Kinetics and mechanism of the Fischer–Hepp rearrangement and denitrosation. Part 9. Ring-methyl substituent effects

Abstract

Rate constants have been obtained for both denitrosation and rearrangement reactions of the 2- and 3-methyl- and 2,6- and 3,5-dimethyl-substituted N-methyl-N-nitrosoanilines. The very large rate reduction (ca. 10³) observed for the 2,6-compound (compared with the unsubstituted nitroso-amine) for both reactions, is attributable to steric hindrance towards protonation of the amino-nitrogen atom. Otherwise methyl substitution in the ring activates the system to denitrosation, by relatively small amounts in hydrochloric acid (and also for the reactions with added bromide ion), but by greater amounts in the sulphuric acid reactions; these results are taken to support an earlier suggestion, that at high acidities, and in the absence of added nucleophiles, another mechanism of denitrosation becomes important, which involves attack by H₃O⁺ at the protonated amino-nitrogen atom, with the concurrent expulsion of NO⁺. Both 2- and 3-methyl substitution also increases the reactivity of the nitroso-amines towards rearrangement (Fischer–Hepp), with the 3-methyl group having, as expected the greater effect. Unexpectedly, the 3,5-dimethyl-is significantly less reactive than the 3-methyl-nitroso-amine, suggesting that some steric factor comes into play in the intramolecular rearrangement process.