OXOVANADIUM(IV) CATALYSIS IN ACID BROMATE OXIDATION OF 2-HYDROXYBUTYRIC ACID

The kinetics and mechanistic studies on the oxovanadium(IV)-catalysed and uncatalysed acid bromate oxidation of 2-hydroxybutyric acid in the presence of Hg(II) ion as a Br- ion scavenger have been carried out under varying conditions. The uncatalysed oxidation exhibits first order each in [bromate] and [substrate], and second order in [HClO4]. Primary kinetic isotope effect has been found to be absent, however, an inverse solvent isotope effect (k(H2O)/k(D2O) = 1.75) is observed. The mechanistic pathway involves formation of a cyclic bromate ester of hydroxy acid in the slow step, which decomposes to propanaldehyde and CO2 through C-C bond cleavage in the fast step. The oxovanadium(IV)-catalysed oxidation exhibits zero order each in [bromate] and [HClO4] and first order each in [substrate] and [oxovanadium(IV)]. The reaction also exhibits a substantial primary kinetic isotope effect (k(H)/k(D) = 4.86) while solvent isotope effect is absent. A mechanism involving transfer of hydride ion from the substrate to the catalyst through C-H bond cleavage in the rate-determining step is postulated. The carbocation thus formed decomposes to CH3CH2COCOOH, a keto acid, in the subseqent fast step.