Studies of nuclear magnetic resonance chemical shifts caused by protonation. Part II. Formamide and some N-alkyl and NN-dialkyl derivatives

Abstract

Chemical shifts caused by protonation of the formyl proton resonance and of the N–CH resonances of formamide and seven N-substituted formamides have been measured in sulphuric acid–water mixtures and their apparent pK values obtained. An account of the changes in the spectra caused by protonation is given, the outstanding observations being the loss of coupling between the formyl proton and the NH and NCH protons, which occurs at quite low degrees of protonation in the spectra of all the formamides, as well as the loss of cis–trans isomerism in the spectra of N-alkylamides. These observations are consistent with N-protonation of these amides. The apparent pK values are therefore discussed in these terms.