Formation of positive ions and other primary species in the oxidation of sulphides by hydroxyl radicals

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Abstract
The oxidation of simple aliphatic sulphides [MeSMe, EtSEt, (CH2)4S] by hydroxyl radicals occurs via a complex reaction mechanism. The first step is addition of the OH· to sulphur to form R2ṠOH radicals. At low sulphide concentrations (< 10–4 M) R2ṠOH rapidly eliminates H2O to form a RSR(–H)· radical which may be described by the mesomeric forms –CH–[graphic omitted]– and –CHṠ–. This radical is ultimately also formed at higher sulphide concentrations but via a different pathway. R2ṠOH increasingly reacts with another R2S molecule to form a short lived (R2S)2OH· radical complex which dissociates to (R2S)2 + and OH–. The (R2S)2 + complex ion seems to be relatively stable and decays essentially via equilibration to the molecular cation R2S+. This ion in its reaction with the solvent, OH– ions, and through a bimolecular process with another R2S+ cation is effectively deprotonated to form the RSR(–H)· radical. The reaction route at high concentration includes the formation of transient species with oxidizing properties; Fe(CN)6 4– is rapidly oxidized by (R2S)2 +[and possibly (R2S)2OH·]. The RSR(–H)· radical partially disproportionates to negative and long lived (>1 ms) positive ions. The stable oxidation product, sulphoxide, has been identified.

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