15N nuclear polarisation in nitration and related reactions. Part 6. A comparison of 15N and 13C CIDNP effects in the nitration of phenols

Abstract

In the nitration of 4-methylphenol in acetic anhydride, the products from nitration at the 2-position and ipso-attack at the 4-position show strong emission signals in the ¹⁵N NMR spectrum during reaction. In the same reaction, the ¹³C NMR spectrum shows emission and enhanced absorption signals for the substrate and both products. The nitration of 2-methyl-6-nitrophenol under similar conditions shows similar ¹⁵N nuclear polarisation and similar ¹³C nuclear polarisation in the substrate but the polarisation in the product is complicated by the fast rearrangement of the ipso-intermediate. The pattern of nuclear polarisation in these substrates identifies the phenoxy radical as one component of the radical pair generating the polarisation and accords with the accepted mechanism for nitrous acid catalysed nitration.