ipso-Attack in the nitration of aromatic amines. Part 1. The reactions of NN-dimethyl-p-toluidine and some related compounds
NN-Dimethyl-p-toluidine, NN-dimethyl-4-ethylaniline and NN-dimethyl-2,4,6-trimethylaniline react with nitric acid in aqueous sulphuric acid (ca. 70%) to form the ipso-Wheland intermediates with an NO2 group at the 4-position (identified by 1H and 13C n.m.r. spectroscopy). These reactions cannot derive from direct attack by nitronium ions since they require the presence of traces of nitrous acid. The first example gives rise to an e.s.r. spectrum identified as that of the cation radical 4-MeC6H4NMe2+˙. Where the ortho-positions are unsubstituted, the formation of the ipso-Wheland intermediate is followed by a 1,3-rearrangement to give the 2-nitro-product. The rate-determining step of this reaction appears to be the proton loss from the 2-position for there is a large isotope effect (kH/kDca. 5) and the rate of the rearrangement decreases with an increase in the acidity of the medium. The 1,3-rearrangement of the nitro-group appears to be mainly intramolecular but studies of isotopic exchange in concurrent rearrangements show that there is also an intermolecular component and this is considered to involve the cation radical (ArNMe2+˙) and nitrogen dioxide. Other aromatic amines present in the solution during the rearrangement do not undergo nitration.