Oxidation of organic sulphides - II. Interaction ofcyclohexyl methyl sulphide with hydroperoxides in hydrocarbons

Abstract
Oxidations of cyclohexyl methyl sulphide by cyclohexenyl and t-butyl hydroperoxides in hydrocarbon solvents simulate those in alcohols (part I) in that the sulphoxide yield is quantitative under all the conditions studied. They differ, however, in their kinetic characteristics, and the complexities of the hydrocarbon systems are found to vary remarkably with the hydroperoxide and with the solvent. The reaction with cyclohexenyl hydroperoxide in benzene or cyclohexane is insensitive to the presence of oxygen, evidences no free radical character, and has orders of one with respect to sulphide and two with respect to hydroperoxide. Acetic acid catalyzes the reaction and changes the hydroperoxide order from 2 to 1. Essentially the same oxygen atom transfer process as postulated in alcohols is believed to occur: in the absence of acid, the oxidant is a bimolecular hydroperoxide complex; in the presence of acid, the component molecule effecting the associated hydrogen transfer is displaced by an acid molecule which can perform this function more efficiently. The reaction with t-butyl hydroperoxide in benzene proceeds faster in air than in a vacuum; the oxygen catalysis is less in cyclohexane and absent in cyclohexene. The reaction stoicheiometry remains the same in all cases. The addition of acetic acid, which catalyzes the reaction as indicated for cyclohexenyl hydroperoxide, completely eliminates the sensitivity to oxygen. Considering oxygen-free systems, the reaction orders with respect to the sulphide are 0$\cdot $7 in benzene, 0$\cdot $4 in cyclohexane, and 0$\cdot $45 in cyclohexene; the order with respect to hydroperoxide is 2 throughout. The reactions are insensitive to free radical inhibitors. It is suggested that oxidation occurs by two concurrent mechanisms; one is the oxygen-atom transfer process referred to above; the other is formulated as a two-stage process involving a hydroperoxide-solvent complex as oxidant, the rate of complex formation being rate-determining. In benzene in the presence of oxygen, the sulphide order is higher (0$\cdot $9) and the hydroperoxide order lower (1$\cdot $4); free radical inhibitors effectively repress oxygen catalysis. A free radical chain mechanism therefore appears to come into play in the presence of oxygen, and the postulated requirements of this to account for the unaltered stoicheiometry and the solvent effects are discussed.