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.
Published Version
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