15N nuclear polarisation in nitration and related reactions. Part 2. p-Nitrophenol

Abstract

The nitrous acid-catalysed nitration of p-nitrophenol by H¹⁵NO₃ in aqueous trifluoroacetic acid gives a strong emission signal during reaction for the 2-¹⁵NO₂ group in the ¹⁵N n.m.r. spectrum of the 2,4-dinitrophenol formed: the enhancement is by a factor of up to 600. The reaction is accompanied by 11% migration of the original 4-nitro group to the 2-position and substitution by the labelled nitro group at the 4-position. The ¹⁵N n.m.r. spectrum during the related reaction using ¹⁵N labelled p-nitrophenol and unlabelled nitric acid shows that the migration of the 4-¹⁵NO₂ group to the 2-position causes a strongly enhanced absorption signal for the 2-¹⁵NO₂ group in the 2,4-dinitrophenol formed. A strongly enhanced absorption signal for this group is also found for the reaction of p-nitrophenol with H¹⁵NO₃ in the presence of sodium azide. The complete set of results is interpreted in terms of nuclear polarisation deriving from the reactions of the radical pair [graphic ommitted] when formed either by diffusion or from a singlet precursor. The reaction between hydrazoic acid and nitric acid is catalysed by p-nitrophenol.