[en] Highlights: • We investigate formanilide and its mono-substituted derivatives. • The reaction enthalpies of the protonation of amidic nitrogen are discussed. • The dependence of pK_BH+ constants on the proton affinities is linear. • The experimental rate constants are correlated with the proton affinities. • Amide group protonation may be rate determining step of acid catalyzed hydrolysis. Various para, meta and ortho substituted formanilides have been theoretically studied. For trans and cis-isomers of non-substituted formanilide, the calculated B3LYP vibration normal modes were analyzed. Substituent effect on the selected normal modes was described and the comparison with the available experimental data is presented. The calculated B3LYP proton affinities were correlated with Hammett constants, Fujita-Nishioka equation and the rate constants of the hydrolysis in 1 M HCl. Found linear dependences allow predictions of dissociation constants (pK_BH⁺) and hydrolysis rate constants. Obtained results indicate that protonation of amide group may represent the rate determining step of acid catalyzed hydrolysis.