Kinetics and mechanisms of oxidations by metal ions. Part VI.1 Oxidation of α-hydroxy acids by cerium(IV) in aqueous nitric acid

Abstract

The kinetics of oxidation of glycolic, malic, tartaric, and citric acids by cerium(IV) ammonium nitrate were investigated in 0.006 mol dm-3 nitric acid. The reaction was catalysed by H+ in the range 0.006-0.016 mol dm-3 at constant [NO-3] (0.02 mol dm-3). The pseudo first-order rate constant kobs was independent of [Ce111] (0.0004-0.002 mol dm-3). The proposed mechanism is based on the assumption that the formation of the precursor Ce(IV) - α-hydroxy acid complex precedes its rate controlling disproportionation, which is assisted by a proton, possibly due to the formation of the activatedstate [H+ - CeIV - HA]† (where CeIV is the reactive cerium(IV) species and HA is the α-hydroxy acid). The free radical R1R2C0H produced in the rate controlling step further reacts with a number of Ce(IV) molecules in the fast step to yield the final oxidation product. The activation parameters for the rate controlling step could be evaluated only in the oxidation of tartaric acid.