Kinetics and mechanism of oxidation of organic sulphides by lead tetraacetate

Abstract

Kinetics of oxidation of thirty-three organic sulphides, comprising monosubstituted aryl methyl sulphides and alkyl phenyl sulphides, by lead tetraacetate (LTA), to yield the corresponding sulphoxides, have been studied. The reaction is first order with respect to both sulphide and LTA and is catalysed by hydrogen ions. The observed solvent effect (m= 0.90 for MeSPh) is explained by a nucleophilic attack of the sulphide sulphur on LTA yielding a polar transition state. The rates of oxidation of meta- and para-substituted phenyl methyl sulphides were correlated with Taft′s and Swain′s dual substituent parameter equations. For the para-compounds, the best correlation is obtained with σ1 and σR+; the meta-substituted compounds correlate best with σ1 and σR0 values. The reaction constants are negative. The oxidation rates of ortho-substituted compounds yield an excellent correlation with a triparametric equation involving Taft′s σ1, and σR+ parameters, and Charton′s steric parameter V. The oxidation of alkyl phenyl sulphides is susceptible to both polar and steric effects of the alkyl groups. A mechanism involving formation of a sulphonium cation, in the rate-determining step, has been proposed.