Kinetics and mechanism of the oxidation of histidine by dodecatungstocobaltate(III) and trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate (III) in aqueous medium

Abstract

The oxidation of histidine with dodecatungstocobaltate(III), CoW₁₂O₄₀^5– and trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III), [Mn^III(cdta)]^– have been investigated in the range pH 3.0–9.5 at variable reductant concentrations, at a constant ionic strength and temperature. Both reactions are found to be dependent on the first powers of the concentration of the substrate and oxidants and follow the general rate law, -d[Ox]/dt= 2k[Ox][Hist], where 2 is the stoichiometric factor. In the reduction of [Mn^III(cdta)]^–, a bell-shaped curve is obtained for the variation of second-order rate constant (K) as a function of pH, and this has been explained by considering the hydroxo form of the complex as being unreactive towards the reductant. An attempt has been made to verify the effect of alkali cation catalysis on the reaction rate. The observed alkali cation catalysis for the reduction of CoW₁₂O₄₀^5– is consistent with a rate law, k=k₀+k_c[M⁺] where k₀ and k_c are the rate constants for the spontaneous and catalysed paths respectively. For the reduction of [Mn^III(cdta)]^–, a negligible dependence of rate on [M⁺] was noted. All the observations are successfully explained by considering outer-sphere mechanistic pathways for both reactions.