Kinetics and mechanism of the oxidation of some α-hydroxy acids by hexamethylenetetramine-bromine

Abstract

The oxidation of lactic acid, mandelic acid and ten monosubstituted mandelic acids by hexamethylenetetramine-bromine (HABR) in glacial acetic acid, leads to the formation of the corresponding oxoacid. The reaction is first order with respect to each of the hydroxy acids and HABR. It is proposed that HABR itself is the reactive oxidizing species. The oxidation of α-deuteriomandelic acid exhibits the presence of a substantial kinetic isotope effect (k H /k D = 5.91 at 298 K). The rates of oxidation of the substituted mandelic acids show excellent correlation with Brown’s σ+ values. The reaction constants are negative. The oxidation exhibits an extensive cross conjugation between the electron-donating substituent and the reaction centre in the transition state. A mechanism involving transfer of a hydride ion from the acid to the oxidant is postulated.