Kinetics and mechanism of the oxidation of α-hydroxy acids by tetrabutylammonium tribromide

Abstract

The oxidation of glycollic, lactic, malic and a few substituted mandelic acids by tetrabutylammonium tribromide (TBATB) in 1: 1 (v/v) acetic acid-water leads to the formation of corresponding oxoacids. The reaction is first order each in

TBATB, and the hydroxy acid. Addition of tetrabutylammonium chloride does not affect the rate. Tribromide ion has been proposed as the reactive oxidizing species. The oxidation of α-deuteriomandelic acid shows the presence of a primary kinetic isotope effect (<i>k</i>_H/<i>k</i>_D<i> </i>= 5.50 at 303 K). The reaction does not

exhibit any solvent isotope effect [<i>k</i> (H₂O)/<i>k</i>(D₂O) = 1.01]. The rate decreases with an increase in the amount of acetic acid in the solvent mixture. A mechanism involving hydride ion transfer to the oxidant is proposed.