Trichloromethylperoxyl Radicals Research Articles

ChemInform Abstract: Absolute Rate Constants for Hydrogen Abstraction from Hydrocarbons by the Trichloromethylperoxyl Radical

Absolute rate constants have been measured for the reactions of trichloromethylperoxyl radicals with cyclohexane, cyclohexene, and hexamethylbenzene. The CCl3O2 radicals were produced by pulse radiolysis of air-saturated CCl4 solutions containing various amounts of the hydrocarbons. The rate constants were determined by competition with the one-electron oxidation of metalloporphyrins, using the rate of formation of the metalloporphyrin radical cation absorption to monitor the reaction by kinetic spectrophotometry. The rate constants for hydrogen abstraction from cyclohexane, cyclohexene, and hexamethylbenzene were found to be 1 × 103, 1.0 × 105, and 7.5 × 104 M−1 s−1, respectively.

Absolute rate constants for hydrogen abstraction from hydrocarbons by the trichloromethylperoxyl radical

AbstractAbsolute rate constants have been measured for the reactions of trichloromethylperoxyl radicals with cyclohexane, cyclohexene, and hexamethylbenzene. The CCl3O2 radicals were produced by pulse radiolysis of air‐saturated CCl4 solutions containing various amounts of the hydrocarbons. The rate constants were determined by competition with the one‐electron oxidation of metalloporphyrins, using the rate of formation of the metalloporphyrin radical cation absorption to monitor the reaction by kinetic spectrophotometry. The rate constants for hydrogen abstraction from cyclohexane, cyclohexene, and hexamethylbenzene were found to be 1 × 103, 1.0 × 105, and 7.5 × 104 M−1 s−1, respectively.

The formation of a novel free radical metabolite from CCl4 in the perfused rat liver and in vivo.

Electron spin resonance spectroscopy has been used to monitor free radicals formed during CCl4 metabolism by perfused livers from phenobarbital-treated rats. Livers were perfused simultaneously with the spin trap phenyl N-t-butylnitrone and with either 12CCl4 or 13CCl4. Perfusate samples and CHCl3:CH3OH extracts of perfusate and liver samples were analyzed for phenyl N-t-butylnitrone radical adducts of reactive free radicals. In the organic extracts, hyperfine coupling constants and 13C isotope effects observed in the ESR spectra indicated the presence of the radical adduct of the trichloromethyl radical. Surprisingly, an additional free radical signal about two orders of magnitude more intense than that of the phenyl N-t-butylnitrone/CCl.3 radical adduct was observed in the aqueous liver perfusate. This adduct was also detected by ESR in rat urine 2 h after intragastric addition of spin trap and CCl4. This radical adduct had hyperfine coupling constants and 13C isotope effects identical with the radical adduct of the carbon dioxide anion radical (CO2-.). Analysis of the pH dependence of the coupling constants yielded a pK alpha of 2.8 for the CO2-. radical adduct formed either in the perfused liver or chemically. Carbon tetrachloride is converted into CCl.3 by cytochrome P-450 through a reductive dehalogenation. The trichloromethyl free radical reacts with oxygen to form the trichloromethyl peroxyl radical, CCl3OO., which may be converted into .COCl and then trapped. This radical adduct would hydrolyze to the carboxylic acid form, which is detected spectroscopically. Alternatively, the carbon dioxide anion free radical could form through complete dechlorination and then react with the spin trap to give the CO2-. radical adduct directly.

Free radical one-electron versus hydroxyl radical-induced oxidation. Reaction of trichloromethylperoxyl radicals with simple and substituted aliphatic sulphides in aqueous solution

One-electron oxidation of organic sulphides leading to sulphur-centred radical cations, R2S+˙, can conveniently be initiated by CCl3O2˙ radicals. Absolute rate constants have been measured for the reaction of CCl3O2˙ with dimethyl sulphide, 2-(ethylthio)ethanol, and methionine to be 1.8 × 108, 4 × 107, and 2.9 × 107 mol–1 dm3 s–1, respectively. Oxidation of dimethyl sulphide and methionine by the halothane-derived CF3CHClO2˙ radical occurs with absolute rate constants of k 6 × 106 and 1.4 × 106 mol–1 dm3s–1, respectively. The R2S+˙ generally equilibrates according to R2S+˙+ R2S ⇌(R2S ∴ SR2)+ to yield sulphur-centred three-electron-bonded radical cations. Amino-substituted sulphides such as 3-(methylthio)propylamine, methionine, and methionine ethyl ester also stabilize as intramolecularly formed S ∴ N three-electron-bonded radical cations. This competing reaction route requires a free electron pair at nitrogen, i.e. an unprotonated amino group, and steric assistance by five- or six-membered ring structures. The pH profiles of S ∴ S and S ∴ N bond formation are significantly different in the oxidation of the amino-substituted sulphides by one-electron oxidants from those for the ˙OH radical-induced process. The underlying mechanisms are compared and discussed.

Reductive oxygenation of carbon tetrachloride: Trichloromethylperoxyl radical as a possible intermediate in the conversion of carbon tetrachloride to electrophilic chlorine

Under aerobic conditions, rat liver microsomes convert carbon tetrachloride to an electrophilic form of chlorine that is trapped with 2,6-dimethylphenol to form 4-chloro-2, 6-dimethylphenol. The mechanism of cytochrome P-450-catalyzed electrophilic chlorine formation from carbon tetrachloride was examined with structure-activity studies of electrophilic halogen formation and chemical and in vitro microsomal studies. 4-Chloro-2, 6-dimethylphenol is not formed as a consequence of a reaction of 2,6-dimethylphenoxyl radical with carbon tetrachloride or carbon tetrachloride-induced lipid peroxyl radical formation. Only tetrahalomethanes were found to yield electrophilic halogens. The chemical oxidants hydrogen peroxide, cumene hydroperoxide, sodium periodate, and iodobenzene diacetate did not support electrophilic halogen formation from carbon tetrachloride, carbon tetrabromide, or hexachloroethane in microsomal studies. The addition of superoxide dismutase, catalase, sodium azide, or glutathione to microsomal incubations did not affect the rate of electrophilic chlorine formation, whereas Paraquat completely inhibited the reaction. The radical spin trap phenyl t-butyl nitrone (14 m m) completely inhibited electrophilic chlorine formation. The rate of electrophilic chlorine formation was highest at 2–5% atmospheric oxygen, whereas anaerobiosis completely inhibited electrophilic chlorine formation, and high oxygen tension impaired electrophilic chlorine formation. These results preclude direct oxidation of carbon tetrachloride or a reaction of superoxide anion radical with carbon tetrachloride as the initial step in electrophilic chlorine formation and suggest that the likely initial step is reductive dehalogenation of carbon tetrachloride to trichloromethyl radical which then traps oxygen to form trichloromethylperoxyl radical. Subsequent reaction of trichloromethyl peroxyl radical leads to electrophilic chlorine. These findings may have important implications concerning carbon tetrachloride-induced lipid peroxidation and carbon tetrachloride hepatotoxicity.