Metal-ion oxidations in solution. Part I. The oxidation of some α-mercaptocarboxylic acids by cerium(IV)

Abstract

The ceric ion oxidations of thiolactic, thiomalic, and thioglycollic acids have been studied at [H⁺]= 0·5M over a temperature range 0–25°, by use of a stopped-flow technique. The rates are consistent with a mechanism involving the formation of organic radicals. Radical dimerisation occurs and subsequent oxidation to the sulphone may take place. The thermodynamic parameters for the reactions at 25° are: thiomalic acid E_a= 7·6 ± 0·8 kcal./mole, ΔS*=–30 ± 4 e.u.; thiolactic acid, E_a= 6·8 ± 0·4 kcal./mole, ΔS*=–31 ± 3 e.u.; thioglycollic acid, E_a= 7·9 ± 0·5 kcal./mole, ΔS*=–27 ± 3 e.u. Linear free-energy relationships obtained are discussed and comparisons with the reactions of the corresponding oxygen analogues are made.