Mechanism of osmium tetroxide catalysed oxidation of 2-bromopropionic acid by hexacyanoferrate(III) in aqueous alkaline medium

Abstract

The kinetics of the osmium tetroxide catalysed oxidation of 2-bromopropionic acid by hexacyanoferrate(III) ion has been studied in aqueous alkaline medium. The reaction follows zero-order kinetics with respect to hexacyanoferrate(III) ion and first order kinetics with respect to osmium tetroxide. The first-order kinetics with respect to 2-bromopropionic acid at low concentrations shifts to zero-order at higher concentrations. First-order dependence of the reaction at low concentrations of hydroxyl ion also shifts to zero-order at higher concentrations. The reaction is considered to proceed through the formation of a 2-bromopropionate-osmium tetroxide complex which decomposes giving the reduced Os(VI), followed by fast oxidation with hexacyanoferrate(III) ion. The suggested mechanism refers only to the initial stage of the reaction. The energy of activation for the oxidation process is 11.4 kcal/mole. The net rate of oxidation of 2-bromopropionic acid as measured by the consumption of hexacyanoferrate(III) ion, is given by −d[Fe(CN) 6] 3− d t = 2 kKK 2[OH −]Os(VIII)total][CH 3CHBrCOOH] 1+KK 2[OH −][CH 3CHBrCOOH]