Mechanism of the oxidation of alkyl aryl sulphides by peroxodiphosphate

Abstract

The kinetics of oxidation of alkyl aryl sulphides by peroxodiphosphate ion were investigated in 50%(v/v) aqueous acetic acid. At constant [H⁺] and ionic strength, the reaction is second-order, first-order in sulphide and in peroxodiphosphate. An analysis of the dependence of the rate of oxidation of methyl phenyl sulphide on [H⁺] shows that H₃P₂O₈^– is the active species involved in the oxidation process. The rate of the reaction is influenced by changing the ionic strength. Acrylonitrile has no effect on the rate of oxidation. In substituted phenyl methyl sulphides, the rate of the reaction is accelerated by electron-donating substituents and retarded by electron-withdrawing substituents, indicating rate-determining nucleophilic displacement by the sulphide sulphur on the peroxide oxygen. The ρ value obtained at 40 °C (ρ–0.619, r 0.994, s 0.03, confidence limit on ρ±0.04) by correlation analysis of the rate constants with σ⁺ and σ^– constants, reveals the existence of an excellent linear free-energy relationship. The activation parameters and the isokinetic temperature have been determined. Studies with different alkyl phenyl sulphides (C₆H₅SR; R = Me, Et, Prⁿ, Prⁱ, or Bu^t) indicate that the reaction rate is retarded by bulky R groups. On the basis of the kinetic studies, a suitable mechanism has been proposed.