Kinetics and mechanism of the oxidation of substituted benzylamines by N-chlorosuccinimide

Abstract

The oxidation of ortho-, meta-, and para-substituted benzylamines by N-chlorosuccinimide (NCS), to the corresponding benzaldehydes, is first-order with respect to NCS and the amine. The pH dependence of the reaction rate suggests that the unprotonated benzylamine is the reductant. There is no effect of added succinimide. NCS itself has been postulated as the reactive oxidising species. The oxidation of [α,α-²H₂]benzylamine exhibited a substantial primary kinetic isotope effect (k_H/k_D= 6.20). The rates of oxidation of the meta- and para-substituted benzylamines were separately correlated in Taft and Swain's dual substituent parameter equations. For the para-substituted compounds, the best correlation is obtained with σ_I and σ_R⁺ values; meta-substituted compounds correlate with σ_I and σ_R⁰ values. The reaction constants have negative values. The oxidation rates of the ortho-substituted compounds yield an excellent correlation in a triparametric equation involving Taft's σ_I and σ_R⁺ values and Charton's steric parameter, V. A mechanism involving transfer of a hydride ion from the amine to the oxidant in the rate-determining step is proposed.