Kinetics and mechanism of the Fischer–Hepp rearrangement and denitrosation. Part VIII. Substituent effects on denitrosation of aromatic N-nitroso-amines

Abstract

The kinetic effects of various N-, meta-, and para-substituents on the denitrosation of aromatic N-nitroso-amines have been measured, both for the halide ion (Cl–, Br–, and in some cases I–) catalysed reaction and also for reaction brought about by the solvent in sulphuric acid solution. The results are discussed in terms of the effect of substitution upon the basicity of the nitroso-amines, as well as the effect upon the rate-determining step of nucleophilic substitution at the nitroso-nitrogen atom. In the sulphuric acid reactions para-substituent effects are reversed when compared with the halide ion reactions: this is taken to support an earlier suggestion of the incursion of a second mechanism of denitrosation, particularly evident at high acidities, where reaction occurs between H3O+ and the protonated form of the nitroso-amine. Steric hindrance to attack by the nucleophile is evident in the halide ion reactions of N-t-butyl-N-nitrosoaniline as shown by the decreasing ratio of the rate constants for the N-methyl and N-t-butyl compounds along the series Cl–, Br–, and I–. Rate constants have also been determined for the rearrangement (to the para-nitroso-isomers) of the N-alkylated series (N-Me, Et, Pri, and But). The results can be explained in terms of the substituent effects on the initial protonation of the reactant, although steric effects are operative for the N-t-butyl compound, which was hitherto thought not to undergo the Fischer–Hepp rearrangement.