Ruthenium(III)-catalysed oxidation of lactic acid by bromate. A kinetic and mechanistic approach

Ruthenium(III)-catalysed oxidation of lactic acid by bromate in feebly perchloric acid medium, exhibits 1.0 order in [bromate], 0.13 order in [lactic acid], and 0.5 order in [Ru(III)], and a dualistic effect with [acid]. Both, kinetic and spectrophotometric data suggest formation of an outersphere complex between lactic acid and the aquochlororuthenium(III). The envisaged mechanism involves oxidation of the formed Ru(III)-substrate complex by bromic acid, through complexation into Ru(V)-substrate intermediate in a rate-determining step, which subsequently decomposes to CH3CHO and CO2 through C-C bond cleavage with regeneration of Ru(III) in a fast step. All the reaction constants (K-sc, K-sco, k(d)) involved in the mechanism were calculated. There is a good agreement between observed and calculated rate constants under different experimental conditions. The activation parameters with respect to each step of the mechanism were evaluated and discussed.