Kinetics and mechanism of the oxidation of organic sulphides by 2,2¢ -bipyridinium chlorochromate

The oxidation of 34 organic sulphides by 2,2´-bipyridinium chlorochromate (BPCC) resulted in the formation of the corresponding sulphoxides. The reaction is first order with respect to both BPCC and the sulphide, and is catalysed by hydrogen ions. The hydrogen-ion dependence has the form: kobs = a + b [H+]. The oxidation was studied in 19 different organic solvents. An analysis of the solvent effect by Swain’s equation showed that the both cation- and anion-solvating powers of the solvents play important roles. The rates of oxidation meta- and p-substituted phenyl methyl sulphides were correlated with Charton’s LDR equation. The rates of the o-compounds showed excellent correlation with the LDRS equation. Oxidation of the p-compounds is more susceptible to the delocalization effect. Oxidation of the mcompounds exhibited a greater dependence on the field effect. In the oxidation of the o-compounds, the contribution of delocalized effect is slightly more than that of the field effect. The oxidation of alkyl phenyl sulphides is subject to both polar and steric effects of the alkyl groups. Polar reaction constants are negative, indicating an electron-deficient sulphur centre in the rate-determining step. A mechanism involving formation of a sulphurane intermediate in the slow step has been proposed.