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.