A synthesis of chloroanhydrides of phenyl-, alkylsubstituted dodecacarboxylic acids has been performed and their electrophilic reactions with o-phenylenediamine explored. It is established that during the reaction the cyclic compound, C-alkyl derivatives of benzimidazole were produced with high yield. It as a result of formation of the product determined that mono-carboxyamide compounds transformed into enol form intramolecular cyclization, C-alkyl derivatives.  Due to this mono-carboxyamide compounds are obtained with low yield