ؒ CBr 2 CO 2 ؊ and ؒ CCl 2 CO 2 ؊ radicals, generated upon one electron reduction of tribromo-and trichloroacetic acids and ؒ CF 2 CO 2 ؊ radicals produced from difluoroacetic acid by reaction with ؒ OH, exhibit optical absorptions in the UV with max at 290 nm ( = 2580 dm 3 mol ؊1 cm ؊1 ), 330 nm ( = 3000 dm 3 mol ؊1 cm ؊1 ) and 310 nm ( ≈ 660 dm 3 mol ؊1 cm ؊1 ), respectively. Mechanistically, the present report focuses on the freeradical-induced degradation of tribromoacetic acid. Absolute rate constants have been determined for the reactions of CBr 3 CO 2 ؊ with e aq ؊ , H ؒ , CO 2 Ϫ , ؒ CH 2 OH, CH 3 ؒ CHOH, (CH 3 ) 2 ؒ COH and ؒ CH 3 radicals to be k = 1.

γ-Radiolysis of aqueous, pH 6 solutions containing CClF2CO2 - (1 × 10-3 to 2 × 10-2 M), HCO2 - (2 × 10-3 M), and O2 (20−100% O2 saturation) or N2O/O2-(4:1 v/v)-saturated solutions of CHF2CO2 - have been used as models to illustrate the high efficiency (70−100 %) of cross-termination between halogenated peroxyl radicals, here •OOCF2CO2 - and O2 •-, as opposed to the self-termination of the respective radicals. Experiments have been conducted at various [•OOCF2CO2 -]/[O2 •-] concentration ratios and with either of the two species in excess. The proposed mechanisms are supported by quantitative material balances. Since the final reaction products derived from CClF2CO2 - are identical in nature (CO2, Cl-, F-, H+, oxalate) and yields are the same irrespective of cross- or self-termination, the conclusions were based on the H2O2 yields which are shown to differ significantly depending on the mechanism. The •OOCF2CO2 - + O2 •- reaction is considered to proceed via an intermediate hydroperoxide, HOOCF2CO2 -, which predominantly decays via C−C cleavage into CF2O and HCO3 -. Only a minor fraction (about 10%) remains as C2-compound and ends up as oxalate. Mechanistically, the results emphasize the significance of superoxide in all systems in which peroxyl radicals are generated. With respect to halogenated hydrocarbons this is considered to be particularly relevant in, for example, the radical- and redox-induced mineralization process under aerobic conditions and in the biological metabolism of such compounds.

Radiation chemistry and photocatalysis are introduced as complementary methods for the study of the radical- and redox-induced degradation of organic substrates. Particular focus is devoted on the reductive and oxidative formation and destruction of halogenated organic acids, as intermediates in the degradation of halo- genated hydrocarbons, in dilute aqueous solutions. The specific aim of this comparative essay is to point out the fundamental similarities between a radiation chemical and photocatalytical approach but, at the same time, to demonstrate that and why significant differences may, nevertheless, be observed. The examples presented and discussed are concerned with (i) the oxidatively and reductively induced degradation of 1,1,1-trichloroethane to organic acids, (ii) corresponding experiments on the degradation of trifluoro-, trichloro- and tribromoacetic acid, (iii) peculiarities of the degradation processes in the absence of oxygen, and (iv) the importance of the cross­termination reaction between halogenated peroxyl radicals and superoxide, O