Electron spin resonance. Part 67. Oxidation of aliphatic sulphides and sulphoxides by the sulphate radical anion (SO4–˙) and of aliphatic radicals by the peroxydisulphate anion (S2O82–)

Abstract

E.s.r. experiments employing in situ photolytic decomposition of the peroxydisulphate anion (S₂O₈^2–) have been carried out to study the reaction of SO₄^–˙ with aliphatic sulphides and sulphoxides. For the former, ‘dimer’ radical cations (R₂SSR₂⁺˙) are detected; these are evidently generated via direct one-electron oxidation of the sulphide (to give firstly R₂S⁺˙). The more complex behaviour of the latter [involving, for example, the reaction of Me₂S(O) to give ˙CH₂S(O) Me (at low pH), Me˙, and MeSO₂·] is interpreted in terms of the initial formation of an (undetected) radical cation R₂S(O)⁺˙, subsequent competing reactions of which include deprotonation and hydration (followed by fragmentation).

Flow-system studies of the generation of radicals with ˙OH (from Ti^III/H₂O₂) in the presence of S₂O₈^2– provide evidence that the latter is an effective oxidant for both unsubstituted and α-oxygen-substituted alkyl radicals. The rate of oxidation is increased by α-Me and α-OH or -OR groups [e.g. for Me˙ the rate constant is 3.3 ×10⁴ dm³ mol^–1 s^–1; for ˙CH₂OH, the value is 1.3 × 10⁵ dm³ mol^–1 s^–1].