Mechanism of oxovanadium (IV)-catalysed and uncatalysed bromate oxidation of lactic acid in acid medium: Evaluation of composite and individual kinetic and thermodynamic parameters

Uncatalysed acid bromate oxidation of lactic acid (LA) exhibits fractional order (1.6) in [acid] and first order each in [Br(V)] and [LA]. Kinetic isotope effect (k(H)/k(D)=1) is absent, however an inverse solvent isotope effect (k(H2O) / k(D2O)=1.78 at 313 K) is observed. The mechanism proposed consists of a rate determining formation of bromate ester due to the nucleophilic attack by the alcoholic I hydroxyl group at the bromine atom of HOBrO2/(H2O2BrO)-O-4, followed by a fast disproportionation of the ester through C-C bond cleavage to form products. Oxovanadium(IV)-catalysed LA-Br(V) reaction exhibits second order in [acid], fractional order in [lactic acid] and first order each in [VOSO4] and [oxidant]. The envisaged mechanism assumes a ternary complex formation involving a binary complex of LA and V(IV) and protonated bromate, which subsequently decomposes to the intermediate/products. The rate constants involved for each step of the reactions are evaluated along with their thermodynamic parameters.