The tautomerism of heteroaromatic compounds with five-membered rings. Part XII. The imidazole 3-oxide versus 3-hydroxy[3H]imidazole equilibrium

Spectroscopic and pKa value comparisons with model compounds indicate that benzimidazole 3-oxide exists predominantly in the NH-form in aqueous solution, but predominantly in the OH-form in non-polar solvents. For 2,4,5-triphenylimidazole 3-oxide, steric hindrance in the N-methyl model complicates matters, but this compound appears to exist as comparable amounts of OH- and NH-forms in aqueous solution and in the OH-form in non-polar solvents. The structures of these compounds in the solid state are discussed.