Oxidation of thioglycolic acid and glutathione by (trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetato)manganate(III) in aqueous media

Abstract

The kinetics of the electron-transfer reactions of the manganese(III) complex of trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetate (cdta^4–) with two thiols thioglycolic acid and glutathione has been investigated at 30 °C in aqueous media in the range pH 2.0–10.33 with varying reductant concentrations at constant ionic strength, I= 0.20 mol dm^–3(NaClO₄). The reactions are first order both in complex and reductants and follow the general rate law, –d[Mn^III(cdta)^–]/dt=K_obs[Mn^III(cdta)^–]=K[Mn^III(cdta)^–][reductant]. Both the reactions have been assumed to proceed via an inner-sphere mechanism with support for this coming from the observation of a rapid initial increase in absorption followed by a slower decay. This indicates the formation of an inner-sphere associated species which decomposes unimolecularly leading to the transfer of the electron from the thiol to the oxidant. Additional support for this mechanism comes from a comparison of the water-exchange rate of [Mn(cdta)(H₂O)]^– with the higher limit of the electron-transfer rates. The pH–rate profiles are bell-shaped curves and were successfully modelled by fitting the experimental data to a computer-fitted program thereby evaluating the reactivity of all the reacting species of the reductants.