Ultraviolet and nuclear magnetic resonance spectra of protonated benzamide and [15N]benzamide in concentrated sulphuric acid and pure fluorosulphuric acid

Abstract

U.v. spectra of benzamide in concentrated sulphuric acid, which show a tautomeric change from N-protonated in 60% sulphuric acid to O-protonated in 100% sulphuric acid, are discussed and the estimated tautomeric ratios are shown to lead to reasonable activity coefficient ratios of the two types of cation. N.m.r. spectra of benzamide and [¹⁵N]benzamide in 100% sulphuric acid and in pure fluorosulphuric acid are reported. The effect of diluting sulphuric acid with water on the spectra is consistent with a conversion of the O-protonated cation into the N-protonated cation, although under conditions of rapid proton exchange n.m.r. spectra do not provide information on the position of the tautomeric equilibrium.