15N nuclear polarisation in nitration and related reactions. Part 1. Mesitylene

Abstract

The nitration of mesitylene by nitric acid in trifluoroacetic acid has been followed by ¹H and ¹⁵N n.m.r. spectroscopy using H¹⁵NO₃(95–99%¹⁵N). With concentrations of mesitylene > 0.5 mol dm ^–3, the direct nitration by the nitronium ion is accompanied by an autocatalytic reaction involving nitrous acid. The nitrous acid-catalysed process is inhibited by azide ions and can be made to predominate by the addition of nitrous acid. Nitration by the nitronium ion gives no nuclear polarisation in the product but the nitrous acid-catalysed reaction gives marked ¹⁵N nuclear polarisation: the ¹⁵N n.m.r. spectrum of nitromesitylene during reaction then appears in emission with a degree of enhancement that can exceed a factor of 200. This nuclear polarisation is attributed to the formation of a radical pair [graphic ommitted] by the diffusion together of the components. The side-products formed in the nitrous acid-catalysed process are consistent with reactions through ArH⁺˙.The direct nitration of mesitylene by the nitronium ion does not appear to involve this cation radical.