Benzimidazoles annular tautomerism

Benzimidazole, Ka = 5.68, is less basic than imidazole, but with i Ka = 12.75 is more strongly NH-acidic [121]. Like imidazoles, benzimidazoles display annular tautomerism in solution, e.g. ... 

Electrophilic attack in imidazole is usually most facile at an annular nitrogen, and there are many examples of Af-aUcylation, -protonation, -acylation, cyanation, -arylation and -silylation. A-Nitration is much less common A-oxidation is virtually non-existent. When an annular nitrogen becomes substituted, tautomerism in the molecules is blocked, and mixtures of isomers are usually formed with substituted benzimidazoles and 4(5)-substituled imidazoles. 

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