No abstract

No abstract

ؒ 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.

Model systems, based on aqueous solutions containing isoflurane (CHF(2)OCHClCF(3)) as an example, have been studied in the presence and absence of methionine (MetS) to evaluate reactive fates of halogenated hydroperoxides and peroxyl and alkoxyl radicals. Primary peroxyl radicals, CHF(2)OCH(OO*)CF(3), generated upon 1-e-reduction of isoflurane react quantitatively with MetS via an overall two-electron oxidation mechanism to the corresponding sulfoxide (MetSO). This reaction is accompanied by the formation of oxyl radicals CHF(2)OCH(O*)CF(3) that quantitatively rearrange by a 1,2-hydrogen shift to CHF(2)OC*(OH)CF(3). According to quantum-chemical calculations, this reaction is exothermic (DeltaH = -5.1 kcal/mol) in contrast to other potentially possible pathways. These rearranged CHF(2)OC*(OH)CF(3) radicals react further via either of two pathways: (i) direct addition of oxygen or (ii) deprotonation followed by fluoride elimination resulting in CHF(2)OC(O)CF(2)*. Route i yields the corresponding CHF(2)OC(OO*)(OH)CF(3) peroxyl radicals, which eliminate H+/O(2)*-. The resulting ester, CHF(2)OC(O)CF(3), hydrolyzes further, accounting for the formation of HF, trifluoroacetic acid, and formic acid with a contribution of 45% and 80% in air- and oxygen-saturated solutions, respectively. A competitive pathway (ii) involves the reactions of the secondary peroxyl radicals, CHF(2)OC(O)CF(2)OO*. The two more stable of the three above mentioned peroxyl radicals can be distinguished through their reaction with MetS. Although the primary CHF(2)OCH(OO*)CF(3) oxidizes MetS to MetSO in a 2-e step, the majority of the secondarily formed CHF(2)OC(O)CF(2)OO* reacts with MetS via a 1-e transfer mechanism, yielding CHF(2)OC(O)CF(2)OO-, which eventually suffers a total breakup into CHF(2)O- + CO(2) + CF(2)O. Quantum-chemical calculations show that this reaction is highly exothermic (DeltaH = -81 kcal/mol). In air-saturated solution this pathway accounts for about 35% of the overall isoflurane degradation. Minor products (10% each), namely, oxalic acid and carbon monoxide originate from oxyl radicals, CHF(2)OC(O)CF(2)O* and CHF(2)OCH(O*)CF(3). An isoflurane-derived hydroperoxide CHF(2)OCH(OOH)CF(3) in high yield was generated in radiolysis of air-saturated solutions containing isoflurane and formate either via a H-atom abstraction from formate by the isoflurane-derived peroxyl radicals or by their cross-termination reaction with superoxide O(2)*-. CHF(2)OCH(OOH)CF(3), is an unstable intermediate whose multistep hydrolysis is giving H(2)O(2) + 2HF + HC(O)OH + CF(3)CH(OH)(2). In the absence of MetS, about 55% of CHF(2)OCH(OO*)CF(3) undergo termination via the Russell mechanism and 27% are involved in cross-termination with superoxide (O(2)*-) and peroxyl radicals derived from t-BuOH (used to scavenge *OH radicals). The remaining 18% of the primary peroxyl radicals undergo termination via formation of alkoxyl radicals, CHF(2)OCH(O*)CF(3).

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.