Kinetic substituent and isotope effects in the acid-catalysed rearrangement of N-phenylhydroxylamines. Are nitrenium lons involved?

Abstract

Acidity–rate profiles have been established for the rearrangement of N-phenylhydroxylamine and its derivatives in aqueous sulphuric acid and also in D₂SO₄–D₂O. The results, particularly the change in the magnitude of the kinetic solvent isotope effect on increasing the acidity, are consistent only with a reaction mechanism involving unimolecular decomposition of the O-protonated species. N-Ethyl substitution has only a very small effect on the overall rate of reaction, whereas the rate constant was increased by ca. 100 by 4-Me substitution. The substituent effects argue against the involvement of a nitrenium ion intermediate but rather suggest that the intermediate is better represented by an imine structure with the positive charge at the 4-position in the aromatic ring.