Kinetics of oxidation of thioacids by tetrabutylammonium-12-tungstocobaltate(III) ion in dimethyl sulphoxide medium

Abstract

The oxidation of thioglycolic(TGA), thiomalic(TMA) and thiolactic(TLA) acids in DMSO is first order in tetrabutylammonium-12-tungstocobaItate (lII) ion, independent of [H⁺] and the plots of k_obs ^-1 versus [thioacid]^-1 are linear. The reactivity is in the order TLA > TMA > TGA. A comparison with the corresponding oxidation of TLA in aqueous medium has manifested areas of resemblance and strong deviations. The area of agreement includes evidence for the formation of the intermediate complex both from the kinetics and rapid scanning of the reaction mixture. The areas of divergence are (a) no effect of [H⁺] on k_obs (K_obs^-1 was linearly dependent on [H⁺] in water), (b) the association constant K_ass increases with temperature (independcnt of temperature in water), (c) value of K_ass in DMSO is about 400 times the value in water and (d) the rate limiting constant in water is about 10⁴ times as high as in DMSO. These differences are explained on the basis of solvation of the ions and the stability and change in reactivity of the solvated ions in protic and aprotic solvents. The solvating nature of the solvent has also explained the differences in the enthalpy and entropy of activation in DMSO and water.