Electrophilic aromatic substitution. Part 26. The kinetics and mechanism of nitration of some reactive benzene derivatives in aqueous phosphoric acid. The identification of steps rate-limited by nitronium ion formation and by diffusion

Abstract

The rates of nitration of benzene and some of its activated derivatives in 83–98% H₃PO₄ at 25 °C have been measured. The rate coefficients show a steep dependence on acidity. With phenol and anisole in excess over nitric acid a step zeroth-order in the concentration of the aromatic has been recognised. This is identified as the step in which the nitronium ion is formed. The nitrations can also be rate-limited by a step which is first-order in the concentration of the aromatic compound, but independent of the nature of the aromatic. This is identified as the conventional diffusion process, and its nature confirmed by the inverse dependence of the rate coefficient upon the viscosity of the medium, and by the magnitude of the half-life of the nitronium ion. The mechanism of nitration is thus wholly analogous to that for nitration in sulphuric acid. The nitration of benzene is diffusion-controlled in >95% H₃PO₄.