Five substituted anilides of general formula R-CO-NH were studied in acidic media (H,SO,--MeOH and HSOJCI-Me,CO). The unsolved problem of the protonation site (0-protonation and N-protonation) is discussed in terms of the observed N M R shifts [(6NH) and 6(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 NO, 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 %.Resume-En etudiant par R M N cinq anilides de formule generale R-CO-NH + J R , en milieu acide, les auteurs ont misen evidence la protonation de I'atome d'azote de I'ion amidonium forme.

In order to describe very accurately the reaction scheme of the acidic hydrolysis of amides (B) it is necessary to determine the value of the acidity constant (pKBH+) of the conjugated amidonium ion. Connected with previous investigations in this area, following groups were examined along this line : a first one, R— CO— NH— C6H5— Y involving two different series (R = CH3— and C2H5— ) and for each one, Y = CH30-, CHS-, H— , Cl-, CH3CO-, N02-. A second one pertains to X— C6H4— CO— NH— C6H4— Y and is developed along five different series varying X (X = CH30— , CH3— , H— , Cl—, N02— ) and Y (Y = CH30-, CH3-, H-, Cl-, N02-). Acidity functions, H0 and HA were used for that purpose and new values of were determined for anilides.

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