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

Abstract

The reaction between 12-tungstocobaltate(III) and thiolactic acid (TLA) is expressed by the stoichiometric equation, 2[Co(III )W]^5- + 2CH₃CH(SH)COOH = CH₃CH(COO)SS(OOC)HCH₃C + 2H ⁺ 2[Co(II)W]^6-,which is consistent with the stoichiometric ratio &Delta;[Co(III)W]^5-/&Delta; [TLA] = 1.00 &plusmn; 0.06. The reaction is first-order in [Co(III )W]^5-&middot;, fractional order in [RSH] and shows a linear correlation between <em>k</em>_obs^-1and [H⁺] with an intercept on the rate ordinate. The formation of a complex between [Co(III)W]^5- and CH₃CH(SH)COO^- is evident both from rapid scanning of the reaction mixture and the kinetics. The formation of the complex from Co(III )W]^5-and CH₃CH(COOH)S^- , suggested in a previous study, is shown to be unlikely. The formation constant (0.025) for the complex is independent of temperature. The activation parameters for the rate limiting step are &Delta;<em>H&nbsp;</em><em>= 15 </em>&plusmn; 1 kJ mol^-1 and &Delta;<em>S&nbsp;</em>=-153 &plusmn; 3 J K^-1 mol^-1. The small &Delta;<em>H&nbsp;</em>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, produced by the homolytic S-H fission, to COO resulting in the formation of CH₃CH(COOH)S&middot; free radical which dimerises to the oxidation product. Thus the electron-transfer is theoretically <em>inner</em>-sphere. Since the central Co^lll is well protected, it is difficult to visualise the manner in which the thiolic proton could form a bridge. It is equally difficult to be specify whether the central Co^lll atom is directly reduced to Co^ll by the inner sphere electron transfer as suggested by the mechanism or it is reduced by the electron first transferred to outer tungstate framework (in outer-sphere pathway) which instantaneously transfers it to the central Co^lll atom. In either case the spectrum of the reduced reaction mixture would be identical with that of 12-tungstocobaltate(II). Hence, the theoretical aspect of the electron transfer in such a case might have some bearing on the nature of the reaction.