Mechanistic Study of the Oxidation of Substituted Benzylamines by N-Bromoacetamide

Abstract

Abstract The oxidation of ortho-, meta-, and para-substituted benzylamines by N-bromoacetamide (NBA), to the corresponding benzaldehydes, is first order with respect to NBA and the amine. The pH dependence of the reaction rate suggests that the unprotonated benzylamine is the reductant. There is no effect of added acetamide. NBA itself has been postulated as the reactive oxidizing species. The oxidation of benzylamine-α,α-d2 exhibited a substantial primary kinetic isotope effect (kH⁄kD=5.81). The rates of the oxidation of the meta- and para-substituted benzylamines were separately correlated in Taft’s and Swain’s dual substituent parameter equations. For the para-substituted compounds, the best correlation is obtained with σI and σR+ values, while meta-substituted compounds correlated best with σI and σR° values. The reaction constants have negative values. The oxidation rates of the ortho-substituted compounds yield excellent correlation in a triparametric equation involving Taft’s σI and σR+ values and Charton’s steric parameter, V. A mechanism involving transfer of a hydride ion from the amine to NBA, in the rate-determining step has been proposed.