Nitrosation and nitrous acid-catalysed nitration of anisole and 2,6-dimethylanisole

Abstract

Rate–acidity profiles have been obtained for the nitrosation of anisole, 2,6-dimethylanisole (DMA), and 2,6-dimethylphenol (DMP) in aqueous sulphuric acid. The phenol is more reactive than the corresponding anisole, and DMA has a more shallow profile than anisole. A deuterium kinetic isotope effect (k_H/k_D) of 4.0 for [4-²H]anisole indicates that the product of nitrosation of anisole in 46.5% sulphuric acid (p-nitrosophenol) is formed by slow proton loss from the Wheland intermediate. Intense colours were associated with the nitrosation of these compounds when nitrous acid was in excess.

The kinetics of nitrous acid-catalysed nitration of anisole were studied in 43.0 and 47.0% sulphuric acid; the product is p-nitrophenol. Nitrosation followed by oxidation by N^V was the major pathway at these acidities. The other pathway has a kinetic form given by k_C=k₃[N^III][N^V], consistent with a process where oxidation by N^V is rate-limiting. Product studies show that p-nitrophenol is formed at lower acidities and o- and p-nitroanisole at higher acidities. A mechanism is suggested involving a radical cation species, which would predominate at higher acidities and account for the k_C pathway at lower acidities.

The nitrous acid-catalysed nitration of DMA gives 2,6-dimethyl-4-nitroanisole in higher yield as the acidity increases.