Electrophilic aromatic substitution. Part XI. The kinetics and mechanism of the nitration of reactive aromatic substrates in acetic anhydride
Abstract
A study of the kinetics of nitration of several aromatic substrates in acetic anhydride containing added nitric acid and added acetic acid is reported. The effects of added nitrates, of substitution of hydrogen by deuterium in both the aromatic substrate and in the added acetic acid, and of the concentrations of the latter and of the added nitric acid, on the rate constants for both zeroth-order and first-order reactions are reported. In the analysis of the kinetics, account is taken of the possible solvation of both the nitronium ion and the nitrate ion, and the most satisfactory mechanism which emerges is one in which the nitronium ion, formed from dinitrogen pentoxide and acetic acid in the step which is rate determining under zeroth-order conditions, reacts directly to give nitro-products if the concentration and reactivity of the aromatic are high enough, but undergoes solvation by a molecule of nitric acid prior to reaction with the aromatic substrate if the concentration of the latter is as low as that generally used in studies of the first-order reaction. This mechanism is consistent with the notion that both the nitronium ion and the species NO2+(HNO3) react at the encounter rate with aromatic substrates as reactive as mesitylene.