Substituent effects of positive poles in aromatic substitution. Part VI. The nitration of substituted anilinium ions through proton loss from the –NH3+ group

Abstract

The rate profile for the nitration of 2-chloro-4-nitroaniline in 85–100% sulphuric acid accords with a reaction of the free amine, and this interpretation is consistent with the rate of nitration relative to other aniline derivatives and with the solvent isotope effect for reaction in deuteriosulphuric acid. Similar evidence indicates that the rate-determining step in the nitration of p-nitroaniline to picramide involves the nitration of the p-nitroaniline molecule. When the concentrations of these amine molecules are calculated from the Hammett acidity function, the relevant second order rate coefficients for nitration are ∼10⁸ l. mole^–1 sec.^–1 in accord with reaction on encounter with nitronium ions. However, the activation energies are unexpectedly high for such encounter reactions; from this discrepancy and from related evidence with other substrates, it is suggested that the lifetime of the free amines may be too short to permit the formation of the free amine and the attack of the nitronium ion to be regarded as discrete steps.