Deamination and decarboxylation in the chromium(III)-catalysed oxidation of L-valine by alkaline permanganate and analysis of chromium(III) in microscopic amounts by a kinetic method

摘要

The kinetics of Cr~(III) - catalysed oxidation of L-valine by permanganate in alkaline medium at a constant ionic strength has been studied spectrophotometrically. The reaction between permanganate and L-valine in alkaline medium exhibits 2:1 stoichiometry (KMnO_4:L-valine). The reaction shows first order dependence on [permanganate] and [chromium(III), and less than unit order dependence each in [L-valine] and alkali concentrations under the experimental conditions. However, the order in [L-valine] and [alkali] changes from first order to zero order as the concentrations change from lower to higher respectively. The results suggest the formation of a complex between L-valine and the hydroxylated species of Cr~(III). The complexes reacts further with 1 mol of alkaline permanganate species in a rate-determining step, resulting in the formation of afree radical, which again reacts with 1 mol of alkaline permanganate species in a subsequent fast step to yield the products. The reaction constants involved in the mechanism were evaluated. The activation parameters with respect to the slow step of the mechanism were obtained and discussed. The title reaction has been utilised to analyse chromium(III) in the 26.0 ng cm~(-3) - 1.0 mug cm~(-3) range.