Kinetics and mechanism of the Fischer–Hepp rearrangement. Part II. Separation of rearrangement and denitrosation

Abstract

The composite observed first-order rate coefficient (k_obs) for the reaction of N-methyl-N-nitrosoaniline in hydrochloric acid was found to decrease with added N-methylaniline to a limiting value while the yield of rearrangement product increased accordingly. This limiting value (k₂) is taken to be the rate coefficient for the rearrangement process alone. The results accord with a previously suggested scheme involving concurrent rearrangement and denitrosation. There was a linear dependence of log k₂ upon H₀(slope –1·2) between 3M- and 6·5M-acid, above which the rate levelled off, and a ring deuterium isotope effect (k₂)_H : (k₂)_D of 2·4. Substituent (meta) effects confirmed the electrophilic nature of the rearrangement and supported the idea of concurrent rearrangement and denitrosation. A more detailed reaction mechanism is suggested for the rearrangement, involving the reversible formation of two intermediates, a σ-complex and probably a π-complex.