Mechanism of the oxidation of N-benzylanilines to benzylideneanilines by halogen or hypohalite in alkaline methanol

Abstract

The mechanism of the oxidation of N-benzylanilines to benzylideneanilines with iodine, chlorine, bromine, or t-butyl hypochlorite in alkaline methanol has been studied by examination of the kinetics and by isolation of intermediates. A kinetic study is possible only with iodine because of the rapid consumption of chlorine, bromine, and t-butyl hypochlorite in organic solvents. The rate is changed by the amount of added iodine, i.e., v=k[PhCH₂NHPh][MeOI] for a large excess of alkali over iodine and v=k′[PhCH₂NHPh]([MeO^–]_t/[I₂]_t– 1) for a small excess of alkali over iodine, where subscript t refers to the total concentration of reagent. The effect of ring substituents on the rate at 25° in the presence of a large excess of alkali gives a ρ value of –1·15 for substituents on the aniline ring and +0·38 for substituents on the benzyl ring. N-Benzylaniline reacts with molecular chlorine or bromine in the absence of a base to give N-benzyl-N-halogenoanilinium halide, which gives N-benzyl-o- and -p-halogenoanilines but no benzylideneaniline upon treatment with alkaline methanol. A mechanism is postulated, which involves a rate-determining attack of hypohalite on both the amine nitrogen atom and a benzyl proton via a cyclic transition state, giving a zwitterion; this zwitterion rearranges to N-α-halogenobenzylaniline, which is dehydrohalogenated to benzylideneaniline.