ipso-Attack in the nitration of aromatic amines. Part 3. Evidence for two mechanisms of ipso-attack

Abstract

Studies on the nitration of NN-dimethyl-4-methylaniline (1a) have been extended to NN-dimethyl-3,4-dimethylaniline (1b) and NN-dimethyl-3,4,5-trimethylaniline (1c). All three amines react with nitric acid in 60–70% sulphuric acid at 0 °C by the nitrous acid-catalysed mechanism to give mainly the ipso-intermediate with an NO₂ group at the 4-position (identified by the ¹H and ¹³C n.m.r. Spectra). Over the range 76–83% sulphuric acid the overall rates of reaction of the first two amines (and probably of the third) increase rapidly because of the attack of the nitronium ion on the conjugate acids of the amines. In these nitronium reactions, the amine (1a) gives no detectable amount of ipso-attack; the amine (1b) gives 16–51%ipso-attack at the 4-position (depending on the acidity), and the amine (1c) gives 85%ipso-attack at the 4-position. These results accord approximately with the predictions of the additivity principle. The rate of rearrangement of the ipso-intermediate from the amine (1b) to yield the 6-nitroproduct has been determined over the range 64–78% H₂SO₄.