Kinetics and Mechanism of the Oxidation of Some (alpha)-Hydroxy Acids by Benzyltrimethylammonium Dichloroiodate

The oxidation of lactic acid, mandelic acid, and ten monosubstituted mandelic acids by benzyltrimethylammonium dichloroiodate (BTMACI) in glacial acetic acid leads to the formation of the corresponding oxo acid. The reaction is first order with respect to hydroxy acid, BTMACI, and zinc chloride. An addition of benzyltrimethylammonium chloride enhances the rate slightly. [PhCH2Me3N]+[IZn2Cl6]- is postulated to be the reactive oxidizing species. The oxidation of (alpha)deuteriomandelic acid exhibited the presence of a substantial kinetic isotope effect (kH/kD = 5.97 at 298 K). The rate of oxidation of the substituted mandelic acids showed an excellent correlation with Brown’s (sigma)+ values. The reaction constants are negative. A mechanism involving the transfer of hydride ion to the oxidant is postulated.