Protonation of the Amide Group: I. The Basicities of Substituted Benzamides

The basicities of benzamide and of 11 meta- and para-substituted benzamides in sulphuric acid media have been determined by a spectrophotometric method. The [Formula: see text] values of the protonated benzamides were found to be proportional to the Hammett σ constants for the substituents, unlike the [Formula: see text] values of the protonated benzaldehydes, acetophenones, and benzoic acids, which are approximately proportional to σ + constants. The lack of conjugation between the protonated amide group and the ring, which is indicated by these results, is most easily interpreted in terms of N-protonation although O-protonation cannot be completely excluded. Various other aspects of the problem of N- versus O-protonation are also discussed.