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

Abstract

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.