Etude de la Protonation du Groupe Amide par Resonance Magnetique Nucleaire. Comportement des Anilides Substitues en Milieu Acide Concentre

Five substituted anilides of general formula were studied in acidic media (H₂SO₄? MeOH and HSO₃Cl? Me₂CO). The unsolved problem of the protonation site (O-protonation and N-protonation) is discussed in terms of the observed NMR shifts [(δNH) and δ(CO? H)] taking into account several factors including hydrogen bonding and solvent effects. New dramatic evidence is presented of N-protonation of the amide group; in the case of 2,4-dinitro formanilide, the NO₂ groups in the molecule enhance the relative basicity of the nitrogen atom in such a way that the amount of N-protonated amide can be detected. N-protonation is a function of the acidity, showing a maximum in the region of 40 to 50%.