Kinetics and mechanism of the Fischer–Hepp rearrangement. Part I. Rearrangement of N-nitroso-N-methylaniline in hydrochloric acid

Abstract

Rate measurements and product analyses have been carried out for the rearrangement of N-nitroso-N-methylaniline in hydrochloric acid in the acid range 2–8M. The main products were p-nitroso-N-methylaniline and N-methylaniline in amounts which depended on the acidity. The reaction was first-order in the nitrosamine and a linear dependence (slope 1·18) of log (observed rate coefficient) upon –H₀ was found up to 6·5M-acid; thereafter the rate became almost independent of the acidity. A solvent isotope effect k_D2O : k_H2O of 2·1 was observed at 3·96M- acid and a ring deuterium isotope effect of 1·7 was found at 5·5 and 7·0M. Rearrangement took place even in the presence of a large excess of urea. These results do not accord with the commonly accepted mechanism involving C-nitrosation by nitrosyl chloride but rather suggest that rearrangement occurs intramolecularly by a unimolecular reaction of the protonated nitrosamine. Further it was found that the nitrosamine can transfer the NO group to a suitable acceptor without its becoming kinetically free. This accounts for early observations of cross-nitrosation products.