A case of competitive catalytic cycles in ruthenium(III) catalysed oxidation of dimethyl sulphoxide by cerium(IV) in aqueous sulphuric acid media: A kinetic study

The kinetics of oxidation of dimethylsulphoxide (DMSO) by cerium(IV) to dimethylsulphone in the presence of ruthenium(III) (ca. 10(-7) mol dm(-3)) has been followed at 30-50 degrees C under the conditions: [DMSO](T)>>[Ce(IV)](T)>>[Ru](T). The rate of disappearance of [Ce(IV)] has been found to be first order in [Ce(IV)]. The pseudo-first order rate constant (k(0)) obtained from -d[Ce(IV)]/dt shows a first order dependence on [Ru](T). In the reaction, the substrate shows a rate retarding effect. The rate law conforms to: k(0)=k(a)[Ru](T)/{k(b)[DMSO](T)+k(c)} where k(a), k(b) and k(c) are the composite rate constants; [Ru](T) and [DMSO], represent the total concentrations of ruthenium added as catalyst and substrate respectively. A reaction scheme involving two competitive catalytic cycles, one involving Ru(III)/Ru(IV) predominant at higher substrate concentration and the other involving Ru(III)/Ru(V) predominant at lower concentration of the substrate has been proposed. The cycle, Ru(III)/Ru(V) is found to be more efficient. To explain the nonexistence of free radical in the oxidation of the two electron donor substrate (i.e. DMSO), it has been proposed that the two electron transfer occurs at a single step.