Kinetics and mechanism of the oxidation of thiolactic acid by 12-tungstocobaltate(III) ion in acetate buffer

The reaction between 12-tungstocobaltate(III) and thiolactic acid (TLA) is expressed by the stoichiometric equation, 2[Co(III)W](5-) + 2CH(3)CH(SH)COO H = CH3CH(COO)SS(OOC)HCH3C + 2H(+) + 2[Co(II)W](6-), which is consistent wi th the stoichiometric ratio Delta[Co(III)W](5-)/Delta[TLA] = 1.00 +/- 0.06. The reaction is first-order in [Co(III )W](5-), fractional order in [RSH] and shows a linear correlation between k(obs)(-1) and [H+] with an intercep t on the rate ordinate. The formation of a complex between [Co(III)W](5-) a nd CH3CH(SH)COO- is evident both from rapid scanning of the reaction mixtur e and the kinetics. The formation of the complex from Co(III)W](5-) and CH3 CH(COOH)S-, suggested in a previous study, is shown to be unlikely. The for mation constant (0.025) for the complex is independent of temperature. The activation parameters for the rate limiting step are Delta H double dagger = 15 +/- 1 kJ mol(-1) and Delta S double dagger = -153 +/- 3 J K-1 mol(-1). The small Delta H double dagger value is suggestive of a multiple process that included preequilibrium reactions. The rate determining intramolecular electron-transfer from COO- is synchronous with the transfer of H atom, pr oduced by the homolytic S-H fission, to COO resulting in the formation of C H3CH(COOH)S- free radical which dimerises to the oxidation product. Thus th e electron-transfer is theoretically inner-sphere. Since the central Co-III is well protected, it is difficult to visualise the manner in which the th iolic proton could form a bridge. It is equally difficult to be specify whe ther the central Co-III atom is directly reduced to Co-II by the inner sphe re electron transfer as suggested by the mechanism or it is reduced by the electron first transferred to outer tungstate framework tin outer-sphere pa thway) which instantaneously transfers it to the central Co-III atom. In ei ther case the spectrum of the reduced reaction mixture would be identical w ith that of 1 2-tungstocobaltate(II). Hence, the theoretical aspect of the electron transfer in such a case might have some bearing on the nature of t he reaction.