ipso-Attack in the nitration of aromatic amines. Part 4. Rate profiles and isotope effects for the rearrangements of ipso-intermediates

Abstract

The conversion of the ipso-intermediates (1) and (2) into the corresponding o-nitroamines in aqueous sulphuric acid at 0 °C has been followed by u.v. Spectrometry. The reactions are first-order and the form of the rate profile shows that either the 1,3-rearrangement of the nitro-group or the subsequent proton loss can be rate-determining depending on the acidity. This is confirmed by the isotope effect (k_H/k_D) which [for intermediate (1)] varies from 1.0 to 4.8 over the range 25.0–75.9% sulphuric acid. The rate of the 1,3-rearrangement is not very sensitive to the acidity but the rate of the subsequent proton loss varies with h_o^–0.7. A meta-methyl group slightly stabilises both the ipso-intermediate and the Wheland intermediate (3) relative to the transition state for the 1,3-rearrangement. With these ipso-intermediates, the rate of the rearrangement step is much faster than the corresponding step in the rearrangement of the cyclohexadienone intermediates derived from the nitration of phenols.