The chemistry of nitroso-compounds. Part IX. General acid catalysed decomposition of N-nitroso-2-pyrrolidone, an example of amide hydrolysis via SN2 displacement on the N-conjugate acid

Abstract

Decomposition of N-nitroso-2-pyrrolidone in mildly acidic aqueous solutions proceeds by concurrent pathways resulting in either denitrosation or deamination. The acid-catalysed component of both pathways is shown to involve a common rate limiting step, which is an unusually slow protonation of the amino nitrogen atom: the N-conjugate acid then rapidly partitions via the two decomposition pathways to products. In aqueous buffer solutions of CF₃CO₂H, Me₃[graphic omitted]CH₂CO₂H, Cl₂CHCO₂H, and ClCH₂CO₂H Brønsted general acid catalysis is observed and the results lead to a common Bronsted plot for both deamination and denitrosation with α= 0·64. With high concentrations of the buffer components, the rates reach a limiting value for both decomposition pathways, which shows that neither deamination nor denitrosation of the N-conjugate acid intermediate requires an additional molecule of base. For denitrosation, decomposition of the N-conjugate acid is a unimolecular process; for deamination, however, decomposition is bimolecular involving the solvent water, which is well able to compete with the weakly basic buffer anions. The results provide the first authenticated example of an amide hydrolysis reaction proceeding via an N-conjugate acid species. Reasons for the preference of this pathway (instead of via the more usual O-conjugate acid) are discussed in terms of the cyclic amide structure rather than the presence of an N-nitroso-group. Further, the results show that N-nitroso-amides may undergo denitrosation as well as deamination under mild conditions which has an important bearing on carcinogenesis by these compounds.