15N nuclear polarisation in nitration and related reactions. Part 4. Durene and nitrodurene

Abstract

As expected from studies on mesitylene, the nitrous acid-catalysed nitration of durene by H¹⁵NO₃ in aqueous trifluoroacetic acid gives a strong emission signal during reaction for nitrodurene in the ¹⁵N n.m.r. spectrum. However, unlike the corresponding reactions of mesitylene, nuclear polarisation is also found when the nitration of durene under these conditions is carried out by nitronium ions: the ¹⁵N n.m.r. signal for nitrodurene during reaction then shows enhanced absorption. The nitration of nitrodurene does not give rise to nuclear polarisation when carried out separately but an emission signal is seen for dinitrodurene in the ¹⁵N n.m.r. spectrum during the nitration of durene by nitronium ions. All these results can be understood in terms of nuclear polarisation deriving from the partition of the radical pair [ArH ⁺˙NO₂˙] between dissociation and recombination. In the nitration of durene by nitronium ions, this radical pair is considered to result from ipso-attack followed by homolysis of the C–N bond.