ozone-protective action of HMAO is determined by the dispersion level and their ability to cover the sample surface [273, 274]. Polymers containing antioxidant groups can be prepared by the following reactions:

296

Degradation and Stabilisation of Rubber 1. Interaction of polydienes with nitroso compounds, i.e., p-nitrosoamines [275], giving PPHDA bound to the rubber [276, 277]:

H2

H

+

H O -N

O =N

NH Ph

NH

NH Ph

NH Ph

The nitroso compound can be easily added to rubber simultaneously with the other ingredients. 2. Interaction of unsaturated polymers containing epoxy-groups with primary aromatic amines [278]: -CH2-CH-CH-CH2- + H2N-Ar

-CH2-CH-CH-CH2 HO NHAr

O

or with 2,6-di-tert-butyl-4-alkylphenols derivatives [279]: t.-But -CH 2 C H CH CH 2 - + HX ( CH2 ) n O X = -NH-, -OO-, -O-; n = 2-3

OH t.-But

H2 C HC X( CH2 ) n HCOH

t.-But OH . t.-But

H2 C

Mostly, the HMAO obtained by modiﬁcation via reaction (1) are equal or superior to N-phenyl-B-naphthylamine (Neozone D) and practically do not differ and even in some cases are superior to N,Na-diphenyl-p-phenylenediamine, 4010NA, etc., while those prepared by reaction (2) are similar in their antiozonants action to 2,6-di-tert-butyl-4-methylphenol (Ionol). There are also other polymer similar conversions for introduction of stabilising groups. During the functionalisation of butadiene-metacrolein copolymer with aromatic amines and phenols [280] or treating of hydrochlorinationed cis-1,4-polyisoprene with 2-tert-butyl-resorsinol, the efﬁciency of the HMAO obtained is higher than that of Neozone D, 2,2-methylene-bis-(4-methyl-6-tertbutylphenol)-2246 and that of the HMAO based on the diene elastomer treated with the antioxidant with aliphatic unsaturation such as N-(4-anilinophenyl)-

297

Ozonation of Organic and Polymer Compounds metacrylamide (CH2=CH-CO-NH-Ph-Ph). The reaction can be conducted in solution, emulsion or in solid polymer. The stabiliser is added through stirring in a mill or closed mixer for 5-10 minutes at a temperature ranging from 20 to 160 oC in the presence of peroxides initiator [281]; phosphorisation of diene rubber with P2S5 and further treatment with aromatic amines [282] resulting in functionalising of the polymer with functional groups of the type: CH3 -CH2-C=C-CH2-CH2S=P-NH-

containing P-S and N-P bonds which are energetically weaker than the C-C and C-H bonds. 3. Copolymerisation of diene monomers (butadiene with styrene, acrylonitrile, isoprene, 2-chlorobutadiene) in the presence of a third monomer with groups capable of inhibiting the oxidation processes [283-285] such as compounds with secondary aminogroups, i.e., Ph-NH-Ph-CO-C(CH3)=CH2 or phenol type antioxidants [286, 287], i.e., HO-Ph-CH2-CH2-O-CO-C(CH3)=CH2. The mechanism of the protective action of HMAO is determined by: 1) The more efﬁcient termination of the chain in oxidative degradation and ozone scavenging as a result of the more uniform distribution of HMAO. 2) Structurising of the polymer during recombination of HMAO molecules and elastomer radicals or its zwitterions [289]. 3) The itramolecular synergetic action in polyfunctional HMAO. Some other factors, like P-conjugation, configuration, conformation and supramolecular structures in elastomers [288] also affects the HMAO efﬁciency. The kinetics of HMAO stabilisation is controlled by the simultaneous presence of the stabiliser and elastomer radical/zwitterion in the kinetic cage [289] rather than by the HMAO diffusion in the polymer matrix which is the case with lowmolecular antiozonants. The protective action of HMAO based on nitroso compounds is due to the deactivation of the impurities catalysing the oxidation, for example iron, as a result of the formation of complex compounds [275]. HMAO are most widely applied for stabilising

298

Degradation and Stabilisation of Rubber vulcanisates which are continuously exposed to the avtion of higher temperatures, various solvents, oils, water and static load under operating conditions.

4.2.4.1 Ethylene-Propylene Rubber With Diene Monomer (EPDM) Recently the EPDM are considered as the most promising HMAO for vulcanisates [290-293]. However, EPDM addition can result in deterioration of PMP of the vulcanisates which is mainly due to: different compatibility of the vulcanising agents in EPDM and the diene elastomer and the insufﬁcient compatibility of the elastomers used. This problem can be successfully solved by the proper choice of the vulcanising groups which can ensure most similar kinetics of elastomers vulcanisation. In our studies we have used Keltan 312 (dicyclopentadiene as a third monomer) as a HMAO since it has a higher rate of vulcanisation at temperatures over 150 oC. Based on literature data three vulcanisation groups have been selected: sulfur/2morpholinothiobenzothiazole (MBS) (I); sulfur/MBS/dibenzothiazoldisulﬁde (Altax) (II) and sulfur/diphenylguanidine (DFG) (III). Their efﬁciency was evaluated on rubber mixtures based on: Bulex, SKD and SKI-3 and their combinations [290, 294-298]. The kinetic curves obtained by the standard method with Moncanto TM-100 rheometer at 155 oC show that similarly to other elastomers Keltan 312 undergoes an effective structurisation under the inﬂuence of the vulcanising systems used (Table 4.8). The addition of Altax to (I) forming a system (II) increases the rate constants of vulcanisation by more that 7% and the structurising effect for all rubbers including Keltan 312 is more than 25% (Table 4.9, deformation at 300% elongation). It has been found that systems I and II are very suitable for covulcanisation of Keltan 312 with Bulex 1500 and SKD and to a smaller extent with SKI-3, namely, due to the different vulcanisation rates. These are large differences in the vulcanisation rates using system III. The small differences in the vulcanisation rate of Keltan 312 with diene elastomers favours the formation of homogenous vulcanisation structure which preserve or improve their PMP. The optimum ratio of Keltan 312/diene elastomers, according to our studies, should not exceed 20/80 as the best ratios are found in the range of 5/95-15:85. On the basis of the results obtained we have proposed rubber compositions for PT protectors in diene rubber/Keltan 312 ratio of 95:5 thus reducing the SKI-3 content. PMP of the vulcanisates with I and II are listed in Table 4.10.

299

Ozonation of Organic and Polymer Compounds

Table 4.8 Rate constants of rubber mixture vulcanisation (k) with various vulcanising groups (I-III) at 155 oC No.

Elastomer

I, k (min-1)

Ratio

II, k (min-1)

Ratio

III, k (min-1)

Ratio

1.

Bulex - M27

0.19

1.46

0.22

1.50

2.30

8.80

2.

Bulex - 1500

0.14

1.10

0.14

1.00

1.97

7.60

3.

SKD

0.31

2.40

0.38

2.70

4.57

13.30

4.

SKI-3

0.75

5.80

0.81

5.80

3.00

11.50

5.

Keltan 312

0.13

1

0.14

1

0.26

1

Table 4.9 PMP of vulcanisates based on Bulex 1500 (1), SKD (2), SKI-3 (3) and Keltan 312 (4) in dependence on the vulcanising system (I and II). Property

Unit

1I

2I

3I

4I

1II

2II

3II

4II

Force of strain

kg/cm2

186

130

222

167

202

140

237

170

Tension at 300% elongation

kg/cm2

75

52

73

91

90

58

87

117

Relative elongation

%

555

460

565

455

540

455

545

445

The ozone resistance of the studied protector compositions increases in the presence of Keltan 312 in the elastomer composition (Table 4.11). It is seen that the type of the vulcanisation group does not affect the ozone resistance of the vulcanisate. The analysis of the results reveals that the amount of Keltan 312 in the PT protector compositions should be up to 5 php. This content ensures high PMP and enhances substantially their ozone resistance. Among the vulcanising groups used, sulfur/MBS/ Altax = 1.9/0.7/0.5 turns to be the most appropriate one.

300

Degradation and Stabilisation of Rubber

Table 4.10 PMP of protective type vulcanisates of elastomer compositions Bulex 1500:SKD:SKI-3 = 50:27:23 (1) and (1):Keltan 312 = 90:10 (2) with vulcanising systems: I - sulfur/MBS = 1.9/1 php and II - sulfur/MBS/Altax = 1.9/0.7/0.5 php 1I

2I

2II

Time of prevulcanisation at 130 oC (min)

48

51

40

Force of strain (kg/cm2)

175

150

162

Tension at 300% elongation (kg/cm2)

102

70

85

Relative elongation (%)

515

490

500

8

14

10

62-63

60

60

Elasticity (%)

25

23

23

Strength of tear (kg/cm)

5.2

4.9

4.9

Gurvich residual deformation (%)

14.6

8.9

10.2

o

40

40

41

Property

Residual prolongation (%) Shore A hardness

Heat formation ( C - according to Gurvich)

Table 4.11 Ozone resistance of elastomer compositions from Table 4.10 stabilised by 4010NA/Flectol H = 1.7 + 1.3, at 6 ppm of ozone, 30 oC Time (min)

1I

2I

2II

20

A1T1

45

A1T2

A1T1

A1T1

130

A1T3

A1T2

A1T2

365

T4 - break down

A1T3

A1T3

T4 - breakdown

T4 - breakdown

660

4.3 Apparatus for Ozone Resistance Determination The ozone resistance of vulcanisates is tested by various accelerated methods both under laboratory and real conditions [299, 300]. The laboratory methods are divided

301

Ozonation of Organic and Polymer Compounds into two groups. The ﬁrst one, using up to 6 ppm ozone, is intended for testing of natural rubber, polybutadiene, polyisoprene, butadiene-styrene rubbers which are usually more susceptible to ozone action and the second one, with 0.01-0.15% ozone concentration, is applied for testing of ozone resistant rubbers such as polychloroprene rubber, butylrubber, and so on. The time of cracks appearance (t0) and breakdown (tC) depends on E and [O3] and in the different standards they are evaluated at various deformations and ozone concentrations. This is the reason why the results from the different tests can not be properly compared [301-303]. The apparatus for testing are with ﬁxed parameters and do not allow the tests to be conducted according to the different standards. This has stimulated us to design new more suitable equipment which could make possible the application of different standards along with selection of appropriate test conditions [304]. The scheme of the apparatus developed by us for evaluation of the vulcanisate’s ozone resistance is depicted in Figure 4.8.

220 V

8

7

9 6-10 kV atm

O2

1

O2

2

3

O2

4

O 3 /O 2

5

6

Figure 4.8 Block scheme for ozone resistance determination of vulcanisates. 1 - oxygen source; 2 - reducing valve; 3 - gas dryer; 4 - ozonator; 5 - test chamber; 6 - UV analyser of ozone; 7 - stabiliser; 8 - autotransfomator; 9 - high voltage transformer

The ozone chamber with capacity of 20 litres is equipped with an ozone inlet and outlet system and ventilator for equalisation of ozone concentration.

302

Degradation and Stabilisation of Rubber The frames for deformation of the samples were designed according to our original schemes. In one frame can be placed up to eight samples. Three frames can be placed in the test chamber, or a total of 24 samples can be tested simultaneously. Ozone was prepared by passing oxygen or air (preliminary dried) through a pipe generator at an electrode voltage of 5-8 kV. The ozone analysis was carried out spectrophotometrically at 254 nm by means of an ADS-3 (double bond analyser, produced at the Institute of Chemical Physics, Russian Academy of Sciences). The device sensitivity is about 10% volume. The samples are rubber strips with size 25 x 5 x 1 mm. The ozone concentration may vary from 4 to 10-5 vol%, and can reach 200%. We have investigated the ozone resistance of Bulex-1500-based vulcanisates by means of the designed test unit. The sample compositions were as follows: Bulex1500 - 100, carbon black (PM-75) - 50, ZnO - 3, PN-6SH - 3, N-cyclohexyl-2benzothiazolsulfenamide (CZ) - 1.2, sulfur - 1.8 and 4010NA - 0, 1.5, 2, 2.5, 3 or 3.5 php, vulcanisation at 160 oC for 10 and 15 minutes. The test results are demonstrated in Figure 4.9.

14

2.0

12

2.5

10

1.5

8

1.0 6

0

4 2 0

0

10

20

30

40

50

, %

Figure 4.9 Dependence of the time of crack breakdown on elongation at 42 ppm ozone, 20 oC and different concentrations of 4010NA

303

Ozonation of Organic and Polymer Compounds As Figure 4.9 shows, the fastest breakdown of the samples takes place at E = 1015% - the range of critical deformation. Before and after that point, the curves are increase relativly fast. That experimental fact can be explained with appearance of critical phenomena [299]. The stabilisation of vulcanisates at higher E is also related to the decrease of the degradation rate as a result of the increase of the system potential energy due to the hindered sp2-sp3 transition [305]. At E << 10-15% the microcracks on the sample’s surface do not grow, not only because of the low values but also because of the accumulation of oxygen containing compounds - a result of the ozone reaction with elastomer which together with the antiozonants impedes the ozone assess [306, 307]. Upon ozone ageing it was found that swellings and microcracks are ﬁrst formed in the surface defects, then being gradually converted into cracks rectangular to the direction of the applied deformation. These changes can be observed ﬁrst by optical means and after that with unaided eye. The crack formation and growth is due to: 1) C=C bonds cleavage under the ozone action; 2) initiation of ozone oxidative degradation and 3) cracks development resulting from the deformations concentration in their lips. Other authors explain this step of elastomers degradation process in a similar way [308, 309]. As degradation proceeds the growth of some microcracks leads to the appearance of more and more deep cracks. The greater portion of the microcracks remains unchanged in longitude and depth up to the moment of breakdown. The cracks occurred at the defects are usually those which grow since these are the places of higher deformations concentration. Similar observations have been registered by other authors in investigating metal corrosion [310] and degradation of some polymers [311]. The optimum amount of 4010NA is 2-2.5 ppm (Figure 4.10) which is in a good agreement with the data on the ozone resistance during rubber product’s exploitation in real conditions [299, 301, 312]. In this sense the apparatus designed by us proves to be quite suitable for accelerated studying the ozone resistance of vulcanisates. The apparatus has the following advantages: it is small-sized, easily operable and allows: 1) the variation of ozone concentration in wide ranges; 2) the continuous registration of the change in ozone concentration during exposure time; 3) continuous monitoring of the changes occurring on the samples surface and their photography; 4) application of static deformations in a wide range of elongations. Thus, this equipment can be used for studying the ozone resistance of any type of elastomers, their mixtures and vulcanisates on their base.

304

Degradation and Stabilisation of Rubber

16 15 14 13 12 11 10 9 0

1

2

3

4

w.p.

Figure 4.10 Dependence of breakdown time on the amount of 4010NA

4.4 Evaluation of Industrial Stabilisers The antioxidant and antiozonant efﬁciency of some industrial stabilisers and their mixtures were studied [313-320] with the purpose of proposing new highly efﬁcient stabilising systems for rubbers. The estimation of their efﬁciency was carried out by applying model compounds, rubbers solutions and real samples upon conditions of oxidative, ozone and atmospheric degradation. For the purpose of the present investigation the following antiozonants were used: UÊ 2,2´-Methylene-bis-(6-tert-4-methylphenol) (I) Plastanox 2246 - Cyanamid Co. product (CY) UÊ 2,2´-Methylene-bis-(6-tert-4-ethylphenol) (II) Plastanox 425 - CY UÊ 2,6-Di-tert-butyl-4-methylphenol (III) - Ionol - Shell; UÊ N,N´-Bis-(1,4-dimethyl-pentyl)-p-phenylenediamine (IV) – Santoflex 77 Monsanto (MO); UÊ N-Phenyl-N´-iso-propyl-p-phenylenediamine (V) - 4010NA - BASF; UÊ N-Phenyl-B-naphthylamine (VI) - Neozone D - DuPont;

305

Ozonation of Organic and Polymer Compounds UÊ 6-Ethoxy-2,2,4-trimethyl-1,2,-dihydroquinoline (VII) – Santoﬂex AW - MO; UÊ 4,4-Thio-bis-(6-tert-butyl-m-cresol) (VIII) – Santwhite Crystal (SC) - MO; UÊ Polymerised 1,2-dihydro-2,2,4-trimethylquinoline (IX) - Flectol H - MO. The stabilisers were investigated without further puriﬁcation. The reactions in liquid phase were conducted in tetrachloromethane and cumene using SKD, NR, SKI-3 and Bulex-1500 rubbers. The latter were used either without puriﬁcation (technical grade) or after triple precipitation with ethanol from CCl4 with pure analysis grade. The cumene oxidation was carried out in a volumetric device [321, 322], the ozonation of antiozonants and their mixtures in CCl4 medium with or without rubber in a bubbling reactor [323], the degradation of the solid specimen - upon atmospheric conditions and in an ozone chamber - 3-6 ppm ozone and ε = 20%.

4.4.1 Antioxidant Action The antioxidant efﬁciency of antiozonants (I-IX) and their mixtures was studied in an initiated oxidation of cumene. The initiation was accomplished by the addition of azo-bis-(isobutyronitrile) (AIBN) ensuring an initiating rate (W) of 2 s 10-6 M/s at 70 o C. According to the classical concept [322] the antioxidant efﬁciency of the stabilisers was estimated by: 1) the induction time - T, and, 2) the coefﬁcient of inhibition (M). Both parameters are deﬁned by the following equation: T = ln{1 - (M/2e).[InH]0/[AIBN]0}.1/ki

(4.6)

where: e is the probability for the initiation radicals to leave the kinetic cage, for AIBN it is equal to 0.6 at 70 oC ; [lnH]0 = 5 and 2.5 mM and [AIBN]o = 50 mM are the initial concentrations of antiozonants and AIBN; ki - the rate constant of the AIBN decay at 70 oC equal to 3.36 x 10-5 s-1. If we substitute the previous values for e, [lnH]o and [AIBN]o and accept the values of 1, 2, 3 and 4 for M, the following respective values are obtained: 44, 90, 142 and 200 minutes for [lnH]o = 5 mM and 22, 43, 66 and 91 minutes for [lnH]o = 2.5 mM. The experimentally observed values for antiozonants ranging from I to IX were: 194, 123, 85, 97, 123, 41, 54, 126 and 66 minutes. The experimental values of M for I-IX (within unity) were 4, 3, 2, 2, 3, 1, 1, 3 and 2. On substituting the experimental values in Equation 4.6 the following values were obtained: 4, 6.6, 2.96, 1.99, 2.33, 2.95, 0.98, 1.30, 3.0 and 1.6. Bearing in mind the

306

Degradation and Stabilisation of Rubber structure of the stabilisers, it can be concluded that these values of M are in compete agreement with the antiozonants structure and properties. Moreover, the reaction of cumene oxidation was used in examining mixtures of two stabilisers. Comparing the experimental values with those estimated by Equation 4.6, it can be seen that systems 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 13 and 14 exhibit a synergistic effect (Table 4.12).

Table 4.12 Induction periods of stabilisers mixtures in the reaction of cumene oxidation No.

Stabilising system

Tadit (min)

Texp (min)

$T (min)

1.

IV/I

134

242

108

2.

IV/II

109

225

116

3.

IV/III

86

119

33

4.

IV/VIII

109

205

96

5.

IV/IX

86

138

52

6.

V/I

157

93

-64

7.

V/II

132

94

-38

8.

V/III

109

154

45

9.

V/VIII

132

172

40

10.

V/IX

109

135

26

11.

VI/I

113

198

85

12.

VI/II

88

76

-12

13.

VI/III

63

130

65

14.

VII/V

88

123

35

15.

VII/II

88

88

0

Tadit: induction period with additives Texp: induction period determined experimentally

Later, we selected systems 1, 4, 5, 6, 9 and 10 for experiments with vulcanisates. The choice was justiﬁed by the strong synergism and the reference data on their antiozonant and antifatigue properties.

307

Ozonation of Organic and Polymer Compounds

4.4.2 Atmospheric Ageing Standard samples (TGL-16890), from mixtures based on Bulex-1500, NR and SKI-3 at a ratio of 50:20:30 were mixed with stabilising systems 1, 4, 5, 6, and 9 in 3 php ratio. We have introduced the antiozonants in slight excess (by about 1-0.5 php more than the optimum) (Figure 4.8) in order to ensure a maximum activity. The samples were ﬁxed in frames at E = 20% and were placed on a building roof in spring - summer period in Soﬁa, Bulgaria. A great number of cracks of 1 mm depth were observed only with system 5 (after ﬁve months exposure). All other samples remain unchanged during the exposure time.

4.4.3 Antiozonant Action Samples (10 each) of the investigated mixtures for atmospheric degradation were also exposed to ozone (ozone concentration of 3 ppm, 30 oC and E = 20%). Figure 4.11 shows the behaviour of the samples from the formation of the ﬁrst cracks (to) to the complete breakdown (tb) and the full lines demonstrate the degradation time.

50

40

9

30

5 10

4

20

10

1

6

0

Stabilizers

Figure 4.11 Dependence of to and tb of the stabiliser systems 1, 4, 5, 6, 9 and 10 at 3 ppm ozone, 30 oC , E = 20%

308

Degradation and Stabilisation of Rubber As seen systems 4, 9, 5, and 10 show higher protective properties as compared to systems 1 and 6. For system 9 (S77/SC) the ﬁrst cracks appear when the other samples have already broken down completely and breakdown of its cracks is not observed during the test time. The effect of the stabilising systems was studied by using rubber solutions containing the stabilising system 10 and reference solutions without a stabiliser. A bubbling reactor (10 ml capacity) was loaded with a 0.5% rubber solution (CCl4) mixed with an amount of the stabilising system corresponding to 3 php antiozonants in the rubber mixture. The ozone concentration was 10-4 M and the rate 0.1 l/min. The stoichiometric coefﬁcients (the ratio of the amount of absorbed ozone per rubber monomer unit) are presented in Table 4.13

Table 4.13 Stoichiometric coefﬁcients of the ozone reaction with technical and pure rubbers in the presence and absence of system 10 No.

Rubber

Purity

System 10

Stoichiometric coefﬁcient

1.

Bulex-1500

Technical grade

-

4

2.

Bulex-1500

Pure

-

2

3.

Bulex-1500

Technical grade

Added

3

4.

Bulex-1500

Pure

Added

1.8

5.

NR

Technical grade

-

2

6.

NR

Pure

-

1

7.

NR

Technical grade

Added

1.7

8.

NR

Pure

Added

1

9.

SKI-3

Technical grade

-

2

10.

SKI-3

Pure

-

1

11.

SKI-3

Technical grade

Added

1

12.

SKI-3

Pure

Added

0.8

13.

Bulex1500:NR:SKI-3

Technical grade

-

3

14.

Bulex1500:NR:SKI-3

Pure

-

1

15.

Bulex1500:NR:SKI-3

Technical grade

Added

1

16.

Bulex1500:NR:SKI-3

Pure

Added

0.9

309

Ozonation of Organic and Polymer Compounds The concentration of rubber was 5 x 10-4 M, with an antiozonants concentration of 1 x 10-5 M and the rate constants of ozone interaction with the double bonds were equal to 105 M-1.s-1 and that with antiozonants 107 M-1.s-1 [323]. It is seen that the stabilising system decreases the stoichiometric coefﬁcient both with the technical grade rubbers and the puriﬁed ones. In all cases it is related to the lowering of the concentration of the reactive double bonds. Considering the pure rubbers, the change of concentration is also associated with the removal of unsaturated olygomers during puriﬁcation. Bearing in mind the structure of stabilisers V and IX, we can assume that the antiozonants screen the C=C bonds which thus, become unaccessible to ozone action. Most probably, the stoichiometry decrease is consistent with deactivation of the double bonds by the stabiliser molecules, note that one molecule of stabiliser deactivates from 2 to 4 double bonds. This model appears to be qualitative but it is in conformity with the reported mechanism of the antiozonant action [324]. It can be summarised that detailed research is necessary to select the most efﬁcient stabilising systems including: invention of model conditions for a primary selection and presence of synergism, further selection by model conditions close to the real ones and ﬁnal selection of stabilisers examined under operating conditions. Simultaneous check of stabiliser’s efﬁciency towards the effect of ozone, oxygen, heat and light is appropriate at each stage of research. It was established that among the stabilisers studied, systems 4, 5, 9 and 10 were the most effective. Figure 4.12 shows the behaviour of these vulcanised samples in the presence of antiozonants IV and V. It is seen that, depending on the concentration, three zones appear on the kinetic curves which can be deﬁned as follows: period of breakdown, relative stability, and complete stability. Comparing these two stabilisers it can be concluded that the both have highly efﬁcient however the former one is slightly superior. This fact can be explained by the strong electrophilic properties of the ozone molecule and the stronger electron-donor ability of the two alkyl substituents compared to that of the arylalkyl substituents. We have studied the behaviour of various individual stabilisers and stabilising mixtures (Table 4.14).

310

Degradation and Stabilisation of Rubber

35 2.IV 2.V

30 25 20

Degradation zone

15

Zone of crack formation

10 1.IV

5

1. V

Zone of full stability

0 0

1

2

3

4

php

Figure 4.12 Antiozonants - IV and Y efﬁciency in dependence on their concentrations. 1. IV, 1.V - time for the ﬁrst crack appearance; 2.IV, 2.V - time of breakdown

Table 4.14 Ozone resistance of vulcanisates based on Bulex 1500:NR:SKI-3 = 50:20:30 for tyre sidewalls (SW) and Bulex 1500:SKI-3:SKD = 50:23:27 for tyre protectors (P) at ozone concentration of 3 ppm, 30 oC and 20% elongation in dependence on the stabilising system (3 php). In double mixture the ratio is 1:1. No.

Stabilising system

Time of ﬁrst crack (min)

Character of cracks

Breakdown (h)

1.

S77/2246 (SW)

70

A1T1

5.5

2.

4010NA/2246 (SW)

30

A3T2

2.3

3.

S77/Flectol H (SW)

370

A1T2

35

4.

4010NA/Flectol H (SW)

220

A1T1

35

5.

S77/SC (SW)

970

A1T2

35

6.

4010NA/SC (SW)

>35 h

-

-

7.

S77/Flectol H (P)

155

A1T1

32

8.

4010NA/Flectol H (P)

140

A1T1

32

9.

S77/SC (P)

23

A1T2

32

10.

4010NA/SC (P)

>32 h

-

-

311

Ozonation of Organic and Polymer Compounds

4.5 Prediction The ageing and deterioration of PMP of manufactured rubber articles during their storage and exploitation is related to the change of the molecular mass of the polymer molecules. Among the numerous chemical and physical factors resulting in polymer degradation, as it has been already mentioned, two basic factors should be outlined.

4.5.1 Oxidation Oxidation proceeds via two reaction routes: a mainly radical pathway, which includes the formation of alkyl (R), alkoxy (RO) and peroxy (RO2) radicals and in some particular cases, ionic which gives cations, anions, metals and metalloid species at different oxidation states. The reactions of monomolecular decomposition, cyclisation and disproportionation are the basic reactions which lead to molecular mass reducing. It is known that any oxidation process begins with the reaction of chain initiation, i.e., the reaction of free valence generation. Some of the initiators of oxidation degradation are: UÊ ÝÞ}i]Êâi UÊ ViV>ÊÌ>ÌÀÃ UÊ Ìi«iÀ>ÌÕÀi UÊ }Ì UÊ À>`>Ì UÊ iV>V>Ê>VÌ UÊ V>Ì>ÞÃÌÃ UÊ `iviVÌÃ UÊ ÃÕÌ>iÕÃÊ>VÌÊvÊÌiÊÌ>ÌÀÃÊ`iÃVÀLi`Ê«ÀiÛÕÃÞ Temperature, defects and mechanical effects are the main initiators for the oxidation of rubber articles - PT and rubber goods during their usage.

4.5.2 Ozonolysis Ozonolysis takes place when ozone attacks the elastomer double bonds. The reaction is accelerated by the presence of defects and application of mechanical deformations 312

Degradation and Stabilisation of Rubber while temperature has an insigniﬁcant effect. The main reactions responsible for occurrence of degradation process are the reactions with participation of polymer zwitterions. They replace the strong C=C bonds with the relatively weak peroxide bonds. The latter are rapidly decomposed thus decreasing the molecular mass. Polymeric zwitterions can also interact with low molecular compounds or undergo monomolecular decomposition which also results in reduction of the molecular mass. On predicting the variations in the service properties of manufactured articles one should bear in mind the proceeding of these two reactions. Considering the prolonged duration (years) of storage and exploitation of rubber goods, it is very important to look for and propose accelerated tests for estimation of their behaviour thus providing reliable information for prediction of the storage and usage terms of rubber products. Some accelerating tests are applied: UÊ iÌiÀ>ÌÊvÊÌiÊ«ÞÃViV>V>Ê«>À>iÌiÀÃÊ**®Æ UÊ ÌÃ«iÀVÊ>}i}Ê>ÌÊÌi«iÀ>ÌÕÀiÃÊvÊÇäÊ°C, 85 °C and 100 °C for 56 days. The estimation of PMP involves the determination of the change of two parameters: tensile strength and relative elongation in % at a deﬁnite temperature: aT= (1 - AT/A0) x 100%

(4.7)

where: Ao and ATare the values of the parameter before and after ageing time T. The experimental results from the accelerated test according to this standard are shown in Figures 4.13a and 4.13b. It is obvious that the stabilising system that preserves the PMP of the samples for a more prolonged time at the three temperatures studied is more efﬁcient in its protective action. We have analysed the results obtained and established that the change of A can be quite reliably described by the exponential law: AT= A0.exp(-kT)

(4.8)

where: k is the rate constant of A change and then: a = 1 - exp(-kT)

(4.9)

313

Ozonation of Organic and Polymer Compounds 40 0

90

0

100 C

35

100 C 80 0

30

85 C

70 0

85 C

25

60 50

20 0

15

0

70 C

40

70 C 30

10 20

5

10

0

0

-5

-10

0

10

20

30

40

50

60

0

10

20

Time, days

30

40

50

60

Time, days

(a)

(b)

Figure 4.13 Variation of tensile strength (a) and relative elongation (b) in time depending on the temperature at E = 20% for Bulex-1500 based vulcanisate with 3 php Santoﬂex 77

The experimental points of Figure 4.13 are linearised when plotting in coordinates ln(1-a)/T) ((Figures 4.14a and 4.14b).

0.0

0.0

-0.1

-0.4 0

70 C

0

70 C

-0.8

-0.2 0

-1.2

85 C

0

-0.3

85 C -1.6

-0.4

0

100 C

0

100 C 0

10

20

30

40

50

-2.0

60

Time, days

(a)

0

10

20

30

40

50

60

Time, days

(b)

Figure 4.14 Semilogarithmic anamorphosis of experimental points of Figure 4.13, where ln(S) is strength and ln(E) is relative elongation

The value of k estimated from the curve tangent is (3, 5 and 7) x 10-3 days-1 and (11, 20 and 20) x 10-3 days-1 for the three temperatures and the two parameters, respectively. The dependence of k on temperature is described by the Arrhenius equation: k = k0.exp(-Ea/RT)

314

(4.10)

Degradation and Stabilisation of Rubber The activation energy of the parameter - tensile strength and that of the relative elongation amounts to 7.25 and 8.3 kcal/mol, respectively, (Figure 4.15). The calculated values of k at 20 oC for the parameters are 5 x 10-4 and 1.4 x 10-3 days-1, respectively. According to the standard requirements, any product whose PMP properties have not been changed by more than 20% during storage, i.e., up to values of ln (Ao/ AT) = 2.23, can be used in practice. Then the time of its suitability upon storage, the temperature of which does not usually exceed 20 oC, can be determined by the following expression: T20 = 2.23/k

(4.11)

where: k is a constant at 20 oC.

-3.6

-4.0

2 -4.4

-4.8

-5.2

1

-5.6

-6.0 2.65

2.70

2.75

2.80 3

2.85

2.90

2.95

-1

1/T.10 , K

Figure 4.15 Arrhenius relationships of the rate constants on temperature

Applying the values of k at T20 for vulcanisates in Figure 4.13 it has been found out that the variation in the tensile strength and relative elongation values by 20% will be reached after 12 and 5 years. These values are in good correlation with those observed by us during the long-term storage of these unforced vulcanisates (stored from 1982 at temperatures of 20-25 oC). We have registered 18% and 34% change of these two parameters after 12 years storage. This observation gives us reason to assume that this accelerated test can be successfully used for predicting the service properties of rubber products in real storage conditions:

315

Ozonation of Organic and Polymer Compounds UÊ }i}ÊÊ>ÌÃ«iÀVÊi`ÕÊÜÌÊVÀi>Ãi`ÊâiÊVÌiÌÊÕ`iÀÊÌiÊvÜ}Ê conditions: temperature of 30 p 2 oC, 3 ppm ozone and tensile deformation E = 20%, registering the time of the ﬁrst crack appearance, the breakdown time and the change of the crack pattern. UÊ }i}ÊÊ>ÀÊ>ÌÊÀi>ÌÛiÞÊ}ÊÃÌÕÀiÊiÛiÃÊ>`ÊiiÛ>Ìi`ÊÌi«iÀ>ÌÕÀi° UÊ }i}ÊÊ>ÀÊ>ÌÊVÞVVÊÌi«iÀ>ÌÕÀi]Ê16Ê>`Ê,ÊÀi}i° UÊ }i}ÊÊÌiÊ«iÊÕ`iÀÊ-w>ÊV>ÌVÊV`ÌÃÊ>ÌÊE = 20%. UÊ iÌiÀ>ÌÊ vÊ «iÕ>ÌVÊ ÌÞÀiÊ ÃÌÀ>}iÊ ÌiÀÊ >VVÀ`}Ê ÌÊ ÌiÊ iÌ`Ê vÊ accelerated thermal ageing. At present the usage and storage terms are determined on the basis of the results observed during the real terms of PT storage and exploitation. These terms are 3 and 5 years for car and truck tyres, respectively, that corresponds to 60,000 and 60,000 km. According to the Russian standard these values amount to 45,000 and 70,000 km for car and truck tyres, respectively. The corresponding terms are guaranteed by the standard PMP values of vulcanisates and PT element specimen, the simulation tests of randomly selected PT from production and the observations related to the real life of PT. The analysis of PMP of the protected mixtures reveals that the modern compositions and materials’ properties can ensure a working guarantee period of 80,000 and 100,000 km for car and truck tyres, respectively. The existent standard guarantees 45% and 60% of them leaving a considerable reserve. The main factor for such a high reserve is due to the insufﬁcient level of the tyres stabilisation. Thus, during the accelerating run test, the serial tyres of 165SR/13 size are broken down at 4,500 p 400 km and of 9.00R/20 size at 2,500 p 400 km, while prepared with new stabilising system give 15.5% and 24% more distance, respectively. Next are presented the recipes and the acceleration run test parameters of the new tyres with new stabilising system: UÊ /iÊÌÞÀiÃÊvÊÃâiÊ£Èx-,É£ÎÊ>`Ê°ää,ÉÓä\ The stabilising system of sidewalls and protectors is uniﬁed and it is: Antiozonants: 4010NA – 1.9 php and Santowhite Crystals – 1.1 php; Parfﬁn wax - Ozonschutzvaxe 111 - 2.0 php;

316

Degradation and Stabilisation of Rubber The two tyre sizes are broken down at 5200 p 300 km run and 3100 p 200 km run at a speed of 80 km/h during the acceleration test, respectively. These data clearly manifest the great signiﬁcance of the stabilisation of rubbers and rubber materials and the effect of the new stabilising systems offered by us. In our opinion the conformity with PMP is a necessary but not sufﬁcient condition for evaluation of the future behaviour of rubber-based articles due to the unobligatory application of accelerated ageing tests. The standard values of PMP guarantee the protection of the rubber products against harmful factors arising from the different road conditions. However, the prediction of the useful life of rubber products could not be laid on a scientiﬁc basis without knowledge of the tendency and namely of the rate of PMP variations under the effect of various degradation agents. The physicomechnical properties are closely related to the elastomers molecular mass and its preservation with time [325, 326]. Based on general considerations the rate of degradation of unstabilised diene elastomer will result in 100-fold loss of molecular mass for one year, i.e., from 105-6 to 103-4, which will lead to its complete breakup of the article. For these reasons the adoption of accelerated methods for evaluation of degradation and for predicting purposes, as obligatory is of great practical importance. Actually it represents a simpliﬁed version of the modiﬁcation developed by us, of BS 8800-77. For that purpose the vulcanisates are subjected to thermal ageing at three temperatures T1, T2 and T3, 50, 70 and 90 oC (100 oC). The ageing duration is determined by the variation of a deﬁned PMP by 80% at the highest temperature applied. First the curves describing the change of PMP with time at the three temperatures are plotted afterwhich they are replotted in coordinates log(1/T)(1/T) where (is the time of 40% change of PMP for the three temperatures. Thus, across the points corresponding to 10, 30 and 50% change of PMP at 70 oC are drawn lines parallel to the line with coordinate (1/T40)(1/T2). Thus the nomogram obtained allows the extrapolation at various temperatures and PMP values. The application possibility of this nomogram is demonstrated by the data in Tables 4.15 and 4.16.

317

Ozonation of Organic and Polymer Compounds

Table 4.15 Experimental (T8090, T8050) and calculated (T2025) storage terms of vulcanisates for protectors and sidewalls containing different stabilising systems T8090 (days)

T8050 (days)

T2025 (days)

Side wall - car tyre*

162

1300

2250

Protector - car tyre*

53

600

1125

Side wall - truck tyre*

126

1010

1900

Protector - truck tyre*

41

410

770

2-5-02A

220

1800

3100

2-5-02B

260

2100

4000

2-5-02C

196

1570

2910

2-5-02D

197

1580

2960

1-5-02A

156

1600

3000

1-5-02B

135

1400

2625

1-5-02C

120

1200

2250

1-5-02D

127

1270

2380

Vulcanisate

Note: *: recipe of the tyre plant ‘Vida’-Bulgaria originally from the French Company - Rhone-Poulenc. Symbols 1-5-02 (A-D) are protector compositions proposed by us for car and truck tyres after an uniﬁed reception for car and truck tyres protectors were prepared containing: Bulex 1500 - 50; SKI-3 - 25; SKD - 25; carbon black PM-100 - 65; ZnO - 3; stearic acid - 2; softener PN-6SH - 13; Rubrax - 5; Pyrolen -2; MBS - 1.2; Sulfur - 1.9; stabiliser - 3 php: A - 4010NA /Flectol H = 1.7:1.3; B - 4010NA/SC = 1.7:1.3; C S77/Flectol H = 1.7:1.3 and D - S77/SC = 1.7:1.3. Symbols 2-5-02 (A-D) denote the vulcanisates composition developed by us for sidewalls of car and truck tyres: NR - plasticated - 10; SKI-3 - 40; Bulex 1500 - 30; Bulex M27 - 20, Carbon black PM-75 - 25; Carbon black PM-50 - 50; ZnO - 3; stearic acid - 2; PN-6SH - 13; Pyrolen - 7; MBS - 0.9; sulfur - 1.9; stabiliser - 3 php: A - 4010AN/Flectol H = 1.7:1.3; B - 4010NA/SC = 1.7:1.3; B - S77/Flectol H = 1.7:1.3 and D - S77/SC = 1.7:1.3).

The simultaneous application of this method and BS 8800-77 for evaluation the behaviour of static dynamic strained specimen coupled with the data from ozone and atmospheric ageing can be regarded as a reliable base for prediction the storage and exploitation terms of tyres.

318

Degradation and Stabilisation of Rubber

Table 4.16 Experimental (T8090, T8050) and calculated (T2025) storage terms of produced tyres (their potectors and sidewalls) containing different stabilising systems T8050 (days)

T2025 (days)

Test run (km)

134 82 127 67

1071 820 960 670

2000 1540 1800 1260

4500 2500

165-SR-13 (SW) - recipe A 165-SR-13 (P) - recipe A 9.00-R-20 (SW) - recipe A 9.00-R-20 (P) - recipe A

188 128 191 132

1500 1150 1530 1320

2810 2400 2870 2480

5100 3100

165-SR-13 (SW) - recipe B 165-SR-13 (P) - B 9.00-R-20 (SW) - B 9.00-R-20 (P) - B

198 154 182 135

1590 1540 1460 1350

2980 2890 2740 2530

5300 3000

165-SR-13 (SW) - C 165-SR-13 (P) - C 9.00-R-20 (SW) - C 9.00-R-20 (P) - C

192 113 300 125

1530 1130 2400 1250

2880 1950 4500 2340

5300 -

165-SR-13 (SW) - D 165-SR-13 (P) - D 9.00-R-20 (SW) - D 9.00-R-20 (P) - D

190 119 300 115

1520 1190 2400 1150

2850 2230 4500 2160

2000

Tyres

T8090 (days)

VIDA - base 165-SR-13 (side wall (SW)) 165-SR-13 (protector (P)) 9.00-R-20 (SW) 9.00-R-20 (P) NEW

4.6 Efﬁciency of Antiozonants Under the Conditions of Various Deformations Data from the literature show that the efﬁciency of antiozonants is decreased at application of large deformations [327]. However, the intimated studies have not been carried out and there is no generally accepted view on this problem. We have investigated the behaviour of the following elastomers: SKI-3 with cis-1,4units content of 95%, unsaturation level - 94-98%; SKS-30, trans-1,4-units - 33%,

319

Ozonation of Organic and Polymer Compounds cis-1,4-units - 43.4%, 1,2-units - 23.3%, ash - 0.2% and residual monomers content of 0.15% wt in a broad range of stretch deformations using some of the most widely used antiozonants: PPHDA, hydroquinoline, dithiocarbamates, mono, bis and trisphenol compounds. The rubbers were pre-puriﬁed as follows: dissolution in CCl4 (1% solution) and precipitation by adding a three-fold volume of methyl alcohol followed by drying. The stabiliser was added to the concentrated rubber solution before preparing the ﬁlms for it’s uniformly distribution over the whole sample volume. Then the rubber specimen without sulfur and promoters were subjected to radiation vulcanisation with a 20 MRad dose (Co source) at intensity of 0.9 MRad/h, while the sulfur-containing samples were vulcanised at 150 - 1700 C for 10 - 20 minutes. The degree of crosslinking the samples was determined by the swelling method [328]. The IR spectra (the method of multiple attenuated total reﬂection (ATR) with KRS-5 crystal number of reﬂections - 20, 45o, penetration 10-40 Mm) were monitored during the exposure duration of the specimen to ozone atmosphere - 10 ppm and 20 oC. The IR-spectra of butadiene-styrene rubber in a transsmition (1) and reﬂection (2) mode is given in Figure 4.16.

Figure 4.16 IR spectra of SKS-30, 1 – transmission 2 – reﬂection.

320

Degradation and Stabilisation of Rubber IR ATR-spectra has a good resolution and can be used for quantitative studies. The correlation time (Tc) that stands for the rotation mobility of the nitroxyl radical was estimated on the basis of the ESR spectra of the paramagnetic marker - 2,2,6,6tetramethyl-pyperidine-1-oxyl - b1017 spin/cm3, introduced through evaporation [329]:

Tc = 6.65.$H.[(I+/I-)1/2-1].10-10

(4.12)

where: $H - the width of the lowﬁeld spectrum components; I+, I- - intensity of the ﬁnal spectrum components, in high and lowﬁeld, respectively. The stress relaxation in the rubber specimen in ozone atmosphere (50 ppm) was measured at a temperature of 17 oC in the 5-160% deformation range. Based on the experimental curves the time of chemical relaxation (T), which is inversely proportional to the chemical relaxation rate vr , can be estimated using the following expression [330]:

S/S0=exp(-t/T) ((1-t/T),

(4.13)

where: S is the equilibrium deformation obtained by extrapolation to zero time in the linear part on the relaxation curve; t - time. The initial parts of the relaxation curve allow the determination of the physical relaxation. In order to study the effect of antiozonants in a strained specimen it is necessary to know the structural changes occurring in the rubbers under the inﬂuence of applied deformations (ratio between the amorphous and the crystalline phase). Though the crystallisation processes in cis-1,4-polyisoprene rubbers are well studied [328], due to the speciﬁc degree of crosslinking at the doses applied by us, we have to deﬁne the deformation range for this particular case. Visible reﬂections of the crystalline phase appear at deformations over 300% (Figure 3.5a, b). All tests were conducted in 0-100% deformation range whereby the rubber crystallisation is of minor signiﬁcance [328]. It is well known, [331] that the rates of many chemical reactions in polymers depend on the segmental mobility of the macromolecules. For studying the inﬂuence of the stretching deformations on the segmental mobility some experiments with rubber samples containing a nitroxyl radical were carried out. The dependence of the correlation time of the marker - radical on the elongation degree E is presented in Figure 4.17.

321

Ozonation of Organic and Polymer Compounds It is seen from the ﬁgure that the value of Tc for SKI-3 samples does not change with E which testiﬁes the lack of change in the rotational diffusion of the marker - radical in the used deformation range. However, with SKS-30 (Russia), a decrease of the rotational diffusion of the nitroxyl radical is observed as the value of Tc rises with the increasing of the E. These results suggest that tensile deformation in SKI-3 up to 100% does not change substantially the value of Tc, and consequently the rotational mobility of the antiozonants, whose molecules are very similar in structure and composition to that of the probe-radical. Moreover the translational mobility of the latter does not change too since the rotation and translational diffusion are interconected [332].

5.5

2

5.0

1 4.5 4.0 3.5 3.0 2.5 0

20

40

60

80

, %

Figure 4.17 Dependence of the marker - radical correlation time of (Tc) on elongation level (E). 1 - SKI-3; 2 - SKS-30

Data from the literature [333] clearly shows the close relationship between the antiozonant’s efﬁciency and its rate constant in its reaction with ozone. In Table 4.7 are listed the rate constants (k) which we have measured by using the bubbling method [334]. The rate constants of ozone reaction with amine stabilisers are about 2-3 orders higher than those with oleﬁns and about 3-4 orders as compared with phenol stabilisers (No.13 and 14). Among all the stabilisers studied only N-dithiocarbamate exhibits a similar antiozonant efﬁciency (No.15). The difference in their reactivity

322

Degradation and Stabilisation of Rubber towards ozone is related to their different structure and various donor properties of the substituents. However, it can be accepted that they exhibit similar protective ability with respect to ozone, particularly if one considers that in real conditions the diffusion of antiozonants plays a very important role. Upon radiation vulcanisation the antiozonants are stable and only in amines with long alkyl groups, the alkyl chain is cleaved (No.4 and 6) with preservation of the PPHDA structure while at 200 oC (the maximum vulcanisation temperature) in an inert atmosphere the amine does not undergo any changes.

Table 4.17 Rate constants of ozone with antiozonants in CCl4 at 25 oC No.

Antiozonant

k x 10-6 (M-1.s-1)

1.

4-Methyl, N,N´-bis (iso-propyl)-m-PHDA

24

2.

N-Phenyl, N´-iso-propyl-PPHDA

12

3.

N-iso-propyl-p-anisidine

14

4.

N,Na-Bis (1,4-dimethylpentyl)-PPHDA

13

5.

N,Na-Bis-(1,3-dimethylpentyl)-PPHDA

100

6.

4-Methyl, N,N´-bis-(2-ethylhexyl)-m-PHDA

7.3

7.

N-Phenyl, N´-1,3-methylbutyl-PPHDA

25

8.

N,N´-(Dinitroso, di-sec-octyl)-PPHDA

5

9.

N-Phenyl-B-naphthylamine

20

10.

N-Phenyl, N´-cyclohexyl-PPHDA

24

11.

N,N´-Bis (phenyl)-PPHDA

27

12.

Diphenylamine

40

13.

2,2a-Methylene-bis-(6-tert-butyl-4 ethylphenol)

5 x 10-3

14.

2,2a-Methylene-bis-(6-tert-butyl-4-methylphenol)

4 x 10-3

15.

Ni-bis-(N-dibutyldithiocarbamate)

16.

2,2,4-Trimethyl-1,2-dihydroquinoline

17.

2,2a-Thio-bis-(6-tert-butyl-4-methylphenol)

1 0.8 4.5 x 10-3

323

Ozonation of Organic and Polymer Compounds The efﬁciency of antiozonants was evaluated by: 1) the kinetics of C=O bonds formation, and, 2) the deformation reduction. The rate of change of the 1720/1640 cm-1 ratio, depending on the stabilising system used is depicted in Figure 4.18.

2

60 50 40 30

3 20

6 10

0

1

7

0

45 0

10

20

30

40

50

, %

Figure 4.18 Initial rates of C=O accumulation in SKI-3 in dependence on antiozonants (1-7) or without antiozonants (0) at [O3] = 1 x 10-6 M, 20 oC

Upon ozonolysis of the SKI-3 samples without antiozonant (0) the accumulation of the reaction products was linear in the whole range of deformations applied. At the same time the constancy in the value of $D for antiozonants No.1, 4, 5, 6 (the designation from Table 4.17) indicates the absence of any visible chemical changes on the sample surface. Antiozonants No.2, 3 7 are also efﬁcient in the absence of deformation. It should be noted that $D rises at relatively small values of (while further increase in stress deformation results in a decrease of the rubber oxidation level (Figure 4.19). The existence of critical deformation was also observed for the pure rubber samples and the specimen containing antiozonants No.2, 3 and 7.

324

Degradation and Stabilisation of Rubber

1.4 1.2 1.0 0.8

2

0.6 0.4 0.2

1

0.0 0

10

20

30

40

50

, %

Figure 4.19 Dependence of the oxidation level of SKI-3 (D1720/1640) on E: 1 without antiozonants; 2 - in the presence of 4010NA

As has already been mentioned this phenomenon is characterised by a maximum crack propagation rate and a minimum longevity being observed in a certain range of tensile deformations [327, 328]. The critical elongations were investigated by means of relaxometer. The relaxation curves of ozone-induced SKI-3 samples are demonstrated in Figure 4.20. Highest rate of stress decay is observed in unstabilised rubber sample at E = 20% (the curve is not shown in Figure 4.20) while in the presence of antiozonants under the same conditions the rate of stress decay declines slightly with time. It has been established that at higher deformations (50%, 100% and 160%) the presence of antiozonants in the rubber specimen reduces their ozone resistance as manifested by the relatively higher relaxation rate than the expected one. This supposition is exempliﬁed by the dependence of logarithm of the chemical relaxation time on the tensile deformation without antiozonants (1) and with antiozonants (2) shown in Figure 4.21. It is clearly seen that the presence of antiozonants exerts a substantial stabilising effect in the initial range of small deformations.

325

Ozonation of Organic and Polymer Compounds

1.0

0.8

0.6 160% 0.4

100% 50%

0.2

20% 0

10

20

30

40

Time, min

Figure 4.20 Stress relaxation (S/S0) in SKI-3 at different E on ozonation duration. Antiozonants - No.1 = 2 s 10-6 M

4.0

1

3.5

2

3.0

2.5 0

50

100

150

, %

Figure 4.21 Dependence of the chemical relaxation on samples without antiozonants (1) and with antiozonant addition (2)

326

Degradation and Stabilisation of Rubber

4.7 Effect of Vulcanisate’s Structure The antiozonant efﬁciency also depends on the chemical structure of rubbers [335, 336]. Different points of view are put forward with respect to this relationship. For example, one of them supposes that the stabiliser activity is related antibatic to its ability to form hydrogen bonds with polymers [337]. For evaluating the role displayed by the chemical nature of elastomers on antiozonants efﬁciency we have investigated several rubbers specimen - NR, SKI-3, SKD and SKS-30. Mixtures with the following composition: Elastomer - 100 php, ZnO - 3.0, Stearin - 1.0, Vulcazit CZ - 1.0, sulfur - 1.8, antiozonants - 3.0 were vulcanised at 160 oC for 15 minutes. We have studied the following antiozonant systems: No.2, 4, 15, 16 and 17 (Table 4.17). We have measured the relaxation time (T) which is inversely proportional to the crack growth rate vp [337]: vp = -u(S/S0)/uT = -(S0/Sd).u(F/F0)/uT,

(4.14)

where: F - the stress applied to the sample cross section S; S0 - initial stress; Sdequilibrium stress resulting from the fast physical relaxation. Figure 4.22 shows typical curves of stress decay in the presence of ozone. Curve 1 is assigned to the occurrence of fast physical relaxation taking place within several seconds, up to one minute and does not change with time. It is characterised by a slight decline at the beginning which is related to the physical re-orientation of the macromolecules and the constancy of the parameter F/F0 with the time after that. This region designated in the ﬁgure as (1) is observed in all curves. In the presence of ozone (curves 2 and 3) the pattern changes radically. Moreover, curve 2 clearly shows three regions: I - fast physical relaxation up to 1 minute, II - chemical relaxation up to 5 minutes, a linear stress decay is observed accompanied by visible changes on the sample surface, and consequently with cracks originating on them, III - after 5 minutes - chemical relaxation which accelerates the decay of stress and is associated with cracks appearance and propagation. If one assumes the rate of region II as unity, then the acceleration for region III will be 3.2 times. Curve 3 is characterised by an absence of region II since immediately after region I (physical relaxation) portions of cracks grow at a constant rate. The slow down of the stress decay is about 1.7 times as compared with the rate for curve 2, region II and remains constant up to 18 minutes. The greater elongations change the surface rheology making it energetically more isotropic and thus the risk of appearance and growth of separate cracks to sizes resulting in sharp decrease of the sample resistance is decreased (region II in curve

327

Ozonation of Organic and Polymer Compounds

2). The addition of antiozonants (curve 4) leads to marked expansion of region II with almost zero value of change rate, up to 15 minutes, and in region III (after 15 minutes) the rate of relaxation decay is by a factor of 2 smaller than that of region II in curve 2.

1.0 1

II I

0.8

III

II

III 4

0.6

3

III

0.4

0.2

2 0

5

10

15

20

25

Time, min

Figure 4.22 Relaxation curves of SKI-3 samples without stabiliser and with stabiliser addition in air and ozone atmosphere - 9 x 10-4 mol/m3 at 20 oC. 1 - without antiozonants, in air, E = 10%; 2 - without antiozonants, in ozone atmosphere, E = 10%; 3 - without antiozonants, in ozone atmosphere, E = 100%; 4 - in the presence of 5 php of 4010NA, in an ozone atmosphere, E = 10%

Another parameter characterising the process of ozone interaction with rubber samples is the change in crack growth rate (vp) with E (Figure 4.23). The rate of cracks growth depends on the ozone concentrations as it is shown in Figure 4.24.

328

Degradation and Stabilisation of Rubber 30 25 20

5

15 10

3 5 0

4

6

2 1 0

20

40

60

80

100

120

140

160

180

, %

Figure 4.23 Dependence of vr on E for SKI-3 without antiozonants (curves 1, 3, 5) and in the presence of antiozonants - 4010NA (curves 2 and 4), S77 (curve 2) and N-iso-propyl-p-anisidine (curves 2, 4, 6) in amounts of 1, 3 and 5 php, respectively, at [O3] = 5 x 10-4 mol/cm3 - (curves 1 and 2), 6.4 s 10-2 (curves 3 and 4) and 9 x 10-4 mol/cm3 (curves 5 and 6). Note: the experimental points for antiozonants - 4010AN and S77 on curves 2 and 4 could not be seen for their values are very close to those of the third antiozonants.

1 -1.0

2

-1.5

-2.0

-2.5

-3.0

-3.5

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

3

lg [O3], mol/m

Figure 4.24 Dependence of log vp for SKI-3 samples, E = 20%, at 20 oC on log [O3]; 1 - without antiozonants; 2 - in the presence of 3 php of 4010AN

329

Ozonation of Organic and Polymer Compounds It is seen that the value of T' is constant in the stationary zone of stress decay and has the same meaning as the relaxation time T' in Tobolski’s formula [338]. Figure 4.25 shows the dependence of the value of log n on the degree of tensile deformation in the vulcanisates SKI-3 (a), NR (b), SKD (c) and SKS-30 (d). A similar pattern is observed only in two cases: SKI-3 and NR. This similarity is not only qualitative but also quantitative both for the control (curve 1) and the protected samples. The shape of all the curves, irrespective of the presence of a stabiliser, is also quite similar; the lowest stability (the highest rate of cracks growth rate of cracks) is observed in the range of critical deformations E = 10-20%. However, with an increase in the degree of strething the stability increases (the parameter Tc rises).

8.5

b a

7.0

8.0

2 0

7.5

6.5 4, 14 17

6.0

7.0

4

16

6.5

5.5

2

6.0

0

5.0

15

16

5.5 4.5

5.0 0

4.0 0

20

40

60

80

100

120

140

20

40

60

160

80

100

120

140

160

, %

, %

8.5

7.5

d

c

8.0

2

7.0 0

15 16

6.5

17 6.0

7.5

4

7.0

5.5

6.5

5.0

6.0

4.5

5.5

4.0

5.0

17 0 15 14 16

4.5

3.5 0

20

40

60

80

, %

100

120

140

160

0

20

40

60

80

100

120

140

160

, %

Figure 4.25 Relationship between log nT and the degree of deformation E in specimen from SKI-3 (a), NR (b), SKD (c) and SKS-30 (d) at ozone concentration of 6.4.10-8 M, (the number of the curves correspond to the number of the stabiliser in Table 4.17)

330

Degradation and Stabilisation of Rubber The stability increase can be explained by the decrease in the reactivity during the extension of cis-fragment -C-C=C-C- in all the reactions accompanied by the rearrangement of two carbon atoms at the double bond from sp2 into sp3 state [339] and it is well known that ozone attacks the double bond with the formation of a primary ozonide. There also exists another interpretation of the ‘critical deformation’ phenomenon, based on the mechanism of orientation strengthening [335]. However, as shown by ESR spectral studies performed with the use of a marker radical [340, 342], at least up to 200%, the tensile deformation of such elastomers of SKI-3, NR and SKD does not change the polymer matrix rigidity (segmental mobility of macrochains remains constant). That is why in these cases orientation strengthening cannot, in our opinion, play any signiﬁcant role. A different situation is registered in the case of SKS-30. The stretching of its vulcanisate raises substantially the rigidity of its polymer matrix [340]. It is with the structuralphysical changes that the rise in (value with the growing E) observed in Figure 4.25c, seems to be associated. An efﬁcient stabiliser for SKI-3 and NR under the chosen testing conditions is the widely applied antiozonant - 4010NA. In the whole range of deformations this stabiliser slows down the growth of cracks, which follows from the comparison of curves 2 and 1 in Figure 4.26 (curve 2 lie above curve 1). This slowing down, however, is not very great: in SKI-3 the vr values decreases by a factor of approximately 1.6, in NR - 1.3. Compositions containing other stabilisers are less resistant to ozone effect under these conditions, as compared with the control samples. The similarity in the results obtained for SKI-3 and NR is caused by the identity of their monomer unit. The signiﬁcant role of the macrochain chemical nature is conﬁrmed by the difference in the results obtained for elastomers with other monomer units. A relatively small difference in the structure of cis-1,4-polyisoprene (the presence of CH3-substituent) leads to a quite signiﬁcant difference in the stabiliser efﬁciency. Thus, it is seen from Figure 4.18c that SKD vulcanisate is stabilised much better than SKI-3 and NR. Apart from 4010AN, other stabilisers prove also to be efﬁcient in the whole deformation range: No.4 (Figure 4.26b, curve 3) and No.15 (the curve is not given because it is identical to curve 1 and lies above it), in the region of small deformations the stabilising effect is manifested by No.16 (up to 60%, curve 4) and No.17 (up to 20%, curve 5). It should be emphasised that for SKD samples especially a decrease in the efﬁciency of stabilisers with the growing of E is a characteristic. The only exception is antiozonants No.15. Table 4.18 illustrates the stabilising effect, expressed as a degree of a decrease

331

Ozonation of Organic and Polymer Compounds in the crack growth rate at various deformations of samples, as compared with samples that contain no stabilisers.

8

8

SKI-3

NR

2

7

2

7

1

5 1

3, 5, 6

6

3, 4, 6, 7 6

7 4 5

5

4

4

3

3

0

50

100

150

0

50

Elpngation, %

100

150

Elongation, %

9

9

SKS-30

SKD 8

2

3

3

8

5

7

7

6

5 5

4 1

4

1

2, 4, 7

6 5

6

4

3

3

0

50

100

150

0

Elongation, %

50

100

150

Elongation, %

Figure 4.26 Dependence of log nT on the degree of deformation E in: a) SKI-3; b) NR; c) SKD; d) SKS-30, [O3] = 6.4 s 10-2 mol/cm3, 20 oC . The numbers of the curves correspond to the numbers of the samples in Table 4.17

Table 4.18 Degree of a decrease in the crack growth rate in SKD samples in the presence of a stabiliser, compared with samples without antiozonants, at different deformations Antiozonant

E= 20%

E= 30%

E= 60%

E= 100%

No.4

-

14.7

5

2.4

No.2

10

3.6

1.4

1.5

No.16

8.1

2.2

1.1

0.7

No.15

1.4

1.6

1.6

1.6

332

Degradation and Stabilisation of Rubber We have observed a decrease in the antiozonant efﬁciency with an increase in the degree of cis-1,4-polyisoprene deformation. Thus, in Figure 4.25b the highest stability effect of 4010NA (curve 2) is displayed for E = 10% (two-fold decrease in vp; and at larger deformations the effect decreases. But, as it follows from the above data, that this effect is much more pronounced in the case of SKD. Therefore, one can speak of a different nature of log nTc = f(E) dependencies obtained for the protected vulcanisates of butadiene and isoprene rubbers (Figure 4.25 a-c). For SKS-30 vulcanisate one can note the following peculiarities. This elastomer, as well as SKD, is stabilised by No.4 in the whole range of E (curve 3). Inhibiting effect is also exerted by No.17 at all deformations (curve 5), although to a smaller extent than No.4. All other stabilisers slow down ozone degradation only at small E (up to 20-30%), and at large deformations their presence has an opposite effect - accelerate the cracks growth. Another similarity of SKS-30 with SKD manifests itself in a decrease in the efﬁciency of stabilisers with the growth of E, although it is not so substantial as in SKD. This is demonstrated by the data on the decrease in vp in strained SKS-30 samples in the presence of antiozonants (Table 4.19).

Table 4.19 Degree of a decrease in the crack growth rate in SKS-30 samples in the presence of a stabiliser, compared with samples without antiozonants, at different deformations. Antiozonant

E = 10%

E = 20%

E = 60%

E = 100%

E = 150%

No.4

10

8

5

4

3

No.17

6.7

-

1.4

1.3

1.7

No.14

2.3

0.8

0.8

0.8

0.8

No.15

2.3

1.2

0.8

0.8

0.8

In our opinion, the results presented in Figure 4.20 cannot be explained by the quenching of ozone molecules by the stabiliser, i.e., with the help of a mechanism based on a higher antiozonants reactivity with respect to ozone in comparison with the polymer C=C bonds. This follows from the data obtained related to the inﬂuence of the macromolecule chemical structure on the stabilisers efﬁciency. Most clearly this is seen for No.2 and 4. The rate constants of their reactions with ozone are practically the same and equal to 1.2 x 107 and 1.3 x 107 M-1.s-1, respectively, [341, 342]. Their protective effect, however, differs considerably and depends on the type of the polymer. Indeed, stabiliser No.2 raises and No.4 lowers the stability of SKI-3 and NR samples, whereas for SKD and SKS-30 stabiliser No.4 is a very efﬁcient one.

333

Ozonation of Organic and Polymer Compounds These facts can only be explained by taking into account the active role played by the polymer matrix itself. It should be noted that this role is hardly caused by the intermolecular interaction of the polymer with the stabiliser, since the elastomers in question are by their nature of low polarity. From the standpoint of intermolecular interaction it is difﬁcult to explain the considerable difference between SKI-3 and NR, on the one hand, and SKD and SKS-30, on the other. Most probably, the stabilisers enter into a chemical reaction with the products of the polymer oxidative degradation (crosslinking of the cut-off fragments). In this case the nature of the monomer unit can predetermine the course of the rate of such a reaction, thereby inﬂuencing the antiozonants efﬁciency. From this point of view it proves possible to explain the No.4 efﬁciency in the two cases of SKD and SKS-30 because they have similar chemical fragments resulting from butadiene polymerisation. And, possibly, it is with the products of ozone oxidation of these very fragments that a given stabiliser interacts effectively. In polyisoprene chains a similar chemical process most probably does not take place at all or occurs with appreciably lower rate. Thus, it seems that, as noted above, two factors (in the case of polyisoprene samples) are being superimposed [343]: the interaction of antiozonants with ozone and the formation of a protective ﬁlm on the polymer surface, whereas in the presence of butadiene units another means of protection appears - crosslinking of the brokendown antiozonant fragments. The latter, though more effective, are quite sensitive to deformation. Indeed, stretching must facilitate the drawing apart of the molecule cut-off ends and thereby impede their crosslinking. The conclusion put forward for the active role of the chemical nature of the polydiene monomer unit is in agreement with the data from the literature data. It is known that the two end aldehyde groups resulting from the ozonation of the polybutadiene monomer unit are capable of reaction with diamines, thus restoring the broken polymer chain. The ozonation of the polyisoprene monomer unit leads to aldehyde and ketone grups formation. That is the reason why diamines cannot restore the length of the polyisoprene macromolecule [345]. In conclusion it should be noted that the ozone degradation of diene polymers is a result of ozone reaction with the C=C bonds in the polymer macromolecule whereby the C=C bond is broken or is replaced by the weak peroxide bonds in ozonides and in di- and polyperoxides. The most susceptible areas to ozone attack are the defects on surface, amorphous and interphase regions in the elastomer structure. The application of external deformations in all cases accelerates the ozone degradation transforming the strain energy during C=C bonds cleavage. The critical deformations, in our opinion, are determined by the initial rates of ozone interaction with the elastomer and the restoration of the degraded surfaces in addition to the mechanisms cited in literature.

334

Degradation and Stabilisation of Rubber The ozone stabilisation of elastomers is accomplished by using antiozonant systems which can impede the harmful ozone effect by exerting simultaneous protective action, both chemically and physically. They include ozone quenching, C=C bonds and ozone deactivators, stucturising bi- and polyfunctional compounds, which connect the cut-off ends of the macromolecules and resulting in fast stress relaxation, compounds that facilitate the antiozonants transport from the volume to the reaction centre, physical deactivators and concentration diluters.

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344

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346

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347

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348

Degradation and Stabilisation of Rubber 210. B.L. Khavkina, S.A. Ovchinnikova, E.V. Safronova, V.M. Gorchakova and R.G. Stefanskaya, inventors; Moscow Textile Insitut, assignee; SU759556, 1980. 211. J.E. Vostovich, inventor; General Electric, assignee; FR2459266, 1981. 212. E.L. Kay, R. Gutieerez and W.R. Hausch, inventors; Firestone Tire & Rubber Company, assignee; US4568711, 1986. 213. H. Nagasaki, Y. Takemoto, T. Yamaguchi, A. Okamoto, H. Okamura and E. Okino, inventors; Sumitomo Chemical Company, assignee; JP60245652, 1985. 214. Y. Hiarata and H. Kondo, inventors; Bridgestone Corporation, assignee; JP61168640, 1986. 215. H. Nagasaki, Y. Takemoto, M. Yoshimura and E. Okino, inventors; Sumitomo Chemical Company, assignee; JP60250051, 1985. 216. Y. Imoto and N. Ichikawa, inventors; Sumitomo Rubber Industries, assignee; JP2008189718, 2008. 217. O. Uegakito, H. Doi, Y. Fukushima, S. Yamakoshi and T. Sakakibara, inventors; Toyoda Chuo Kenkyusho KK and Aisin Seiki, assignees; JP58108236, 1983. 218. H. Nagasaki and G. Honda, inventors; Sumitomo Chemical Company, assignees; JP57117545, 1982. 219. A. Zelenskij, N.M. Kolesnikov, V.G. Suzanskij, V.P. Kavalerchik and L. Tishchikova, inventors; Bruss Ti Kirova, assignee; SU594138, 1978. 220. T. Tada, H. Kishimoto, T. Mabuchi and K. Shiga, inventors; Sumitomo Rubber Industries, assignee; JP2008195893, 2008. 221. Y. Minagawa and N. Yagi, inventors; Sumitomo Rubber Industries, assignee; JP2007321041, 2007. 222. R.S. Frenkel and M.A. Krakshin, inventors; R.S. Frenkel and M.A. Krakshin, assignees; SU954404, 1982. 223. H.C. Beadle, inventor; RT Vanderbilt Company, Inc., assignee; US3969315, 1976.

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Degradation and Stabilisation of Rubber 296. V.D. Voronchikhin, I.A. Iliin, D.V. Ershov, K.A. Dubkov, D.P. Ivanov, S.V. Semikolenov and G.I. Panov, inventors; Institut Kataliza Imeni GK Boreskova Sibirskogo Otdeleniya Rossiiskoi Akademii Nauk, assignee; WO2009022941, 2009. 297. T.A. Ustinova, T. Polyakova and S. Luk’yanenko, Kauchuk i Rezina, 1972, 4, 16. 298. S.E. Khamin, L.G. Angert and A.S. Kuzminskii, Kolloid Journal, 1975, 37, 99. 299. V.S. Zacheev and A.I. Zvonkova, Production of Tyres, Rubber and Asbestos Goods, 1974, 1, 11. 300. N.I. Troitskaya and Yu.S. Zuev, Kauchuk i Rezina, 1974, 6, 18. 301. J. Campbell and R. Thurn, inventors; EI DuPont de Nemours, assignee; US3817884, 1974. 302. D.L. Turk, inventor; The Goodyear Tire & Rubber Company, assignee; US3704741, 1972. 303. International Synthetic Rubber Company, inventor; International Synthetic Rubber Company, assignee; GB1357318, 1974. 304. M.Y. Mix, European Rubber Journal, 1975, 6, 27. 305. E.H. Andrews, Rubber Chemistry and Technology, 1967, 40, 633. 306. S.E. Khaninya, L.G. Angert and A. S. Kuzminskii, Kauchuk i Rezina, 1974, 1, 30. 307. U.S. Maloshuk, V.N. Kuleznev and S.E. Khanin, Colloid Journal, 1973, 35, 408. 308. B.T. Ashworth, B.N. Leisland and P.M. Quan in Proceedings of the International Rubber Conference, Brighton, UK, 1972. 309. M.E. Cain and P.J. Watson, Journal of the Rubber Research Institute of Malaysia, 1969, 22, 289. 310. J.I. Cunneen, Revue Generale des Caoutchoucs et Plastiques, 1972, 49, 6, 521. 311. V.P. Kirpichov and N.S. Domnina, Kauchuk i Rezina, 1972, 7, 20.

355

Ozonation of Organic and Polymer Compounds 312. V.P. Kirpichov and N.S. Domnina, Vestnik Penzenskogo Gosudarstvennogo Universiteta, 1978, 3, 16, 126. 313. T.Ya. Gordon, Stabisation of Synthetic Polymers, Publishing House – Khimicheskaja Literatura, Moscow, Russia, 1963. 314. The Goodyear Tire & Rubber Company, inventor; The Goodyear Tire & Rubber Company, assignee; GB1498707, 1978. 315. G.F. Bebikh and V.P. Saraeva, Vysokomolekulyarnye Soedineniya Seriya A, 1976, 18, 461. 316. R.H. Kline and Y.R. Miller, Rubber Chemistry and Technology, 1973, 46, 98. 317. G.E. Mayer, R.W. Kavchok and F.J. Naples, Rubber Chemistry and Technology, 1973, 46, 196. 318. R. Levy, Revue Generale des Caoutchoucs et Plastiques, 1974, 51, 243. 319. R.H. Kline, inventor; Goodyear Tire & Rubber Company, assignee; US3714122, 1973. 320. R.H. Kline, inventor; The Goodyear Tire & Rubber Company, assignee; US3962187, 1976. 321. Kinetics and Mechanism of the Formation and Transformation of Molecules, Ed., N.A. Plate, Nauka Publishing House, Moscow, 1968. 322. M.A. Askarov and A.S. Bank, Chemical Stabilisation of Polymers, Uzbekistan Academy of Sciences, Tashkent, Uzbekistan Soviet Socialist Republic, 1974. 323. B.N. Gorbunov, Y.A. Gurvich and I.P. Maslova, Chemistry and Technology of Stabilisators for Polymeric Materials, Khimiya Publishing House, Moscow, 1981. 324. N.M. Emanuel and A.L. Buchachenko, Chemical Physics of the Degradation and Stabilisation of Polymers, Nauka Publishing House, Moscow, 1983. 325. N.M. Emanuel and D. Gal, Oxidation of Ethylbenzene, Nauka Publishers, Moscow, 1984. 326. G. Scott and E. Setoudeh, Polymer Degradation and Stability, 1983, 5, 11. 327. J. Rotschova and J. Pospisil, Collection of Czechoslovakian Chemical Communications, 1981, 47, 2501.

356

Degradation and Stabilisation of Rubber 328. L. Taimr and J. Pospisil, Polymer Degradation and Stability, 1984, 8, 1, 23. 329. J. Furukawa, S. Yamashita and T. Kotani in Stabilisation of Polymers and Stabiliser Processes, Advances in Chemistry Series, No.85, ACS, Washington, DC, USA, 1968, p.113. 330. S.D. Razumovskii, V.V. Podmasterev and G.E. Zaikov, Doklady Akademii Nauk SSSR, 1982, 129. 331. A.A. Popov and G.E. Zaikov, Journal of Macromolecular Science C, 1983, 23, 1, 1. 332. C.S. Amsden, Annual Ozone Test Lecture at the AGM of the Institute of Rubber Technologists, Sweden, 1967. 333. N.Ya. Rapoport and G.E. Zaikov, Developments in Polymer Degradation, 1985, 6, 207. 334. H. Chtourou, B. Riedl, B.V. Kokta, A. Adnot and S. Kaliaguine, Journal of Applied Polymer Science, 1993, 49, 2, 361. 335. J.R. Ebdon and S. Rimmer, Journal of Polymer Science A: Polymer Chemistry, 1997, 35, 15, 3255. 336. J. Couve, R. Dobrevaschue, J. Sledz, F. Schue and P. Nicol, European Polymer Journal, 1997, 33, 2, 137. 337. P. Gatenholm, T. Ashida and A.S. Hoffman, Journal of Polymer Science A: Polymer Chemistry, 1997, 35, 8, 1461. 338. W.S. Kim and T.S. Huh, Bulletin of the Korean Chemical Society, 1997, 18, 5, 543. 339. Rimmer, J.R. Ebdon and M.J. Shepherd, Reactive & Functional Polymers, 1995, 26, 1-3, 145. 340. E.T. Kang, K.G. Neoh, X. Zhang, K.L. Tan and D.J. Liaw, Surface and Interface Analysis, 1996, 24, 1, 51. 341. T. Fargere, M. Abdennadher, M. Delmas and B. Boutevin, Macromolecular Chemistry and Physics, 1994, 195, 5, 1673. 342. E.J. Ebdon, N.J. Flint and S. Rimmer, Journal of Macromolecular Science, Pure and Applied Chemistry A, 1995, 32, Supplement 5-6, 603.

357

Ozonation of Organic and Polymer Compounds 343. S. Varghese, B. Kuriakose and S. Thomas, Polymer Degradation and Stability, 1994, 44, 1, 55. 344. M. Giurginca, T. Zaharescu and A. Meghea, Polymer Degradation and Stability, 1995, 50, 1, 45. 345. S.G. Hong and C.M. Liao, Polymer Degradation and Stability, 1995, 49, 3, 437. 346. A.M. Evtushenko, I.P. Chikhacheva, G.V. Timofeeva, S.D. Stavrova and V.P. Zubov, Vysokomolekulyarnye Soedineniya Seriya B, 1993, 35, 6, 312. 347. K.M. Bulanin, J.C. Lavalley and A.A. Tsyganenko, Journal of Physical Chemistry B, 1997, 101, 15, 2917.

358

5

Quantum Chemical Calculations of Ozonolysis of Organic Compounds

Quantum chemical calculations of organic compound ozonolysis are continuously expanding and cover more and more new compounds and separate stages in already accepted mechanisms of interaction [1-10]. The reaction of ozone with C=C bonds [11-20] is one of the reactions that has attracted much attention. Here should be mentioned the fundamental contributions of Godart [20-22], Cremer [6, 7, 24], Rouz [17, 19], etc., who have applied various ab initio methods. A number of important and interesting ozone reactions, however, have remained out of the scope of research interest [21-25]. In this connection we devoted our studies on quantum chemical calculations of the ozonolysis of organic compounds containing H, O, S, N and P atoms. The transition states (TS) depending on the atom to which the ozone attack is directed, are denoted in the following manner: hydrogen bound to carbon atom - TS1, hydrogen bound to heteroatom - TS2 and heteroatom - TS3. We have carried out the calculations by using a MOPAC6 program, of which the details, parameterisation and practical use are described in references [26-83].

5.1 Alkanes The calculations were performed according to the method of the reaction coordinate, whereby all coordinates of the reagents are optimised with the exception of the ﬁxed reaction coordinate. The latter varied from 3-4 to 5 Å with a gradient from 0.2 to 0.05 Å. The transition states for the reactions investigated are described as follows: CH3-(H2)C-H...O3

(CH3)2(H)C-H...O3

(CH3)3C-H..O3

(1) Linear transition states that correspond to the hydrogen-atom abstraction mechanism; the C-H bond, being the bond of lowest bond energy, is the reaction centre of the molecule: CH3-H2C-H...O3

(CH3)2-HC-H...O3

(CH3)3C-H..O3

359

Ozonation of Organic and Polymer Compounds (2) Cyclic transition states, which accounts for the 1,3-cycloaddition mechanism, including a transitory state with 5-coordinated carbon and resulting in hydrotrioxide formation in one step:

C

H O

O O

A summary of the results from calculating the kinetics of the ozone reaction with methane, ethane, propane and iso-butane with linear transition states of the TS1 type are shown in Figure 5.1. The calculations with cyclic transition states are not given because we failed to observe the formation of any structure resulting from the C-H bond cleavage and formation of two new bonds C-O and O-H. The data in Figure 5.1 support the ozone reaction with C-H bonds. For methane, the C-H bond cleavage occurs at a reaction coordinate within 0.95 Å and 0.9 Å with a simultaneous drop of $H from 96.3 kcal to 75.3 kcal. In the case of ethane this takes place at a reaction coordinate from 1 Å to 0.95 Å and a change of $H from 79.8 kcal to 11.9 kcal. For the ozone reaction with propane the C-H bond break occurs at a reaction coordinate from 1.05 Å to 1 Å and $H changes from 61.7 kcal to 46 kcal. The reaction coordinate for iso-butane C-H bond ﬁssion lies between 1.05 Å and 1 Å and $H changes from 54.2 to –25.9 kcal. The analysis of the foregoing results reveals the following sequence of increasing reactivity of the studied alkanes: methane<ethane<propane
360

Quantum Chemical Calculations of Ozonolysis of Organic Compounds

100

0.8

1.0

1.2

60

1' 2' 3'

40

4'

80

1.4

1.6

1.8

2.0

CH3-H...O3 (CH3)2CH-H...O3

20

C2H5-H...O3

0

(CH3)3C-H...O3

-20 -40 3.6

(CH3)2CH-H...O3 3

3.2

4 (CH3)3C-H...O3

2.8

2

2.4 2.0

1 1.6

C2H5-H...O3 CH3-H...O3

1.2 0.8

1.0

1.2

1.4

1.6

1.8

2.0

Reaction coordinate, A

Figure 5.1 Interaction of ozone with methane, ethane, propane and iso-butane. 1, 2, 3, 4 - dependence of the C-H bond length on the reaction coordinate, respectively, and 1', 2', 3' and 4' - dependence of $H on the reaction coordinate, respectively

361

Ozonation of Organic and Polymer Compounds

Table 5.1 Reaction of methane with ozone - localisation of TS Gradient norm minimised using NS01A SCF ﬁeld was achieved Parameters Heat of formation Electronic energy Core-core repulsion Gradient norm RMS force Dipole No. of ﬁlled levels Ionisation potential Molecular symmetry Molecular weight SCF calculations Computation time

Values 28.597233 kcal/mol –2761.007473 eV 1623.351869 eV 0.079615 0.018765 1.78872 debye 13 9.738345 eV c1 64.041 149 1845.20 s

Final geometry optimisation AM1 precise NS01A t = 60 m PULAY ITRY = 1200 Methane + ozone TS localisation Charge *1 2

5

6

7

8 9 10

0.000000 0 0.000000

0

0.000000

0

0 0 0

–0.2678

H 1.111572 1 0.000000

0

0.000000

0

1 0 0

0.0649

H 1.111566 1 109.469708 1

0.000000

0

1 2 0

0.0634

H 1.111589 1 109.449504 1

119.975690

1

1 2 3

0.0648

H 1.112658 1 109.474622 1

–120.050073 1

1 2 3

0.0749

XX 1.000000 0 90.000000

0

0.000000

0

5 1 2

O 2.409391 1 89.586115

1

179.856964

1

5 6 1

–0.2786

O 1.160081 1 168.570623 1

114.303161

1

7 5 6

0.5535

O 1.160081 1 120.954422 1

–168.405569 1

8 7 5

–0.2752

0

0.000000

0 0 0

C

3 4

0.000000 0 0.000000

0

0

1 – chemical symbols of the atoms formed the investigated molecule 2 – atomic distance in angstroms between this atom and the atom connected with it 3 – Program to does (1) or not (0) the optimisation of the atomic distance 4 – Valent angle in degrees 5 – Optimisation of this parameter: yes 1 or no 0 6 – Dihedral angle 7 – Optimisation: yes 1 or no 0 8 – the number of neighbouring atoms with that distance is formed 9 – the number of atom that takes part in the formation of the valent angle 10 – the number of atom with that the dihedral angle is formed. TS = transition state SCF = self-consistent ﬁeld

362

Quantum Chemical Calculations of Ozonolysis of Organic Compounds In the transition state any substantial change in the length of the C-H bond to which the ozone attack is directed, is not observed as shown by its negligible change (by 1%) and the reaction coordinate is 2.4 Å. Practically, it can be assumed that the transition state is not localised. This compelled us to use other key words for localisation. Looking for the transition states as a biradical the results are obtained as shown in Table 5.2.

Table 5.2 Reaction of methane with ozone - localisation of TS Geometry optimised using Eigenvector following (EF) SCF ﬁeld was achieved Parameters Heat of formation Electronic energy Core-core repulsion Dipole No. of ﬁlled levels No. of open levels Ionisation potential Molecular weight SCF calculations Computation time

Values 68.776672 kcal –2913.845415 eV state: singlet a 1770.900535 eV 2.90916 Debye Symmetry: c1 12 2 5.330939 eV 64.041 49 8 min and 51.680 s

Final geometry obtained MNDO PULAY t = 500 m TS biradical Methane + ozone Charge C 0.00000000 0 H 1.09600787 1 H 1.09630043 1 H 1.09587419 1 H 1.29083650 1 XX 1.00000000 0 O 1.25191432 1 O 1.26258040 1 O 1.20288730 1

0.0000000 0.0000000 113.5850685 113.8612890 104.5292213 90.0000000 85.9170670 112.7809188 114.7207682

0 0 1 1 1 0 1 1 1

0.0000000 0 0.0000000 0 0.0000000 0 132.4350059 1 –113.6886841 1 0.0000000 0 179.9731171 1 147.2814073 1 –93.6461817 1

0 1 1 1 1 5 5 7 8

0 0 2 2 2 1 6 5 7

0 0.0011 0 0.0257 0 0.0217 3 0.0230 3 0.1295 2 1–0.2060 6 0.2789 5–0.2740

It follows from this matrix Z that the transition state is more probably a biradical producing a CH3· radical and ·O3H radical with 18% extension of the C-H bond. 363

Ozonation of Organic and Polymer Compounds The difﬁculties on transition state localisation with parafﬁns can be related to two reasons: parameterisation of the semiempirical calculations and the low probability of these reactions proceeding. In spite of this the observed tendency shows that the transition state may be related to the hydrogen-atom abstraction.

5.2 Oxygen-containing Compounds 5.2.1 Water It can be assumed that the ozone reaction with water may take place through the following transition states, TS2 and TS3: H

O H

H 2

O

O

O2 H

O

O2

3

At TS2 the reaction coordinate is the H...O bond, while at TS3 it is the O...O bond. The changes in the bond lengths and heats of formation ($H) depending on the reaction coordinate are shown in Figure 5.2. The analysis of Figure 5.2 reveals that the ozone reaction with water through the O-H route is inefﬁcient as the decrease in the reaction coordinate from 2.5 Å to 0.9 Å leads to a 27% change of the O-H bond length (from 0.96 Å to 1,232 Å). The heats of formation of the transition states rise smoothly without any sharp leap. The interaction in the oxygen atom direction is quite different. The change of the reaction coordinate from 3 Å to 1.3 Å does not change the O-O2 bond up to 1.5 Å (1.16-1.19 Å) but in the range of 1.5-1.3 Å its length rises abruptly up to 2.14 Å (i.e., by 84%) and $H is reduced from 46.2 to 17.1 kcal. The presence of such a sharp transition in the bond length and the signiﬁcant positive difference in $H in this case indicates the great probability of this reaction pathway for the reaction between ozone and water. According to this route the reaction products should be H2O2 and O2. In fact some amounts of hydroperoxide have been experimentally observed in water ozonolysis.

5.2.2 Methanol The ozone reaction with methanol may proceed through the formation of the following transition states: through attack on the C-H bond, TS1, through attack on the O-H

364

Quantum Chemical Calculations of Ozonolysis of Organic Compounds bond, TS2, and attack on the oxygen atom, TS3. The ﬁrst presumes the formation of HOCH2· and HO3· (HO· + O2). This mechanism also assumes the formation of HOCH2OOOH, HOCH2OH or HOCH2OO· in the second step. In the case of TS2, CH3O·, HO3·(HO· + O2) will be formed ﬁrst and then CH3OOH. For TS3, HOCH3OOOH will be obtained. The quantum chemical calculations on the three routes are presented in Figure 5.3.

80 2.2

3

2'

60 2.0 40

1.8

3'

1.6

20

1.4

0

2 H2O...O3

1.2

H2O...O3 HO-H...O3

1.0 0.8

1.2

1.6

2.0

2.4

HO-H...O3

-20

-40

2.8

Reaction coordinate, A

Figure 5.2 Ozone reaction with water: 2 - dependence of the O-H bond length on the reaction coordinate at TS2; 3 - dependence of the O-O2 bond length on the reaction coordinate at TS3; 2′ - dependence of $H (TS2) on the reaction coordinate; and 3′ - dependence of $H (TS3) on the reaction coordinate

The analysis of the results show that TS1 is the preferred route for this reaction. At a reaction coordinate variation from 2.5 to 1.1 Å the C-H bond length changes smoothly from 1.12 to 1.226 Å (i.e., by 9%). When the reaction coordinate reaches the value of 1.0 Å the C-H bond length rises sharply to 2.973 Å and $H drops abruptly thus suggesting a C-H bond break. However, at this moment no change in the O-O2 bond length is observed on the TS1 diagram. Thus two particles, i.e., HOCH2· and HO3· are obtained in this step. The variations that are observed at TS2 and TS3 are ﬂuent without any indications of qualitative changes in the system. This suggests

365

Ozonation of Organic and Polymer Compounds that these two routes are quite inefﬁcient. The probability for TS1 has already been experimentally proved in Chapter 2. Similar results were obtained on calculations of the ozone reaction with ethylene glycol. The procedure for the TS1 search is given in Table 5.3. The transition state shows that the vibration with pseudofrequency is along the C-H bond. It is raised by 37% which corresponds to the HOCH3· and HO3· formation and A-H-atom cleavage.

80

0.8

1.2

3'

60

1.6

2.0

2.4

2.8

3.2

CH3(H)=O...O3

2' 40 20 0 -20

CH3O-H...O3

-40 -60

HO-CH2-H...O3

1'

-80 3

1

HO-CH2-H...O3

2

3

CH3(H)=O...O3

2 1

CH3O-H...O3 0.8

1.2

1.6

2.0

2.4

2.8

3.2

Reaction coordinate, A Figure 5.3 Ozone interaction with methanol. Lower panel: dependence of the C-H bond length at TS1 - 1, O-H at TS2 - 2, and O-O2 at TS3 - 3, on the reaction coordinate; upper panel: dependence of the corresponding heats of formation 1′, 2′ and 3′ on the reaction coordinate

366

Quantum Chemical Calculations of Ozonolysis of Organic Compounds

Table 5.3 Reaction of methanol with ozone - localisation of TS1 gradient norm minimised using NS01A SCF ﬁeld was achieved Parameters

Values

Heat of formation

17.940267 kcal/mol

Electronic energy

–4287.852106 eV

Core-core repulsion

2831.052188 eV

Gradient norm

0.087353

RMS force

0.019062

Dipole

3.63216 debye

No. of ﬁlled levels

16

Ionisation potential

9.681064 eV

Molecular symmetry

c1

Molecular weight

80.040

SCF calculations

157

Computation time

1434.06 s

Final geometry optimisation AM1 precise NS01A t = 60 m PULAY ITRY = 1200 Methanol + ozone TS localisation

Charge

H 0.000000 0 0.000000

0 0.000000

0

0 0 0

0.2684

O 0.982484 1 0.000000

0 0.000000

0

1 0 0

–0.2345

C 1.344494 1 110.628859 1 0.000000

0

2 1 0

–0.0344

H 1.103293 1 119.800700 1 7.734395

1

3 2 1

0.1408

H 1.107941 1 112.091391 1 156.622107 1

3 2 11.058 0.1446

H 1.477202 1 100.900700 1 –101.173304 1

3 2 1

XX 1.000000 0 90.000000 0 0.000000

0

6 3 2

O 1.137345 1 63.905024 1 –146.142569 1

6 7 3

–0.2217

O 1.241895 1 110.317690 1 38.642660

1

8 6 7

0.2668

O 1.183390 1 113.205484 1 56.471877

1

9 8 7

–0.4670

0 0.000000 0 0.000000

0

0 0 0

0 0.000000

0.1470

367

Ozonation of Organic and Polymer Compounds

5.2.3 Ethylene Glycol The introduction of a second OH group does not change the preferred reaction route - C-H bond break (Figure 5.4).

3.5

-20

1 3.0

-40

2.5

-60

HOCH2C(OH)H-H...O3

2.0

-80 1.5 -100

1'

1.0

-120 0.8

1.0

1.2

1.4

1.6

Reaction coordinate, A

Figure 5.4 Reaction of ozone with ethylene glycol. 1 - dependence of the C-H bond length (TS1) on the reaction coordinate; 1′ - dependence of $H on the reaction coordinate

5.2.4 Formaldehyde and Acetone The procedure for searching the most favourable transition state for the ozone reaction with formaldehyde and acetone is depicted in Tables 5.4 and 5.5.

368

Quantum Chemical Calculations of Ozonolysis of Organic Compounds

Table 5.4 Reaction of formaldehyde with ozone - localisation of TS gradient norm minimised using NS01A SCF ﬁeld was achieved Parameters

Values

Heat of formation

46.477233 kcal/mol

Electronic energy

–3849.714576 eV

Core-core repulsion

2421.463447 eV

Gradient norm

0.028602

RMS force

0.007385

Dipole

2.75136 debye

No. of ﬁlled levels

15

Ionisation potential

9.318082 eV

Molecular symmetry

c1

Molecular weight

78.024

SCF calculations

43

Computation time

277.15 s

Final geometry optimisation AM1 precise NS01A t = 60 m PULAY ITRY = 1200 Formaldehyde + ozone TS localisation

Charge

O 0.000000 0 0.000000

0

0.000000

0

0 0 0

–0.1762

C 1.200896 1 0.000000

0

0.000000

0

1 0 0

0.1458

H 1.095919 1 135.459847 1 0.000000

0

2 1 0

0.1698

H 1.412302 1 116.529164 1 167.377984 1

2 1 3

0.1580

O 1.222269 1 138.224492 1 93.827421 1

4 2 1

–0.2356

O 1.235351 1 104.033312 1 –28.230674 1

5 4 2

0.2906

O 1.194135 1 112.289186 1 51.238553 1

6 5 4

–0.3524

0 0.000000 0 0.000000

0 0 0

0

0.000000

0

369

Ozonation of Organic and Polymer Compounds

Table 5.5 Reaction of acetone with ozone – localisation of TS Gradient norm minimised using NS01A SCF ﬁeld was achieved Parameters

Values

Heat of formation

40.786156 kcal/mol

Electronic energy

–6149.223501 eV

Core-core repulsion

4409.650496 eV

Gradient norm

0.031238

RMS force

0.005446

Dipole

2.71796 debye

No. of ﬁlled levels

21

Ionisation potential

9.286127 eV

Molecular symmetry

c1

Molecular weight

106.078

SCF calculations

142

Computation time

2611.29 s

Final geometry optimisation AM1 precise NS01S t = 60 m PULAY ITRY = 1200 Acetone + ozone TS localisation

Charge

O 0.000000

0

0.000000 0

0.000000

0

0 0

0

–0.3010

C 1.239801

1

0.000000 0

0.000000

0

1 0

0

0.2262

C 1.495467

1

122.540300 0

0.000000

0

2 1

0

–0.2620

C 1.466488

1

121.581600 1

178.781883

1

2 1

3

–0.2715

H 1.118573

1

109.193989 1

116.963962

1

3 2

1

0.1163

H 1.116646

1

109.978078 1

–123.626602 1

3 2

1

0.0976

H 1.116895

1

110.546095 1

–3.045697

1

3 2

1

0.1235

H 1.099567

1

117.606477 1

–165.264690 1

4 2

1

0.1349

H 1.108390

1

115.688326 1

52.082983

1

4 2

1

0.1373

H 1.541413

1

113.400998 1

–39.639822

1

4 2

1

0.2274

O 1.130087

1

141.632512 1

–69.485801

1

10 4

2

–0.2428

O 1.254625

1

105.153145 1

26.878349

1

11 10 4

0.2996

O 1.181268

1

112.192736 1

–51.302198

1

12 11 10

–0.2731

0 0.000000

0

0.000000 0

0.000000

0

0 0

0

370

Quantum Chemical Calculations of Ozonolysis of Organic Compounds It is seen that the length of the C-H bond in the transition state is increased by 29% resulting in its rupture and formation of OHC· and HO3· radicals. In the transition state the length of the A-C-H bond is extended by 40% which leads to its cleavage producing CH3COOCH2·- and HO3·- radicals.

5.2.5 Dimethylether The ozone interaction with dimethyl ether may take place through both TS1 and TS3. The results from the calculations with TS1 – the ozone attack on C-H bond – are shown in Figure 5.5. The calculations on TS3 are not given because they are similar to those carried out on methanol with TS3. It should be noted that in this case the rise of the energy is smooth and the change of the O-O2 bond length in the ozone molecule is negligible. The latter is even smaller than the equilibrium values for the O=O bond. 2.4

1

60

2.2 40

2.0 1.8

20

1.6 0

CH3OCH2-H...O3

1.4

-20

1.2

1' 1.0 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

-40 2.6

Reaction coordinate, A

Figure 5.5 Ozone reaction with dimethylether: 1 – dependence of the C-H bond length (TS1) on the reaction coordinate; 1′ - dependence of $H on the reaction coordinate The experimental data from the study on the mechanism of this reaction shown in Chapter 2 reveals that it takes place via A-hydrogen-atom abstraction. The calculations illustrated by Figure 5.5 with TS1 correspond completely to the concepts developed in Chapter 2.

371

Ozonation of Organic and Polymer Compounds The procedure for searching the transition state of the reaction dimethylether + ozone gives the results shown in Table 5.6.

Table 5.6 Reaction of dimethylether with ozone – localisation of TS Gradient norm minimised using NS01A SCF ﬁeld was achieved Parameters

Values

Heat of formation

28.955315 kcal/mol

Electronic energy

–5470.166525 eV

Core-core repulsion

3858.306637 eV

Gradient norm

0.039366

RMS force

0.007187

Dipole

3.91911 debye

No. of ﬁlled levels

19

Ionisation potential

9.228045 eV

Molecular symmetry

c1

Molecular weight

94.067

SCF calculations

116

Computation time

1720.04 s

Final geometry optimisation AM1 precise NS01A t = 60 m PULAY ITRY = 1200 Dimethylether + ozone TS localisation C 0.000000

0

0.000000

Charge 0

0.000000

0

0 0 0

–0.1013

O 1.433392

1

0.000000

0

0.000000

0

1 0 0

–0.1533

C 1.354323

1

115.333687

1

0.000000

0

2 1 0

–0.1314

H 1.115735

1

110.385725

1

57.868975

1

1 2 3

0.0795

H 1.118623

1

109.048981

1

–64.464029

1

1 2 3

0.0949

H 1.118623

1

103.796319

1

177.135163

1

1 2 3

0.1235

H 1.101094

1

119.564651

1

–19.535388

1

3 2 1

0.1209

H 1.102459

1

111.353778

1

–168.722445 1

3 2 1

0.1735

H 1.550694

1

98.692414

1

79.895322

1

3 2 1

0.1816

O 1.103082

1

134.122537

1

22.683035

1

9 3 2

–0.2696

O 1.261974

1

109.117689

1

16.943166

1

10 9 3 0.2668

O 1.183390

1

113.205484

1

56.471877

1

11 10 9 –0.3853

0 0.000000

0

0.000000

0

0.000000

0

0 0

372

0

Quantum Chemical Calculations of Ozonolysis of Organic Compounds In the transition state the length of the A-C-H bond is stretched by 41% which leads to its cleavage to CH3COOCH2·- and HO3·- radicals.

5.3 Sulfur-containing Compounds The study of the reactions of sulfur-containing compounds with different reagents, including ozone, is of great importance both from a practical point of view for environment protection, atmospheric processes, treatment of drinking and waste water, ageing and stabilisation of elastomers and from a theoretical interest for the reaction ability theory, namely for ozone chemistry. Applying the approach for calculating the interaction between oxygen-containing compounds and ozone we will follow this reaction with sulfur-containing compounds.

5.3.1 Hydrogen Sulﬁde The ozone reaction with hydrogen sulﬁde may take place via TS2 and TS3: HS-H...O3

H2S...O-O2

2

3

The results from the calculations on this reaction are presented in Figure 5.6. In the transition state the length of the A-C-H bond is increased by 40% which leads to its cleavage producing CH3COOCH2·- and HO3·- radicals. The course of curves 2 and 2′ stand for the change in the S-H bond length and the value of the energy of TS2 formation demonstrates the inefﬁciency of this reaction route. There is a constant rise of the formation energy at an insigniﬁcant change in the bond length. Thus at the change of the reaction coordinate from 2.0 to 0.9 Å the S-H bond length is increased by 30% and the heat of formation rises by 150%. The calculations with TS3 (curves 3 and 3′) give a quite different picture. The reaction coordinate variation from 1.65 to 1.6 Å results in a qualitative sharp change in the O-O2 bond length from 1.285 to 2.574 Å, and the heat of formation drops from 46.4 to –15 kcal. Thus, it can be concluded that this pathway is actually the preferred reaction route. The compound containing oxygen is most probably an unstable intermediate which is rapidly decomposed to water and elemental sulfur as shown next: H2S + O3

H2S...O-O2

#

H2O + S + O2

H2O + SO2

373

Ozonation of Organic and Polymer Compounds This mechanism is in agreement with the easy oxidisability of hydrogen sulﬁde to sulfur or sulfur dioxide.

2.6

2'

100

HS-H...O3

2.4

80

3 2.2

60 2.0 40 1.8

2

H2S...O3

HS-H...O3

20 1.6 0

1.4

3'

H2S...O3

1.2 0.8

1.0

1.2

1.4

1.6

1.8

-20

2.0

Reaction coordinate, A

Figure 5.6 Ozone reaction with hydrogen sulﬁde: 2 - dependence of the S-H bond length (TS2) on the reaction coordinate; 3 - dependence of the O-O2 bond length on the reaction coordinate at TS3; 2′ - dependence of $H (TS2) on the reaction coordinate; 3′ - dependence of $H (TS3) on the reaction coordinate

5.3.2 Methylsulﬁde Upon ozonolysis of methylsulﬁde the reaction may proceed via the formation of three different transition states:

HSCH2-H...O3 1

374

CH3S-H...O3

CH3(H)S...O-O2

2

3

Quantum Chemical Calculations of Ozonolysis of Organic Compounds Figure 5.7 exempliﬁes the results from the calculations of the ozone reaction with methylsulﬁde considering the three probable transition states. It is obvious that routes 2 and 2′ are inefﬁcient. At a reaction coordinate variation from 1.6 to 0.9 Å the S-H bond is stretched by about 30% and $H is increased by 100%. Upon TS1- and TS3-based calculations a qualitative different pattern is observed. In the ﬁrst case (TS1) at the reaction coordinate change from 1.1 to 1 Å the C-H bond approaches the value of 2.982 Å and the heat of energy is reduced from 77.4 to –8.3 kcal. As to the second case (TS3) the reaction coordinate variation from 1.7 to 1.65 Å leads to an increase in the O-O2 length to 1.289 Å and the heat of formation is sharply reduced from 33.3 to –25.6 kcal. Thus the both transition states appear to be quite probable as reaction routes. However taking account of the reaction coordinate and $H magnitudes at which the qualitative changes occur in the systems one can assume that the route with TS3 is more favourable. Here the interaction between the two molecules (ozone and methylsulﬁde) starts at a greater distance (1.7 Å) and the value of $H at the beginning is lower although the gain from the transition - 58.9 kcal - is smaller than that at TS1 which is equal to 85.7 kcal. The preference for TS3 is additionally supported by the results from thiosemicarbazide ozonolysis whereby the changes in the system were followed by SO2 evolution.

5.3.3 Dimethylsulﬁde The reaction in this case can take place through two transition states - TS1 and TS3:

375

Ozonation of Organic and Polymer Compounds

100

0.8

2' 80

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

MeS-H...O3

60 40

HSCH2-H...O3 20 0

1'

Me(H)S...O3

-20

3

3'

1 3

2

2 MeS-H...O3

Me(H)S...O3

1

HSCH2-H...O3 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Reaction coordinate, A

Figure 5.7 Ozone reaction with methylsulﬁde. Lower panel: dependence of the C-H bond length (TS1) - 1, S-H (TS2) - 2 and O-O2 (TS3) - 3, on the reaction coordinate; upper panel: dependence of the corresponding heats of formation 1′, 2′ and 3′ on the reaction coordinate

The calculations for the two reaction routes are presented in Figure 5.8. Based on these results one can assume the reaction proceeds via the both transition states. Actually at TS1 the reaction coordinate change from 1 to 0.9 Å leads to a C-H bond break with a $H drop from 111.6 to 23.3 kcal and at TS3 the reaction coordinate change from 1.7 to 1.65 Å results in O-O2 cleavage and a $H drop from 48.8 to –8.4 kcal. If we apply the same approach as upon the methylsulﬁde ozonolysis then we should give preference to the route with TS3 as the ozone interaction with

376

Quantum Chemical Calculations of Ozonolysis of Organic Compounds the S atom begins at 1.7 Å and $H = 48.8 kcal. The results of the search procedure for TS3 are outlined in Table 5.7.

120 2.4

3 100

1

2.2

80 2.0

MeSCH2-H...O3 60 1.8

Me2S...O3

40

1.6

1'

20

1.4

3'

Me2S...O3 0

1.2

MeSCH2-H...O3

1.0 0.8

1.2

1.6

-20

2.0

Reaction coordinate, A

Figure 5.8 Ozone reaction with dimethylsulﬁde: 1 - dependence of C-H bond length (TS1) on the reaction coordinate; 3 - dependence of the O-O2 bond length on the reaction coordinate at TS3; 1′ - dependence of $H (TS1) on the reaction coordinate; 3′ - dependence of $H (TS3) on the reaction coordinate

5.4 Nitrogen-containing Compounds 5.4.1 Ammonia The ozone reaction with ammonia can occur via two reaction pathways - TS2 and TS3: H2N-H...O3 2

H3N...O3 3

377

Ozonation of Organic and Polymer Compounds The calculations for the two reaction routes are presented in Figure 5.9.

120

2'

2.4

3 100

2.2 80

2.0 1.8

H3N...O3 H2N-H...O3

1.6 1.4

60

40

2

20

H3N...O3

3'

1.2

0

H2N-H...O3

1.0 1.0

1.2

1.4

1.6

1.8

2.0

Reaction coordinate, A

Figure 5.9 Ozone reaction with ammonia: 1 - dependence of N-H bond length (TS2) on the reaction coordinate; 3 - dependence of O-O2 bond length on the reaction coordinate at TS3; 2′ - dependence of $H (TS2) on the reaction coordinate; 3′ - dependence of $H (TS3) on the reaction coordinate

For the TS2 route the reaction coordinate variation from 1.6 to 1.0 Å leads to an increase of the N-H bond length by about 31% and $H by 130% while for the TS3 route the reaction coordinate change from 1.5 to 1.4 Å causes the break of the O-O2 bond and $H is reduced from 64.7 to 10.1 kcal. Thus the preferred route appears to be via TS3 which is accompanied by nitrogen oxide formation: H3N + O3

378

H3N...O3

#

H3N

O

+ O2

Quantum Chemical Calculations of Ozonolysis of Organic Compounds

Table 5.7 Reaction of dimethylsulﬁde with ozone - localisation of TS gradient norm minimised using NS01A SCF ﬁeld was achieved Parametres

Values

Heat of formation

25.184250 kcal/mol

Electronic energy

–4933.327276 eV

Core-core repulsion

3548.879170 eV

Gradient norm

0.299787

RMS force

0.054787

Dipole

1.39911 debye

No. of ﬁlled levels

19

Ionisation potential

8.077920 eV

Molecular symmetry

c1

Molecular weight

110.128

SCF calculations

60

Computation time

28943.49 s

Final geometry optimisation AM1 precise NS01A t = 60 m PULAY ITRY = 1200 Dimethylsulﬁde + ozone, S – attack TS localisation

Charge

C 0.000000

0

0.000000

0

0.000000

0

0 0 0

–0.3423

S 1.747691

1

0.000000

0

0.000000

0

1 0 0

0.1713

C 1.746493

1

103.745290

1

0.000000

0

2 1 0

–0.3428

H 1.114349

1

109.532289

1

108.120998

1

1 2 3

0.1078

H 1.117283

1

108.566800

1

–133.275373 1

1 2 3

0.1047

H 1.113394

1

113.329057

1

–13.388482

1

1 2 3

0.0946

H 1.121340

1

106.783666

1

151.793107

1

3 2 1

0.0913

H 1.112606

1

110.777722

1

–89.026801

1

3 2 1

0.1004

H 1.113669

1

113.544434

1

33.875997

1

3 2 1

0.1173

O 1.813371

1

99.140829

1

91.044800

1

2 1 6

–0.2567

O 1.238066

1

122.537804

1

179.670447

1

10 2 1

0.4984

O 1.189194

1

115.514231

1

83.962040

1

11 10 2

–0.3437

0 0.000000

0

0.000000

0

0.000000

0

0 0 0

379

Ozonation of Organic and Polymer Compounds

5.4.2 Methylamine Figure 5.10 illustrates the results from the calculations on ozone reaction with methylamine.

0.8

1.2

100

1.6

2.0

2.4

2.8

3.2

MeHN-H...O3

80 60

MeH2N...O3

40

2'

H2NCH2-H...O3

20

3'

0

1' -20

1

3

3 2 2

Me(H2)N...O3 1

H2NCH2-H...O3

MeHN-H...O3 0.8

1.2

1.6

2.0

2.4

2.8

3.2

Reaction coordinate, A Figure 5.10 Ozone reaction with methylamine. Lower panel: dependence of the length of C-H bond (TS1) - 1, N-H (TS2) - 2, and O-O2 (TS3) - 3, on the reaction coordinate; upper panel: dependence of the corresponding heats of formation 1′, 2′ and 3′ on the reaction coordinate

Analysing the results obtained, it may be concluded that the three transition states lead to qualitative changes in the system but when ozone attacks the nitrogen atom the interaction between the two molecules begins at the biggest reaction coordinate. The TS1 geometry, after applying the procedure for transition state localisation, is

380

Quantum Chemical Calculations of Ozonolysis of Organic Compounds shown in Table 5.8. However it does not allow geometry optimisation for the other two transition states. The data in Table 5.8 indicate that even in the case of TS1 the optimisation of its geometry does not allow precise conclusions for the reaction route. Most probably, the calculation method with AM1 Hamiltonian and the parametrisation procedure are nor quite suitable for this reaction although the use of MINGO/3, MNDO and PM3 also do not provide satisfactorily results.

5.4.3 Dimethylamine The calculation results are presented in Figure 5.11.

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

120

100

Me2(H)N...O3 80

1' 60

3

3'

2'

Me2N-H...O3 MeHNCH2-H...O3

2 1 3

2

MeHNCH2-H...O3 Me2(H)N...O3 1

MeHN-H...O3 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Reaction coordinate, A

Figure 5.11 Ozone reaction with dimethylamine. Lower panel: dependence of the length of C-H bond (TS1) - 1, N-H (TS2) - 2, and O-O2 (TS3) - 3, on the reaction coordinate; upper panel: dependence of the corresponding heats of formation 1′, 2′ and 3′ on the reaction coordinate 381

Ozonation of Organic and Polymer Compounds It should be noted that in this case, similar to methylamine ozonolysis, three transition states are possible. The analysis of the results indicate that the three transition states lead to qualitative changes in the system but among them the most probable are those related to the ozone attack on the nitrogen atom and the C-H bond. For the former it is due to the highest reaction coordinate value and for the latter to the lowest heat of formation.

Table 5.8 Reaction of methylamine with ozone - localisation of TS gradient norm minimised using NS01A SCF ﬁeld was achieved Parameters

Values

Heat of formation

90.540787 kcal/mol

Electronic energy

–3759.668722 eV

Core-core repulsion

2403.735661 eV

Gradient norm

0.064749

RMS force

0.013217

Dipole

1.92064 debye

No. of ﬁlled levels

16

Ionisation potential

9.137121 eV

Molecular symmetry

c1

Molecular weight

79.055

SCF calculations

206

Computation time

2391.24 s

Final geometry optimisation AM1 precise NS01A t = 60 m PULAY ITRY = 1200 Methylamine + ozone, C-H - attack TS localisation

Charge

H 0.000000

0

0.000000

0

0.000000

0

0 0 0

0.1364

N 1.000903

1

0.000000

0

0.000000

0

1 0 0

–0.3447

C 1.431355

1

111.011420

1

0.000000

0

2 1 0

–0.1134

H 1.001973

1

108.692875

1

–121.676995

1

2 1 3

0.1499

H 1.122637

1

109.544687

1

64.286981

1

3 2 1

0.0806

H 1.121132

1

109.393412

1

–176.353838

1

3 2 1

0.0896

H 1.136511

1

114.333359

1

–54.714275

1

3 2 1

0.0517

O 1.924459

1

105.179765

1

–72.092692

1

7 3 2

–0.3928

O 1.683558

1

169.260487

1

170.308011

1

8 7 3

0.2709

O 1.080573

1

1o8.404019

1

80.752208

1

9 8 7

0.0717

0 0.000000

0

0.000000

0

0.000000

0

0 0 0

382

Quantum Chemical Calculations of Ozonolysis of Organic Compounds The TS1 optimisation procedure (Table 5.9) indicates a C-H bond elongation by 30% which obviously results in its breakage producing an HO2·- radical. However the reaction proceeding via an ozone attack on the nitrogen atom with a biradical or bipolar transition state could not be excluded.

Table 5.9 Reaction of dimethylamine with ozone - localisation of TS gradient norm minimised using NS01A SCF ﬁeld was achieved Parameters

Values

Heat of formation

61.781140 kcal/mol

Electronic energy

–5217.677611 eV

Core-core repulsion

3704.959820 eV

Gradient norm

0.037013

RMS force

0.006443

Dipole

5.27252 debye

No. of ﬁlled levels

19

Ionisation potential

8.149698 eV

Molecular symmetry

c1

Molecular weight

93.082

SCF calculations

37

Computation time

462.23 s

Final geometry optimisation AM1 precise NS01A t = 60 m PULAY ITRY = 1200 Dimethylamine + ozone, C-H - attack TS localisation

Charge

H 0.000000

0

0.000000

0

0.000000

0

000

0.2218

N 0.999125

0

0.000000

0

0.000000

0

100

–0.2429

C 1.428077

1

115.318063

0

0.000000

0

21 0

–0.1341

C 1.361194

1

116.972078

1

–153.160480

0

21 3

–0.1043

H 1.121377

1

109.122271

1

44.679142

1

32 1

0.0979

H 1.120953

1

110.381140

1

165.086646

1

32 1

0.0948

H 1.140249

1

110.486763

1

–74.783096

1

32 1

0.1381

H 1.110186

1

116.406114

1

–179.454319

1

42 1

0.1365

H 1.111330

1

115.502029

1

–37.635213

1

42 1

0.1295

H 1.437328

1

109.086276

1

70.120314

1

42 1

0.1227

O 1.149069

1

154.869200

1

–21.025365

1

10 4 2

–0.2502

O 1.236603

1

112.786814

1

82.411015

1

11 10 4

0.2400

O 1.193909

1

115.359758

1

93.890320

1

12 11 4

–0.4560

0 0.000000

0

0.000000

0

0.000000

0

0 0 0

383

Ozonation of Organic and Polymer Compounds

5.4.4 Trimethylamine The calculations on the ozone reaction with trimethylamine are given in Figure 5.12. As seen, only TS1 results in qualitative changes with a decrease of the reaction coordinate. TS3 is characterised by a constant rise of the energy, and no change in the bond lengths of the complexes is observed. Principally this result is confusing as ozone acts as an electrophilic agent in its reactions, and in this case the three electron donor methyl groups should favour the attack on the nitrogen atom, which, however, is not observed. It should be noted that the occurrence of steric hindrances at this interaction could be regarded as a reasonable explanation of this fact.

2.4

140

3'

1 2.2

120

2.0 100 1.8

Me3N...O3 80

Me2NCH2-H...O3

1.6

60 1.4

3 Me3N...O3

1'

1.2

Me2NCH2-H...O3

1.0 0.8

1.2

1.6

2.0

2.4

2.8

40

20 3.2

Reaction coordinate, A

Figure 5.12 Ozone reaction with trimethylamine: 1 - dependence of C-H bond length (TS1) on the reaction coordinate; 3 - dependence of O-O2 bond length on the reaction coordinate at TS3; 1′ - dependence of $H (TS1) on the reaction coordinate; 3′ - dependence of $H (TS3) on the reaction coordinate

5.5 Phosphorus-containing Compounds 5.5.1 Phosphine The results from the quantum chemical calculation on the ozone interaction with phosphine are shown in Figure 5.13.

384

Quantum Chemical Calculations of Ozonolysis of Organic Compounds It is only the attack on the phosphorus atom that results in the occurrence of a saddle point at reaction coordinate 1.72 Å. This observation completely corresponds to the experimental results from the study on the interaction mechanism between phosphorus-containing compounds and ozone. In all cases the basic product is phosphine oxide.

100

2' 3.0

3

80

H2P-H...O3

H3P...O3

60

2.5 40 20

2.0

2

0

H2P-H...O3

1.5

-20

H3P...O3 3'

-40

1.0 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Reaction coordinate, A

Figure 5.13 Ozone reaction with phosphine: 2 - dependence of P-H bond length (TS2) on the reaction coordinate; 3 - dependence of O-O2 bond length on the reaction coordinate at TS3; 2′ - dependence of $H (TS2) on the reaction coordinate; 3′ - dependence of $H (TS3) on the reaction coordinate

5.5.2 Methylphosphine The results from the calculations are given in Figure 5.14. It should be emphasised that the route via TS3 again causes the appearance of an inﬂection point on the potential surface but at reaction coordinate 1.82 Å, i.e., the methyl group favours the interaction between ozone and P to be carried out at a longer distance.

385

Ozonation of Organic and Polymer Compounds

100 80

0.8

1.0

1.2

2'

1.4

1.6

1.8

2.0

2.2

2.4

2.6

H2PCH2-H...O3

60

Me(H2)P...O3

40

MeHP-H...O3

20 0

1'

-20 -40

3'

-60 2.8 2.4

1

3

2.0

2 1.6

MeHP-H...O3 Me(H2)P...O3

1.2

H2PCH2-H...O3 0.8 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Reaction coordinate, A

Figure 5.14 Interaction between ozone and methylphosphine. Lower panel: dependence of the length of C-H bond (TS1) - 1, P-H (TS2) - 2 and O-O2 (TS3) – 3, on the reaction coordinate; upper panel: dependence of the corresponding heats of formation 1′, 2′ and 3′ on the reaction coordinate

The procedure on TS localisation is demonstrated by Table 5.10. It is seen that the result from this procedure does not show the distinct cleavage of ozone O-O bond and the P-O bond formation although this fact has been experimentally conﬁrmed.

386

Quantum Chemical Calculations of Ozonolysis of Organic Compounds

Table 5.10 Reaction of methylphosphine with ozone - localisation of TS geometry optimised using Eigenvector following (EF) SCF ﬁeld was achieved Parameters

Values

Heat of formation

42.443592 kcal

Electronic energy

-3686.869478 eV

Core-core repulsion

2408.444761 eV

Dipole

3.49638 debye

No. of ﬁlled levels

16

Ionisation potential

9.587821 eV

Molecular symmetry

c1

Molecular weight

96.022

SCF calculations

202

Computation time

128.078 s

Final geometry obtained AM1 PULAY t = 500 m TS Methylphosphine-ozone TS Geometry optimisation P 0.0000000

0

0.000000

C 1.7256227

1

0.000000

H 1.1086171

1

112.321176

H 1.1097753

1

108.414256

H 1.1114342

1

H 1.3641962

1

H 1.3645814 XX 1.00000

Charge 0

0.000000

0

0 0 0

0.1509

0

0.000000

0

1 0 0

–0.3316

1

0.000000

0

2 1 0

0.0713

1

–121.846850

1

2 1 3

0.0906

106.630222

1

120.000361

1

2 1 3

0.0939

99.851673

1

61.538462

1

1 2 3

–0.0398

1

100.087818

1

–36.138920

1

1 2 3

–0.0282

0

90.000000

0

180.000000

0

1 2 3

O 2.5445821

1

79.329204

1

–96.026745

1

1 8 2

–0.2737

O 1.1626121

1

173.730888

1

–95.253083

1

9 1 8

0.5424

O 1.1596079

1

121.040862

1

–36.666604

1

10 9 8

–0.2757

5.5.3 Dimethylphosphine Figure 5.15 exempliﬁes the results from the calculations on ozone interaction with dimethylphosphine.

387

Ozonation of Organic and Polymer Compounds

100

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

MeHPCH2-H...O3

80 60

Me2P-H...O3

40 20

Me2HP...O3

2'

0

1'

-20

3'

-40 3.2 -60 2.8 2.4

2

3

1

2.0

Me2P-H...O3

1.6

Me2HP...O3 1.2

MeHPCH2-H...O3 0.8 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Reaction coordinate, A

Figure 5.15 Ozone reaction with dimethylphosphine. Lower panel: dependence of the length of C-H bond (TS1) - 1, P-H (TS2) - 2 and O-O2 (TS3) – 3, on the reaction coordinate; upper panel: dependence of the corresponding heats of formation 1′, 2′ and 3′ on the reaction coordinate

It is clearly seen that the addition of a new methyl group does not result in any substantial change in the reaction route but reaction coordinate is 1.85 Å.

5.5.3 Trimethylphosphine The results from the calculations on the ozone interaction with trimethylphosphine are given in Figure 5.16. The data in Figure 5.16 indicate that the further introduction of a methyl group does not affect the reaction route but the value of the reaction coordinate is 2.10 Å. Summarising the results obtained for ozone reaction with phosphine and its methyl derivatives one can conclude that the P atom is the reaction centre in these compounds; the ozone attack is electrophilic and promoted by the presence of electron donor groups.

388

Quantum Chemical Calculations of Ozonolysis of Organic Compounds

1'

Me2PCH2...O3

80

2.5

60

3

Me3P...O3 40 20

2.0

0

1 Me2PCH2...O3 -20

3'

1.5

-40

Me3P...O3 1.0

-60 -80

0.8

1.2

1.6

2.0

2.4

Reaction coordinate, A

Figure 5.16 Ozone interaction with trimethylphosphine: 1 - dependence of C-H bond length (TS1) on the reaction coordinate; 3 - dependence of O-O2 bond length on the reaction coordinate at TS3; 1′ - dependence of $H (TS1) on the reaction coordinate; 3′ - dependence of $H (TS3) on the reaction coordinate

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389

Ozonation of Organic and Polymer Compounds 7. R. Gutbrod, E. Kraka, R.N. Schindler and D. Cremer, Journal of the American Chemical Society, 1997, 119, 31, 7330. 8. L. Mariey, J. Lamotte, P. Hoggan, J.C. Lavalley, K. Boulanine and A. Tsyganenko, Chemistry Letters, 1997, 8, 835. 9. Z.M. Lu and M.E. Kellman, Journal of Chemical Physics, 1997, 107, 1, 1. 10. S.M. Anderson, P. Hupalo and K. Mauersberger, Journal of Chemical Physics, 1993, 99, 1, 737. 11. F. Cacace and M. Speranza, Science, 1994, 265, 5169, 208. 12. C. Lee and H.R. Kim, Chemical Physics Letters, 1995, 233, 5-6, 658. 13. J.A. Joens, Chemical Physics Letters, 1994, 227, 6, 688. 14. A. Banichevich and S.D. Peyerimhoff, Chemical Physics, 1993, 174, 1, 93. 15. W. Koch, G. Frenking, G. Steffen, D. Reinen, M. Jansen and W. Assenmacher, Journal of Chemical Physics, 1993, 99, 2, 1271. 16. C. Boursier, C. Boulet, F. Menardbourcin, J.M. Flaud and C. Camypeyret, Europhysics Letters, 1993, 24, 4, 259. 17. R.A. Rîuse, Journal of the American Chemical Society, 1973, 95, 34600; 1974, 96, 5095. 18. P.C. Hiberty, Journal of the American Chemical Society, 1976, 98, 6088. 19. R.D. Rouse, International Journal of Quantum Chemistry Symposia, 1973, 7, 289. 20. W.A. Goddard, III, T.H. Dunning, Jr., W.J. Hunt and P.J. Hay, Accounts of Chemical Research, 1973, 6, 368. 21. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1975, 97, 6293. 21b. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1975, 97, 6300. 21c. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1976, 98, 6093.

390

Quantum Chemical Calculations of Ozonolysis of Organic Compounds 22. F.M. Bodrowicz and W.A. Goddard, III in Modern Theoretical Chemistry: Methods of Electronic Structure Theory, Volume 3, Ed., H.F. Schaefer, Plenum Press, New York, NY, USA, 1977, p.79. 23. L.B. Harding and W.A. Goddard, III, Journal of the American Chemical Society, 1978, 100, 7180. 24. D. Cremer, Journal of the American Chemical Society, 1981, 103, 13, 3619. 25. P.S. Bailey and T.M. Ferrell, Journal of the American Chemical Society, 1978, 100, 899. 26. P.L. Cummings and J.E. Gready, Journal of Computational Chemistry, 1989, 10, 939. 27. M.J.S. Dewar and D.A. Liotard, Journal of Molecular Structure: Theochem, 1990, 206, 123. 28. J. Baker, Journal of Computational Chemistry, 1986, 7, 385. 29. B.H. Besler, K.M. Merz, Jr., and P.A. Kollman, Journal of Computational Chemistry, 1990, 11, 431. 30. M.J.S. Dewar and W. Thiel, Journal of the American Chemical Society, 1977, 99, 4899. 31. M.J.S. Dewar and W. Thiel, Journal of the American Chemical Society, 1977, 99, 4907. 32. W. Thiel, Quantum Chemical Progress Exchange Bulletin, 1982, 438, 2, 63. 33. M.J.S. Dewar and H.S. Rzepa, Journal of the American Chemical Society, 1978, 100, 3, 777. 34. M.J.S. Dewar and M.L. McKee, Journal of the American Chemical Society, 1977, 99, 16, 5231. 35. M.J.S. Dewar and H.S. Rzepa, Journal of the American Chemical Society, 1978, 100, 1, 58. 36. L.P. Davis, R.M. Guidry, J.R. Williams, M.J.S. Dewar and H.S. Rzepa, Journal of Computational Chemistry, 1981, 2, 433. 37. M.J.S. Dewar, M.L. McKee and H.S. Rzepa, Journal of the American Chemical Society, 1978, 100, 3607.

391

Ozonation of Organic and Polymer Compounds 38. M.J.S. Dewar, J. Friedheim, G. Grady, E.F. Healy and J.J.P. Stewart, Organometallics, 1986, 5, 375. 39. M.J.S. Dewar and C. H. Reynolds, Journal of Computational Chemistry, 1986, 7, 140. 40. M.J.S. Dewar and H.S. Rzepa, Journal of Computational Chemistry, 1983, 4, 158. 41. M.J.S. Dewar and K.M. Merz, Organometallics, 1986, 5, 1494. 42. M.J.S. Dewar, G.L. Grady and E.F. Healy, Organometallics, 1987, 6, 186. 43. M.J.S. Dewar and E.F. Healy, Journal of Computational Chemistry, 1983, 4, 542. 44. M.J.S. Dewar, E.F. Healy and J.J.P. Stewart, Journal of Computational Chemistry, 1984, 5, 358. 45. M.J.S. Dewar, G.L. Grady and J.J.P. Stewart, Journal of the American Chemical Society, 1984, 106, 6771. 46. M.J.S. Dewar, G.L. Grady, K. Merz and J.J.P. Stewart, Organometallics, 1985, 4, 1964. 47. M.J.S. Dewar, M. Holloway, G.L. Grady and J.J.P. Stewart, Organometallics, 1985, 4, 1973. 48. R.C. Bingham, M.J.S. Dewar and D.H. Lo, Journal of the American Chemical Society, 1975, 97. 49. M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, Journal of the American Chemical Society, 1985, 107, 3902. 50. M.J.S. Dewar, C. Jie and E.G. Zoebisch, Organometallics, 1988, 7, 513. 51. M.J.S. Dewar and E.G. Zoebisch, Journal of Molecular Structure: Theochem, 1988, 180, 1. 52. M.J.S. Dewar and A.J. Holder, Organometallics, 1990, 9, 508. 53. M.J.S. Dewar and C. Jie, Organometallics, 1987, 6, 1486. 54. M.J.S. Dewar and C. Jie, Journal of Molecular Structure: Theochem, 1989, 187, 1. 55. M.J.S. Dewar and K.M. Merz, Jr., Organometallics, 1988, 7, 522.

392

Quantum Chemical Calculations of Ozonolysis of Organic Compounds 56. M.J.S. Dewar and C. Jie, Organometallics, 1989, 8, 1544. 57. J.J.P. Stewart, Journal of Computational Chemistry, 1989, 10, 221. 58. A.V. Mitin, Journal of Computational Chemistry, 1988, 9, 107. 59. M.J.S. Dewar, J.A. Hashmall and C.G. Venier, Journal of the American Chemical Society, 1968, 90, 1953. 60. M.J.S. Dewar and N. Trinajstic, Journal of the Chemical Society D, 1970, 11, 646. 61. M.J.S. Dewar and N. Trinajstic, Journal of the Chemical Society (A), 1971, 1220. 62. P. Pulay, Chemical Physics Letters, 1980, 73, 393. 63. J.J.P. Stewart, P. Csaszar and P. Pulay, Journal of Computational Chemistry, 1982, 3, 227. 64. P.G. Perkins and J.J.P. Stewart, Journal of Chemical Society, Faraday Transactions 2, 1981, 77, 000. 65. Y. Beppu, Computers and Chemistry, 1982, 6. 66. D.R. Armstrong, R. Fortune, P.G. Perkins and J.J.P. Stewart, Journal of Chemical Society, Faraday Transactions 2, 1972, 68, 1839. 67. C.G. Broyden, Journal of the Institute for Mathematics and Applications, 1970, 6, 222. 68. R. Fletcher, Computer Journal, 1970, 13, 317. 69. D. Goldfarb, Mathematics of Computation, 1970, 24, 23. 70. D.F. Shanno, Mathematics of Computation, 1970, 24, 647. 71. D.F. Shanno, Journal of Optimisation Theory and Applications, 1985, 46, 87. 72. H.A. Kurtz, J.J.P. Stewart and K.M. Dieter, Journal of Computational Chemistry, 1990, 11, 82. 73. H.A. Kurtz, International Journal of Quantum Chemistry: Symposium, 1990, 24, 791.

393

Ozonation of Organic and Polymer Compounds 74. M.J.S. Dewar and G.P. Ford, Journal of the American Chemical Society, 1977, 99, 7822. 75. A. Komornicki and J.W. McIver, Chemical Physics Letters, 1971, 10, 303. 76. A. Komornicki and J.W. McIver, Journal of the American Chemical Society, 1971, 94, 2625. 77. M.S. Gopinathan, P. Siddarth and C. Ravimohan, Theoretica Chimica Acta, 1986, 70, 303. 78. D.R. Armstrong, P.G. Perkins and J.J.P. Stewart, Journal of the Chemical Society, Dalton, 1973, 838. 79. M.S. Gopinathan and K. Jug, Theoretica Chimica Acta, 1983, 63, 497. 80. M.J.S. Dewar, E.F. Healy and J.J.P. Stewart, Journal of Chemical Society, Faraday Transactions 2, 1984, 3, 227. 81. A.D. Buckingham, Quarterly Reviews of the Chemical Society, 1959, 3, 183. 82. J.J.P. Stewart, New Polymeric Materials, 1987, 1, 53. 83. H.E. Klei and J.J.P. Stewart, International Journal of Quantum Chemistry, 1986, 20, 529.

394

A

bbreviations for Ozone

13

Carbon-13 NMR

1

Proton nuclear magnetic resonance

AC

Activated complex(s)

AcAc

Acetic acid

AcO

Acetate

ACP

Acetophenone

ACT

Activated complex method

AdAc

Adipic acid

AIBN

Azoisobutyronitrile

ATR

Attenuated total reﬂection

AW

Santoﬂex

ax

Axial

BA

Barium

BAO

Barium oxide

BND

Bond energy

bp

Boiling point

BuOH

Butanol

CaO

Calcium oxide

CC

Cyclic activated complex

CCl4

Carbon tetrachloride

CDCl3

Deuterated chloroform

Ce2O3

Cerium oxide

CEF

Compensation effect

CHCl3

Chloroform

CHP

Cumylhydroperoxide

CL

Chemiluminescence

CO

Carbonyl oxide

C-NMR

H-NMR

395

Ozonation of Organic and Polymer Compounds Cr2O3

Chromium oxide

CT

Collision theory

Cu/ZnO

Copper/zinc oxide

CuCrO2

Copper chromium oxide

CuO

Copper oxide

CY

Cyanamid Company product

CZ

N-cyclohexyl-2-benzothiazolsulfenamide

D

Optical density

DBE

n-Dibutylether

DChM

Dichloromethane

DCHP

Dicumylhydroperoxide

DCLEtE

Dichlorodiethylether

DEE

Diethyl ether

DFG

Diphenylguanidine

DMFA

N,N-dimethylformamide

DMPC

Dimethylphenylcarbinol

DMSO

Dimethyl sulfoxide

DPO

Diperoxide

DSC

Differential scanning calorimetry

EF

Eigenvector following

EOH

α-Hydroxyether

EOOOH

α-Hydrotrioxyether

EPDM

Ethylene propylene diene terpolymer

eq

Equatorial

ERM

Estimation of reaction mechanism

ESR

Electron spin resonance

ETAS

Microporous titanosilicate

EtOH

Ethyl alcohol

exo

Internal

HexOH

Hexanol

HI

Hydrogen iodide

HMAO

Highmolecular antioxidants

HMDS

Hexamethyldisilazane

HPLC

High-performance liquid chromatography

396

Abbreviations for Ozone HTO

Hydrotrioxides

iAME

Di-iso-amylether

iPrE

Di-iso-propylether

iPrOH

Iso-propanol

IR

Infrared

IR

Isoprene rubber

KetAlc

Cyclohexanol-2-one

KI

Potassium iodide

KOH

Potassium hydroxide

KRS-5

Thallium bromoiodide

LC

Linear activated complex

LD50

Lethal dose 50%

MBS

2-(Morpholinothio)benzthiazole

MEK

Methylethylketone

MeOH

Methyl alcohol

MG

Methyl-glucoside

MMD

Mass-molecular distribution

MMD

Molecular mass distribution

MMXE

Alinger energy

Mn

Number average molecular weight

MO

Monsanto

MOR

Morpholinemercaptobenzthiazole

mp

Melting point

Mw

Average molecular weight

n-AME

Di-n-amylether

n-ButE

Di-n-butylether

NC

Without pH control

nd

Not detected

NMR

Nuclear magnetic resonance

NR

Natural rubber

O3

Ozone

P

Tyre protectors

PC

Phthalocyanine

PCR

Polychloroprene rubber

397

Ozonation of Organic and Polymer Compounds Pd/C

Palladium/carbon catalyst

PE

Polyethylene

PH3P

Triphenylphosphine

PhOH

Phenol

php

Parts per hundred polymer

phr

Parts per hundred rubber

PM

1,2,5,6-O-di-iso-propylmannitol

PMP

Physicomechanical properties

PO

Primary ozonide(s)

PP

Polypropylene

PPHDA

N,N´-substituted-p-phenylenediamines

ppm

Parts per million

PrOH

Propanol

PS

Polystyrene

PT

Pneumatic tyres

PVC

Polyvinylchloride

PVCH

Polyvinylcyclohexane

R

Alkyl group

RO

Alkoxy group

RO2

Peroxy group

rpm

Revolutions per minute

SBR

Styrene-butadiene rubber(s)

SC

4,4´-Thio-bis (6-tert-butyl)-m-cresole

SCF

Self-consistent ﬁeld

SE

Steric energies

SE

Strain energy

SKD

Butadiene rubber (Russia)

SKI

Synthetic cis-polyisoprene rubber (Russia)

SW

Tyre sidewalls

THydF

Tetrahydrofurane

THydP

Tetrahydropyrane

TOR

Torsion energy a

TS

Transition states

UV

Ultraviolet

398

I

ndex

A A-Methylbenzylmethylether HTO 112 Activated complex theory 26, 100-101, 135 Linear 29 Acceleration test 317 Acetals, acyclic 280 Acetone 368 Acetophenone 47, 55-56, 64-68, 117 Acrylic acid ozonolysis 191, 204 Adams catalyst 165 Ageing Atmospheric 292-294, 308, 318 Duration of 317 Thermal 316 Time of 313 Alcohol ozonolysis 100, 150 Aldehyde formation 234 Alinger energy 201 Alinger method 31 Aliphatic alcohols 94, 100 Alkane ozonolysis 11, 84 Alkyl ethers 125 Alkylation 273, 280-281 Catalytic 267 Alkynaphthenes 281 Amine stabilisers 322 Aminomethlene derivatives of furane 282 Aminoorganosilanes 285 Analysis, thermochemical 11 Anisole 145 Antifatigue Additives 263 Agents 252 Inhibitors 277

399

Ozonation of Organic and Polymer Compounds Antioxidant Action 306 Efﬁciency 306 Antiozonants 251-252, 261-267, 269, 277, 279, 282-284, 286, 291, 297, 304306, 308-311, 310, 319-329, 331-335 Bifunctional 264 Efﬁciency of 290, 319, 324 For polychloroprene rubber 285 Industrial 290 Low molecular 298 Lower-toxicity 271 Odourless 279 Reaction 266 Solutions 266, 307 Strong 282 Sulfonate 283 Arrhenius Equation 28, 129, 252, 314 Parameters 189, 195, 210, 236 Relationships 315 Aryl derivatives, synthesis of 127 Attenuated total reﬂection 320 Autocatalytic process 61 Avogadro’s number 23

B Bailey mechanism 184 Benzenes 145 Bis-alkylaminophenoxy alkanes 280 Bis-cyclopentadienyl compunds 281 Bond energy 201 Boltzmann’s constant 26 Bubbling method 97, 107, 231, 322 Bulex 299, 300 Butadiene styrene rubbers 282, 302 Butadiene nitrile rubber (SKN 40) 230, 296 Butanediol 153-154 Consumption 164 Butyl rubber 302

C Carbohydrates, oxidation of 149 Chemical reaction, ﬁrst order 212 Chemical relaxation 326-327

400

Index Chemical stabilisation 140 Chemiluminescence intensity 46, 118-119, 190 Chemiluminescence kinetic curve 49 Chloroprene rubber ozonolysis 229 Cis-1,4-polyisoprene 227 13 C-nuclear magnetic resonance 157, 159, 161-163 Collision theory 28, 100 Commercial antiozonants 267 Compensation effect 22 Condensation 279 Copolymerisation 298 Crack Formation 256, 258-260, 264, 304 Growth 332-333 Growth rate 327, 332 Propagation rate 325 Cracking 258-259 Criegee mechanism 182-183, 222, 260 Critical deformation phenomenon 331 Crystallinity, degree of 69, 72-73 Crystallisation 258, 321 Cumene 47, 59 HTO decomposition 117 Oxidation 41, 83, 307 Ozonolysis 46-47, 52, 58, 62 Acetic acid ozonolysis 52 Cumene hydroperoxide 64 Cumylhydroperoxide 47-49, 52, 54, 56, 58-67 Cyanostilbene ozonolysis 209, 214-215 Cyclic acetals 280 Cyclic complex formation 142 Cyclisation 188, 312 Cycloalkane ozonolysis 22, 30 Cyclohexane ozonolysis 36, 41-42 Cyclohexanediol ozonolysis 156 Cyclooctane ozonolysis 24 Cyclo-oleﬁns 201-202 Cycloparafﬁns 20-23, 34

D Decomposition, photochemical 182 Decomposition, thermal 118, 128-129, 130, 226 Deformation, degree of 330, 332

401

Ozonation of Organic and Polymer Compounds Degradation agents 252, 317 Degradation, atmospheric 293-294, 308 Degradation reaction 253 Derivatives of isothiocarbame 282 Dialkylsulfoxides 120 N-dibutyl ether 131 Decomposition of 132, 135 Ozonation of 134 Dichlorodiethylether 135 Dichloromethane 112 Dicumylhydroperoxide 47, 48 Diene monomers 298 Diethylether 130, 135 Differential scanning calorimetry spectra 69, 72, 127, 226 Analysis 128, 229-230 Method 71, 73 Dihydroxybenzene ozonolysis 144 Dihydroxybenzenes 141, 148 Di-iso-amylether 135 Di-iso-propylether 135 Dimerisation 229 Dimethlphenylcarbinol 47-49, 52, 54, 56, 58-68, 117 Dimethylpyrols, N-substituted 276 Dimethyl sulfoxide 119 Di-N-amylether 135 Di-N-butylether 135 Disproportionation 312 Dithiol derivatives 284

E Elastomer Composition 300-301 Degradation 253, 266, 304 Ozonolysis 224 Stabilisation 263, 296, 373 Vulcanisation 299 Electron-acceptor effect 203 Electrophilic agent 104, 384 Electrophilic process 252 Enamines 276 Enolethers 279 Electron spin resonance 82, 331 Cuvette 44

402

Index Intensity 46 Resonator 44 Signal 44, 46-47 Spectrum 45, 57 Estimation of reaction mechanism 26, 28-29, 35, 100, 135, 146 Application of 100, 135, 146, 209 Ethanol, ozonolysis of 93, 152 Ethers 279 Ethylenepropylene rubbers 235, 296

F Frank-Condon’s principle 27-28 Free rotation energy 138 Fumaric acids 208

G Gas chromatography 107 Gas chromatography – mass spectrometry 132 Gas phase 188, 193 Gel chromatography infrared spectrometry 219 Gel formation 232 Globe swelling 217 Glucose formation 160 Glyceric acid formation 164

H Halogenated oleﬁn ozonolysis 190 Halogens 271 Hammett Inductive constants 213-214, 216 Relationship 198, 200 Hammond’s empirical rule 17, 28 Henry’s coefﬁcient 219, 231 Henry’s law 13 Heterogeneous catalysts 165 Hexahydropyrimidines, N,N´–substituted 276 High molecular antioxidants 296-299 High pressure liquid chromatography 152-154, 157, 159, 161-163 Hydrotrioxides 110, 112, 115-122 Decomposition 115 Stabilisation 111 Hybridisation 196 Hydrogenation 269, 271

403

Ozonation of Organic and Polymer Compounds Hydrogen-atom abstraction mechanism 359, 364, 371 Hydrolysation 157, 160, 164 Hydroperoxide 117 Hydroquinone 140 A-Hydrotrioxyether 132-133, 137 Hydroxybenzenes 140 A-Hydroxyether 132-133 Hydroxyphenol ozonation 140 Hypsochromic effect 17

I Infrared – attenuated total reﬂectance spectra 321 Infrared spectroscopy 205-206, 233-234, 257, 320 Interphase catalysis 126 Iodometry 219 Isomerisation 190 Iso-parafﬁns 12, 17 Isoprene rubber 282, 287, 333 Isopropanol 152 Isotope effect 26

K Keltan 312 300 Ketals 280 Keto-enol tautomerism 107-108 Kinetic curve 25, 36, 48, 51-52, 60-63, 68, 78, 95, 105, 109, 131-132, 142-143, 192, 234, 299, 310 Kinetics, ﬁrst order 204 Kinetics, second order 191 Kissinger relationship 129

L Lactams 282 Lewis acid 274 Light rubbers 284 Liquid phase 191, 193, 219, 224 Liquid-liquid system 124 Liquid-solid system 124

M Macromolecule degradation 265 Maleic acid 208 Mannitol 164

404

Index Mass-molecular distribution 80, 84, 217 Methylamine ozonolysis 382 Methyl ether ketone 103 A-D-Methyl-glucoside 160 Methylphosphine 385 Methylsulﬁde 376 Michaelis-Menten mechanism 15 Microcracks 304 Molecular mechanics theory 201 Monomolecular decomposition 312-313 MOPAC6 program 359

N Natural rubber 278-279, 282, 294, 302, 330-331, 333-334 Neutralising agent 284 Nickel catalysts 269. Nitrile antiozonants 285 Nitrile rubber 230-233, 287 Nitrobenzene hydrogenation 272-273 Nitroxyl radical formation 264 Nonstaining antiozonants 276, 280, 282, 285 Normal parafﬁns 12 Nuclear magnetic resonance spectrometry 110, 118, 121, 133, 230, 234

O Oleﬁn 179, 322 Concentration 191, 193, 195, 207-208 Ozonolysis 180-182, 185, 188, 191, 194-198, 208 Structure 198 Oligodiphenylsiloxyquinone 285 O-Quinone formation 142 Oxidation 40, 63, 70, 93, 119-121, 133, 151, 258, 278, 298, 312, 325 Catalytic 162 Degradation 312 Photochemical 261 Oxidising agents 119 Oxolanes 124 Oxygen-containing compounds 93 Ozonation 51, 71, 73-74, 80, 84, 122, 136, 142, 144-146, 148, 158, 160, 255 Alkene 179 Antiozonants 306 1, 2, 5, 6-O-di-iso-propylmannitol 158 Ethers 137

405

Ozonation of Organic and Polymer Compounds A-D-glucose 158 Methyl ethyl ketone 102 Polyisoprene 334 Reaction 148, 154 Starch 161 Ozone Absorption 143, 164, 68, 82 Action 302, 304 Ageing 292, 294-295, 304 Analysis 190, 303 Concentration 106, 131, 142, 191-193, 206, 212, 221-224, 232, 254, 258-259, 261, 295, 302, 304, 309, 311, 328 Consumption 99, 105, 158 Cracking 256 Decomposition 263 Degradation of 216, 253-254, 256, 261, 293-294, 333-334 Diene polymers 334 Elastomers 265 Effect 256, 335 Flow rate 75 Insertion mechanism 130 Oxidation 334 Ozone Induced SKI-3 325 Ozone Interaction 141, 262, 265, 310, 328 Dimethylether 371 Dimethylphosphine 387 Methanol 366 Phosphine 384 Trimethylphosphine 388-389 Ozone oleﬁn complex 193 Ozone reaction with Ammonia 377-378 Dimethylamine 381 Dimethylamine 383 Dimethylphosphine 388 Dimethylsulﬁde 377 Ethylene glycol 366 Hydrogen sulﬁde 373-374 Methanol 364 Methylamine 380 Methylphosphine 387 Methylsulﬁde 375-376 Phosphine 385

406

Index Trimethylamine 384 Water 365 Formaldehyde 368 Ozone resistance 262, 266, 278, 283, 300-304, 311 Ozone resistant Rubbers 302 Ozone stabilisation of Elastomers 335 Ozone titration 107 Ozone-gas curve 106 Ozonide 52, 64-66, 68, 129-130, 181, 183, 190, 203, 224, 331 Cross 182 Curve 51 Cycle 203, 206 Foreign 181-182 Formation 48-49, 59, 63, 66, 68, 76-77, 82,184-185, 188, 208, 212, 223, 227229, 260-261 Stereochemistry 182 Structure 182 Ozonised Keltan 778 Ozonised Rubber SKD 226 Ozonolysis of 1,2,5,6-O-Di-Iso-Propylmannitol Ozonolysis 157 1,2,5,6-Di-O-Mannitol 164 1,2-Cyclohexanediol 155, 164 1,4-cis-Polyisoprene 234 4,4-Dinitro-Cyanostilbene 212 Acrylic Acid 195 Alcohols 93 B-Cyclodextrine 161 Cis Oleﬁns 186 Cumene 48, 51, 54, 57, 60 Denka M40 222, 228-230 D-Glucose 159 Divinylbenzene 202 Elastomers 252 Ethylene 189 Ethylene propylene diene terpolymer 236 Isopropanol 152 Ketones 103, 105, 109 Maleic anhydride 195, 208 Mannitol 156 Methylsulﬁde 374 Nitrile rubbers 234 Oleﬁn 127, 179, 184, 199, 203, 252 Polydienes 222 407

Ozonation of Organic and Polymer Compounds Polymeric materials 217 Saturated polymers 219 SKN-18 232, 234-235 SKI-3 324 Starch 161-162 Trans-oleﬁns 183 Tetradecane 38 Thiosemicarbazide 375 Water 364 Ozonolysis Degradation 295 Ozonolysis Kinetics 189, 191

P Paraphenylenediamine 267 Pauling’s formulae 32 Peroxides, organic 128 Phenol ozonation 140, 145 Phosphates 285 Phosphites 285 Photostabilisers 252 Physical relaxation 327 Physicomechanical parameters 301, 313, 315-317 Pitzer’s tables 33 Planck’s constant 23 Primary ozonide Decomposition mechanism 213, 252 Formation 192-193, 260 Polybutadiene rubbers 219, 227, 302 Polychloroprene rubber 228, 302 Polydiene Consumption 221 Degradation 251 Stabilisation 251 Polydispersity 69, 73-74 Polyethylene 68-71, 74 Polyisoprene 302, 334 Polymer Antiozonant 275 Degradation 217, 312 Dispersion 296 Matrix 69, 236, 262, 266, 296, 298, 331, 334 Organic 286 Orientation 256

408

Index Structure 68 Zwitterions 253, 313 Polymerisation 48, 229, 235-236 Polyoleﬁns 44, 69 Polyperoxide Formation 236 Polypropylene 68 Polystyrene 74 Polyvinylcyclohexane 44 Polypropylene ozonolysis 69-72, 74 p-Phenylenediamine 263, 265-266, 268, 270-271, 274, 290, 296-297, 320 Antiozonant preparation 267 Antiozonants 267 Structure 323 Prilezhaev reactions 196 Polystyrene ozonolysis 76 Pseudo ﬁrst-order kinetics 221 Polyvinylcyclohexane ozonolysis 46 Pyrocatechol 140, 142 Consumption 143 Ozonation 142, 145

Q Quantum chemical calculation 214, 365, 384

R Radiation vulcanisation 323 Razumovskii mechanism 142 Reaction coordinates 359-361, 363-367, 371, 373-378, 380-382, 384-386, 388389 Reaction mechanisms 9, 26 Reaction of Acetone with ozone 370 Dimethylether with ozone 372 Dimethylsulﬁde with ozone 379 Formaldehyde with ozone 369 Ozone with ethylene glycol 368 Reﬂected-light optical spectroscopy 259 Rehybridisation 219 Sp2-Sp3 252 Relaxation curve 321, 325, 328 Relaxation time 325, 330 Relaxometer 325 Resorcinol ozonation 140

409

Ozonation of Organic and Polymer Compounds Rubber Composition 294-296 Degradation 251 Diene 35 NFA 227 Industry 264 Oxidation level 324 Products 251-252, 284, 313, 317 Pure 266, 309-310, 324 Stabilisation of 251 Synthetic 261, 288, 279, 286 Rubine dye 46

S Scraup method 275 Semilogarithmic anamorphosis 25, 109, 314 Strain energy 22, 201 Singlet oxygen 117 SKD solution 221, 227, 229, 292, 299-300, 330-334 Ozonides 224, 229 Solution 224, 226 SKI-3 257, 299, 322, 324, 326, 328-331, 334 SKI-3S 222, 227 SKN-26 234 SKN-40 234 SKS-30 320, 330-334 Solvents, active 205 Soxhlet extractor 219 Stabilisers, industrial 305 Stabilising system 286, 295, 308-311, 313, 316-317, 324 Stability, thermal 119 Stereochemistry 184 Steric energy 23 Stoichiometric coefﬁcient 309-310 Stoichiometry 76, 106, 121, 192, 206 Stop-ﬂow-technique 64, 146, 193 Stress relaxation 321, 326, 335 Stretching, degree of 69, 71, 330 Structural-kinetic investigations 12 Synergetic stabilishing system 290, 292, 294-295 Oxolanes, 124

T Thermodynamic elastomer solutions 217

410

Index Thermostabiliser 285 Titration 108, 121, 164 Toluene 145 Torsion energy 201 Transition state 359, 364, 366, 368, 372, 374-375, 380-381, 383 Triazinedisulﬁde derivatives 284 Triphenylphosphine 121 Reduction 219 TS localisation 386 TS1 optimisation procedure 383 Tunnelling 27 Probability 27

V Van Der Waals radii 138, 147 Viscosimetry 219 Viscosity 221, 236, 292 Vulcanisate’s structure 327 Vulcanisation 263, 295, 299-300

Z Zeolite catalyst 165 Zwitterion 181-183, 213, 223, 225-227, 232, 234, 252, 260, 264, 298 Formation 261 Isomerisation 224-225

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Ozonation of Organic and Polymer Compounds

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Published by Smithers Rapra, 2009

The study of the kinetics and mechanism of ozone reactions is an important field in modern science and is closely related to the solution of the problem of ‘ozone holes’. The development of physical, organic, inorganic, polymer and biochemistry reactions using ozone, chemical kinetics, theory and utilisation of the reactivity of chemical compounds towards ozone, development of new highly efficient technologies for the chemical and electronics industries, solution of ecological and medical problems by using ozone, degradation and stabilisation of organic, polymer, elastomer and biological materials, against its harmful action are all very important with the formation of holes in the atmospheric ëozone layer. The antibacterial properties of ozone are promoted for the use in drinking water treatment and sterilisation of medical equipment and also for treatment of contaminated soils. Novel methods and improvement of well known methods for its generation and analysis, and for its more effective application have been developed. A number of laboratory and industrial methods for its synthesis have been proposed and are discussed in this book.

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