ؒ 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.
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