Mechanism of picolinic-acid-catalysed chromium(VI) oxidation of alkyl aryl and diphenyl sulphides

Abstract

The kinetics of picolinic acid (PA)-catalysed Cr^VI oxidation of organic sulphur compounds have been studied with several alkyl aryl sulphides and diphenyl sulphides in acetic acid–water mixtures. The PA-catalysed oxidation follows third-order kinetics, first order each in oxidant, sulphide, and catalyst, at constant [H⁺] and ionic strength. A good correlation exists btween log k₂ and Hammett σ constants for both aryl methyl and diphenyl sulphides, and the reaction constants are negative. Steric congestion at the reaction centre, sulphur, has been revealed by studies with C₆H₅SR (R = Me, Et, Pr, Prⁱ, and Bu^t). Three mechanisms are proposed to account for the observed experimental results. In aryl methyl sulphides the rate benefit (k_cat–k_uncat)/k_uncat is the least for p-methoxyphenyl methyl sulphide ad the largest for p-nitrophenyl methyl sulphide and this is in accordance with the reactivity–selectivity principle.