Dual mechanism of oxidation of DL-methionine by diperiodatoargentate(III) in aqueous alkaline medium (stopped flow technique)

The kinetics of oxidation of DL-methionine by diperiodatoargentate(III) (DPA) has been studied spectrophotometrically in a wide range, 0.01-1.0 mol dm(-3), of alkali at constant ionic strength of 0.50 mol dm(-3). In a lower range, 0.01-0.20 mol dm(-3) and in the higher range 0.3-1.0 mol dm(-3) of OH- ion concentrations the reaction exhibits first order with respect to DPA concentration. But in the lower and higher range of OH- ion concentration the order with respect to DL-methionine concentration changes from fractional order to zero order. In the lower range of OH- ion concentration the rate is independent of periodate concentration and in the higher range of OH-ion concentration, periodate retarded the rate of the reaction. At constant DPA, DL-methionine, periodate and at constant ionic strength, the order with respect to OH- ion concentration changes from negative fractional (-0.36) to positive fractional (0.44). The increase in dielectric constant of the medium, increases the rate of the reaction in lower and higher range of the OH- ion concentration. The added products did not have any significant effect on the rate of reaction. The main products were identified by spot test and IR spectroscopy. A dual free radical mechanisms have been proposed for both lower and higher range of OH- ion concentrations. The active species of Ag(III) was found to be diperiodatoargentate(III) in lower range of OH- ion concentration and monoperiodatoargentate(III) in higher range of OH- ion concentration. The rate constants of the slow step of the mechanisms were determined. The activation parameters were evaluated and discussed. (C) 2007 Elsevier B.V. All rights reserved.