Homogeneous catalysis of aquachlororuthenium(III) complex and evaluation of individual kinetic and thermodynamic parameters of bromate-lactic acid redox reaction at low and high acid strengths

Aquachlororuthenium(III) complex catalysed oxidation of lactic acid (LA) by bromate at low and high acid strengths has been studied and different reaction mechanisms have been proposed, depending on the composition of the mixture. At low acid strength, the reaction exhibits 1.0 order in [bromate], 0.13 order in [LA], 0.5 order in [catalyst] and a dualistic effect in [acid]. Primary kinetic isotope effect (k(H/)k(D) = 1) has been found to be absent. However, an inverse solvent isotope effect (k(D2O)/k(H2O) = 0.75) is observed. The proposed mechanism involves oxidation of the formed RU(III)-substrate complex by bromic acid, through complexation, to Ru(V)-substrate intermediate in the rate-determining step, which subsequently decomposes into CH3CHO and CO2 through C-C bond cleavage with regeneration of Ru(III) in the fast step. The same reaction at high acid strength exhibits first order each in [bromate] and [LA], 1.5 order in [catalyst], and an inverse fractional (-0.25) order in [acid]. The reaction also exhibits solvent isotope effect, while primary isotope effect is absent. The rate law is interpreted by a mechanism involving oxidation of Ru(III) to Ru(V) by Br(V) followed by the ternary complex formation between the formed Ru(V) and the preformed Ru(III)-substrate binary complex, which decomposes into products in the rate-determining step with regeneration of the catalyst. Spectroscopic data and the effects of dielectric constant and ionic strength are also in accordance with the mechanisms proposed. All the individual rate/formation constants involved in the mechanism proposed have been determined at different temperatures and their thermodynamic parameters have been evaluated and discussed.