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

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.