Kinetics of oxidation of para-substituted phenyl methyl sulphides by pyridinium chlorochromate in dipolar protic and aprotic solvents

Abstract

The kinetics of oxidation of phenyl methyl sulphide (PMS) and several para-substituted phenyl methyl sulphides by pyridinium chlorochromate have been studied in binary solvent mixtures of 60%(v/v) aqueous acetic acid and 50%(v/v) chlorobenzene–nitrobenzene. Acrylonitrile has no effect on the rates of oxidation. In aqueous acetic acid, there is overall second-order reaction. The Hammett p value of –2.12 ± 0.09 (r 0.985, s 0.104) at 50 °C indicates an electron-deficient transition state. In chlorobenzene–nitrobenzene there is an association equilibrium prior to the rate-limiting step, follwed by irreversible decomposition of the complex into products. The complex formation constants (K) and the complex decomposition of rate constants (k₁) have been computed from the double reciprocal plots of 1/K_obsversus 1/[PMS]₀. The decomposition rate constants (k₁) correlate well with Hammett σ values (p–1.23 ± 0.054, r 0.989,s 0.068 at 50 °C). On the basis of the results of the kinetic studies, a suitable mechanism has been proposed.