Mechanism and evaluation of individual kinetic and thermodynamic parameters of acid bromate-lactic acid redox reaction

Department of Chemistry, Kakatiya University, Warangal-506 009, Andhra Pradesh, India
E-mail : chsrkuc@yahoo.co.in
Manuscript received 24 July 2006, revised 2 February 2007, accepted 28 March 2007

Kinetic investigations on the bromate-lactic acid redox reaction in aqueous perchloric acid medium have been carried out under varying conditions. The redox reaction exhibits first-order each in [bromate] and [lactic acid] and 1.6 order in [mineral 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 = 0.66 ± 0.03) is observed. Kinetic data indicate that both the species, HOBrO₂ and HOBr⁺O(OH) of acid bromate are involving in the oxidation of lactic acid in 1 : 4 ratio, at all the studied temperatures. The mechanism proposed involves slow formation of lactic ester of bromic acid due to the nucleophilic attack by the alcoholic/hydroxyl group at the bromine atom of HOBrO₂/HOBr⁺O(OH), which dissociates rapidly through C-C bond cleavage to form products, CH₃CHO and CO₂• Solvent and ionic strength effects are also in accordance with the mechanism proposed. Reaction constants involved in the mechanism have been evaluated along with their thermodynamic parameters. There is a good agreement between observed and calculated rate constants under different experimental conditions.