Kinetics and mechanistic study of manganese(II)-catalyzed cerium(IV) oxidation of thiamine hydrochloride in aqueous perchloric acid medium by stopped flow technique

Abstract

The kinetics of the manganese(II)-catalyzed oxidation of thiamine hydrochloride by cerium(IV) in aqueous perchloric acid medium at a constant ionic strength of 1.10 mol dm−3 was studied spectrophotometrically at 15, 25, 35, and 45 °C by the stopped flow technique. The reaction between thiamine hydrochloride and cerium(IV) in the acid medium exhibits 1:3 stoichiometry. The main products were identified by spot test, IR, 1H NMR, and GC–MS studies. The reaction is first order in cerium(IV) and manganese(II) and has less than unit order in thiamine hydrochloride. As the acid concentration increases the rate of reaction decreases. The added product cerium(III) retards the rate of reaction. The active catalyst and oxidant species were identified as [Mn(H2O)4]2+ and [Ce(OH)]3+, respectively. A probable mechanism involving free radicals and the formation of a complex between substrate and catalyst is proposed. The reaction constants, activation parameters, and thermodynamic quantities are calculated and discussed.