Radical-mediated degradation mechanisms of tribromo- and other trihalogenated acetic acids in oxygen-free solutions as studied by radiation chemistry methods

Abstract

˙CBr₂CO₂^- and ˙CCl₂CO₂^- radicals, generated upon one electron reduction of tribromo- and trichloro-acetic acids and ˙CF₂CO₂^- radicals produced from difluoroacetic acid by reaction with ˙OH, exhibit optical absorptions in the UV with λ_max at 290 nm (ε = 2580 dm³ mol^-1 cm^-1), 330 nm (ε = 3000 dm³ mol^-1 cm^-1) and 310 nm (ε ≈ 660 dm³ mol^-1 cm^-1), respectively. Mechanistically, the present report focuses on the free-radical-induced degradation of tribromoacetic acid. Absolute rate constants have been determined for the reactions of CBr₃CO₂^- with e_aq^-, H˙, CO₂^˙-, ˙CH₂OH, CH₃˙CHOH, (CH₃)₂˙COH and ˙CH₃ radicals to be k = 1.8 × 10¹⁰, 1.5 × 10¹⁰, 2.8 × 10⁹, 1.6 × 10⁹, 2.3 × 10⁹, 3.0 × 10⁹ and 3.0 × 10⁷ dm³ mol^-1 s^-1, respectively. The major fate of ˙CBr₂CO₂^- is self-termination to yield tetrabromosuccinic acid which, however, is unstable and thermally decomposes to HBr, CO₂ and tribromoacrylic acid. Dibromofumaric acid, dibromomaleic acid and carbon monoxide were found as minor secondary products, formation of which is explained by a small yield of reductive decomposition of the transient tetrabromosuccinic acid. A complete and mechanistically satisfying material balance is provided for several systems in which CBr₃CO₂^- has been degraded via a variety of radicals under various conditions. ˙OH Radicals do not react directly with CBr₃CO₂^-. They have been shown, however, to contribute indirectly to the degradation of this acid via their reaction with reductively liberated bromide ions. The Br˙ atoms formed in this process are considered to abstract a bromine atom from CBr₃CO₂^- or oxidize the carboxyl function in a one-electron transfer process. The formation of free Br˙ atoms has been recognized by pulse radiolysis through their conjugate dimer radical anions Br₂^˙-. With respect to the other trihalogenated acids it is noteworthy that CCl₃CO₂^- is efficiently reduced by CO₂^˙- radicals and that CF₃CO₂^- exhibits a high stability toward γ-irradiation and practically resists any reductive attack.

References

1. C. R. Pearson, in The Handbook of Environmental Chemistry, vol. 3, Part B, Antropogenic Compounds, ed. O. Hutzinger, Springer-Verlag, Berlin, Heidelberg and New York, 1982, pp. 69–82.
2. J. Franklin, Chemosphere, 1993, 27, 1565.
3. UNEP, 1991, Montreal Protocol 1991 Assessment, Report of the Technology and Economic Assessment Panel, December 1991.
4. M. Anbar and E. J. Hart, J. Phys. Chem., 1965, 69, 271.
5. M. Exner, H. Herrmann, J. Michel and R. Zellner, Proceedings of the AFEAS Workshop on Atmospheric Wet and Dry Deposition of Carbonyl and Haloacetyl Halides, Brussels, Belgium, September 22, 1992.
6. Y. Mao, C. Schöneich and K.-D. Asmus, J. Phys. Chem., 1991, 85, 10 080.
7. A. Chemseddine and H. P. Boehm, J. Mol. Catal., 1990, 60, 295.
8. Y. Morisawa, K. Konishi and M. Kataoka, A method of preserving organic materials from fungal attack and a composition for use in such a method. Japan patent 3364/81, C 07 C 69/65.
9. E. Ott, Liebigs Annalen, 1912, 392, 267.
10. K.-D. Asmus, Methods Enzymol., 1984, 105, 167.
11. E. Janata, Rad. Phys. Chem., 1992, 40, 437.
12. J. Mönig and K.-D. Asmus, J. Chem. Soc., Perkin Trans. 2, 1984, 2057.
13. G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross, Phys. Chem. Ref. Data, 1988, 17, 513–886.
14. (a) K.-D. Asmus, H. Möckel and A. Henglein, J. Phys. Chem., 1973, 77, 1218; (b) K.-D. Asmus, in Reactive Oxygen Species in Chemistry, Biology, and Medicine, ed. Alexandre Quintanilha, Plenum Publishing Corporation, New York, London, 1988, p. 37.
15. P. Wardman, Phys. Chem. Ref. Data, 1989, 18, 1637.
16. D. Veltwisch, E. Janata and K.-D. Asmus, J. Chem. Soc., Perkin Trans. 2, 1980, 146.
17. M. Lal, C. Schöneich, J. Mönig and K.-D. Asmus, Int. J. Radiat. Biol., 1988, 54, 773.
18. M. Anbar and E. J. Hart, J. Phys. Chem., 1965, 69, 271; Discussions Faraday Soc., 1963, 36, 193.
19. R. Mertens and C. v. Sonntag, J. Chem. Soc., Perkin Trans. 2, 1992, 2181.
20. M. Lal, J. Mönig and K.-D. Asmus, Free Radical Res. Commun., 1986, 1, 235.
21. J. K. Kochi, Free Radicals, Wiley, New York, 1973, vol. 1, p. 410.
22. R. H. Schuler, A. L. Hartzell and B. Behar, J. Phys. Chem., 1981, 85, 192.
23. D. Zehavi and J. Rabani, J. Phys. Chem., 1972, 76, 312.
24. V. Nagarajan and R. W. Fessenden, J. Phys. Chem., 1985, 89, 2330.
25. G. Hug, Natl. Stand. Ref. Data Ser., U.S. Natl. Bur. Stand., Report No. 69, 1981.
26. M. S. Matheson, W. A. Mulac, J. L. Weeks and J. Rabani, J. Phys. Chem., 1966, 70, 2092.
27. H. A. Schwarz and R. W. Dodson, J. Phys. Chem., 1984, 88, 3643.
28. R. Fliount, O. Makogon, D. M. Guldi and K.-D. Asmus, unpublished results.
29. A. Rafi and H. C. Sutton, Trans. Faraday Soc., 1965, 61, 877.
30. C. v. Sonntag, The Chemical Basis of Radiation Biology, Taylor & Francis, London, 1987.
31. J. Mönig, D. Bahnemann and K. D. Asmus, Chem. Biol. Interact., 1983, 47, 15.
32. R. Köster and K.-D. Asmus, Z. Naturforsch., Teil B, 1971, 26, 1104.
33. H. Hungerbühler, K.-D. Asmus, O. Makogon and R. Fliount, AFEAS-Report, Contract No. CTR93-41/P93-110, Brussels, Belgium, 1993.