Nitration of dimethoxybenzenes, mesitylene and toluene with nitric acid and mixed acid studied by 15N-CIDNP

Abstract

During nitration of 1,2- and 1,4-dimethoxybenzene 1,2 with ¹⁵N-labelled nitric acid in acetic acid, acetonitrile and CCl₄ at room temperature, the ¹⁵N NMR signals of the nitration products 5, 6 exhibit emission which is built up in radical pairs formed by encounters of independently generated radical cations l˙⁺, 2˙⁺ and NO₂. It is proved by a quantitative analysis of the ¹⁵N-CIDNP effect during nitration of 1 that the radical pathway is the main reaction, which is a nitrous acid catalysed nitration first proposed by Ridd. Mesitylene 3 and toluene 4 are not nitrated under these conditions. During nitration of 3 with mixed acid in acetic acid, a weak ¹⁵N-CIDNP effect is observed in nitromesitylene 7. ¹⁵N-CIDNP is not found with 4 in the mtrotoluenes 8–10, but in the side products 12 and 13. The nitration of 3 with mixed acid is described to more than 95% and of 4 exclusively by Ingold's classical mechanism.