Heteroaromatic reactivity. Part VII. The kinetics, products, and mechanism of nitration of some 1-arylpyrazoles and 1-phenylimidazole

Abstract

The nitration in sulphuric acid of some 1-arylpyrazoles, and of 1-phenylimidazole, to give substitution in the carbocyclic rings is shown to involve the cations of the bases. Methyl groups at N-2 or C-5 in the 1-arylpyrazoles reduce reactivity in nitration by steric hindrance to the coplanarity of the aryl and heterocyclic group. Nitration of 1-arylpyrazoles in acetic anhydride generally gives poor yields of mononitro-compounds substituted at C-4 of the pyrazole ring. However, 5-methyl-1-phenylpyrazole gives both 5-methyl-3- and 5-methyl-4-nitro-1-phenylpyrazole. The poor yields prevent a definite assignment of mechanism; some possibilities are briefly discussed.