The nitrous acid-catalysed nitration of phenol

Abstract

The reaction of phenol with nitrous acid (10^–4M < [N^III] < 10^–2M) and nitric acid (10^–3M < [N^V] < 0.3M) in aqueous sulphuric acid (19–45% H₂SO₄) at 25 °C gives rise to p-nitrophenol by a nitrosation–oxidation pathway, and concurrently to a 57 : 43 mixture of o- and p-nitrophenol by catalysed nitration. The latter reaction is the major one, and is first-order in phenol with a first-order rate coefficient given by [N^III][N^V]/(x[N^III]+y[N^V]). The x and y are constants for a given concentration of sulphuric acid. The dependence of x and y upon acidity, and comparison of the reactivity of phenol with that of hexadeuteriophenol and anisole, leads to a proposed mechanism for catalysed nitration. In this there is pre-equilibrium formation, from phenol and N^III, of a intermediate with the formula PhONO. This gives rise to a phenoxyl radical and nitric oxide, a step which is rate limiting when the rate is fully enhanced by [N^V]. Nitric oxide is reversibly oxidised by N^V to give NO₂ and N^III. Reaction is completed by combination of the phenoxyl radical and NO₂, in a step which is rate limiting when the rate is fully enhanced by [N^III].