Nuclear magnetic resonance spectra of [15N]acetamide in sulphuric and fluorosulphuric acid. The question of the cation tautomeric equilibrium

Abstract

N.m.r. spectra of [¹⁵N]acetamide in 100% sulphuric acid and in fluorosulphuric acid are reported over a range of temperatures, as well as the effect of acid concentration in >75% sulphuric acid on the spectra of acetamide in these media. The low activation energies for exchange of the two NH sites are accounted for in terms of a tautomerization of the O-protonated cation into the N-protonated cation, which is present in negligible concentration in anhydrous acids. Its concentration increases upon dilution of sulphuric acid with water and exchange of NH protons with the solvent becomes rapid in 75% sulphuric acid. The O-protonated cation of acetamide is also not observable in 72% perchloric acid. The implications of the tautomeric equilibrium for the mechanism of acid hydrolysis of amides are discussed.