A 1H nuclear magnetic resonance study of the protonation of methacrylamide and of the tautomerism of its cation

Abstract

Chemical shifts of the methyl and the vinyl protons of methacrylamide in sulphuric acid–water mixtures have been measured. The resonance of the methyl protons shifts downfield by 0.07₅ p.p.m. up to 60% acid and then remains virtually unchanged up to 100% acid. The resonances of the vinyl protons show large downfield shifts (the cis-proton by 0.55 p.p.m. and the trans-proton by 0.52 p.p.m.) up to 60% acid and then a substantial further shift to low field of 0.08 p.p.m. between 60% and 100% acid. This indicates a redistribution of charge in the cation, consequent upon the conversion of the N-protonated cation into the O-protonated cation. Half-conversion occurs in 78.5 ± 1.5% acid.