Mechanism of formation of alkyl aryl sulphoxides by oxidation of alkyl aryl sulphides with bromine

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Abstract
Bromine reacts with substituted methyl phenyl sulphides and isopropyl phenyl sulphide in 2 : 1 methanol–water (v/v) to give the corresponding sulphoxides. The reaction follows second-order kinetics, first order in bromine and first order in sulphide. The reaction rate is increased by the presence of NaOAc and decreased by addition of NaBr in solution. The reaction is very sensitive to structural factors. It is favoured by electron-donating substituents in the phenyl ring (ρ=–3·2). On the other hand isopropyl phenyl sulphide reacts about 30 times more slowly than methyl phenyl sulphide. A mechanism involving preliminary fast formation of a bromosulphonium cation followed by its rate-determining hydrolysis is suggested.

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