Benzimidazole exhibits a relative weakness as a base but a greater strength as an N–H acid compared to imidazole. These patterns are replicated in other benzo-azoles as well, where the bicyclic systems display diminished basicity when compared to their corresponding monocyclic heterocycles.

Reacion-1:

The sole documented C- substitutions within the heterocyclic rings of any benzo-azole involve the 2-bromination of benzimidazole using N-bromosuccinimide.

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Reacion-2:

The reaction of benzimidazole-2-thiol with bromine yields 2-Bromobenzimidazole.

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Reacion-3:

Benzothiazoles and benzimidazoles, when lithiated at the hetero-ring 2-position with nitrogen blocked or protected, can undergo subsequent reactions with the typical variety of electrophiles.

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References:

[1]Wright J B. The chemistry of the benzimidazoles[J]. Chemical reviews, 1951, 48(3): 397-541.

[2]Alamgir M, Black D S C, Kumar N. Synthesis, reactivity and biological activity of benzimidazoles[J]. Bioactive heterocycles III, 2007: 87-118.

[3]Preston P N. Synthesis, reactions, and spectroscopic properties of benzimidazoles[J]. Chemical Reviews, 1974, 74(3): 279-314.