Nitration in aqueous nitric acid: the rate profile and the limiting reaction rates

Abstract

Rate coefficients for the nitration of a series of quaternary ammonium ions have been used to establish a rate profile for reaction in 63.7–100% nitric acid at 25 °C and to estimate the concentration of nitronium ions in the aqueous media. The kinetics of nitration of a series of reactive aromatic compounds (mainly phenolic ethers) in aqueous nitric acid have been analysed in terms of a first-order rate coefficient and the zeroth-order rate of formation of the nitronium ion. The first-order rate coefficients approach a limiting value as the reactivity of the aromatic substrate is increased and this value is as expected for the rate-limiting formation of an encounter pair (ArH·NO₂⁺). The lifetime of nitronium ions in 60.4% nitric acid (t_½ca. 5 × 10^–8 s) has been calculated from the zeroth-order rate at 25 °C and used to show that the formation of the encounter pair (ArH·NO₂⁺) occurs by the diffusion together of the components, not by pre-association. The studies on the more reactive aromatic compounds were carried out in the presence of hydrazine since this was shown to prevent the nitrous-acid catalysed reactions previously observed.