Kinetic studies on the oxidation of aryl methyl sulfides and sulfoxides by dimethyldioxirane; absolute rate constants and activation parameters for 4-nitrophenyl methyl sulfide and sulfoxide

Abstract

The oxidations of methyl 4-nitrophenyl sulfide and sulfoxide by dimethyldioxirane, in acetone and mixtures of acetone with water, methanol, acetonitrile and hexane, have been followed by UV-Vis spectroscopy to monitor the decay of the substrates. The data show that, under all the conditions studied, both oxidations obey second-order kinetics. Grunwald-Winstein and Kamlet-Taft analyses of the influence of solvents on the second-order rate constants have been used to obtain mechanistic information on the two reactions. Activation parameters for the two oxidations in acetone and aqueous acetone have been calculated from rate constants for reactions in the temperature range 283-313 K and compared with those from sulfide and sulfoxide oxidations with other oxidants. For sulfoxide oxidations in acetone and 1-20% v/v water in acetone, the results support a concerted nucleophilic displacement by sulfur of oxygen from dimethyldioxirane with the rate being dependent on the solvent's polarity. Sulfide oxidations in acetone and 1-5% v/v water in acetone also proceed by a concerted mechanism. However, in the most polar solvent system studied, 20% v/v water in acetone, the mechanism changes in favour of a two-step reaction involving a betaine intermediate. Importantly, the sulfide oxidation shows a different solvent dependence to that of the sulfoxide, with the rate of oxidation being determined by the hydrogen bond donor capacity and electron-pair donicity of the solvent.