Mechanism of oxygenation of aryl methyl and diaryl sulphoxides by peroxomonophosphoric acid

The kinetics of oxygenation of the title sulphoxides by peroxomonophosphoric acid in aqueous acetic acid follows an overall second-order kinetics, first-order in each reactant. An analysis of the influence of [H+] reveals that H3PO5 is the active species in the oxidation. The structure–reactivity studies with different substituents on the phenyl ring give evidence for the formation of an electron deficient sulphonium ion intermediate as the correlation between log k2 and σ gives a negative ρ value (ρ=−0.47±0.09, r=0.988, s=0.03 at 35°C for aryl methyl sulphoxides and ρ=−0.54±0.11, r=0.983, s=0.07 at 35°C for diaryl sulphoxides). It is proposed that the mechanism involves the nucleophilic attack of the sulphoxide sulphur on the peroxo-oxygen of H3PO5 in the rate-limiting step.