15N nuclear polarisation in nitration and related reactions. Part 3. Reactions initiated by ipso-attack in the nitration of amines
The reactions of N,N,4-trimethylaniline (1a), N,N,2,4,6-pentamethylaniline (1b), and N,N,3,4,-tetramethylaniline (1c) with nitric acid in aqueous sulphuric acid have been followed by 15N n.m.r. spectroscopy using 15N-labelled compounds. When the amines react with H15NO3 by the nitrous acid-catalysed pathway of ipso-attack, a strong emission signal is seen at the start of reaction for the 15NO2 group in the ipso-intermediates formed. When the same reaction is carried out using the 15N-labelled amines (1a, b) the signal for the amino group in the 15N n.m.r. spectrum of the ipso-intermediates shows enhanced absorption. The ipso-intermediates from the amines (1a, b) undergo a nitro-group exchange reaction with nitric acid in the solvent and this can lead to either emission or enhanced absorption signals for the 15NO2 group in the ipso-intermediate, depending on the direction of the exchange. The 1,3-rearrangement of the nitro group in the ipso-intermediate from the amine (1a) can also lead to either an emission or an enhanced absorption signal for the 15NO2 group in the N,N,4-trimethyl-2-nitroaniline formed, depending on the conditions of reaction. The phase of the nuclear polarisation in the amino and nitro groups from ipso-attack, exchange, and rearrangement reactions is shown to be consistent with the partition of the radical pair [Ar[graphic omitted]Me2˙NO2˙] between dissociation and combination. The e.s.r. spectrum of the radical cation of the amine (1b) has been observed in solutions of the corresponding ipso-intermediate in aqueous sulphuric acid in the presence of hydrazine.