The nitration of the NN-dimethylanilinium ion. A new mechanism for catalysis by nitrous acid

Abstract

The reaction of the NN-dimethylanilinium ion with nitric acid in ca. 85% sulphuric acid is catalysed by nitrous acid but this catalysis does not occur through the formation and oxidation of the C-nitroso derivative. The initial products of the catalysed reaction are p-nitro-NN-dimethylaniline and NNN′N′-tetramethylbenzidine, the proportion of the former decreasing with decreasing acidity. The catalysed reaction is first order with respect to the NN-dimethylanilinium ion and the nitrosonium ion but zeroth order with respect to nitric acid for [HNO₃] > 0.15 mol dm^–3; this kinetic form and rate are the same as those for the nitrous acid catalysed mechanism for hydrogen isotope exchange at the nitrogen pole of the amine. These observations are interpreted as involving the rate-determining formation of the PhNMe₂⁺·NO˙ radical pair followed by the fast reactions of this species with nitronium ions and molecular nitric acid.