There are various methods reported in the literature for the synthesis of benzimidazole from o-phenylenediamine and benzoic acid. This reaction is usually reported to undergo in acidic conditions (mineral or Lewis acids; Beilstein J. Org. Chem. 2012, 8, 259). However, due to the considerable interest in developing greener protocols Paliakoff et. al. have reported this reaction in water. (Green Chem. 2003, 5, 187).

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thank you so much sir.. Your information helps a lot.

With bezioc acid .... there really a lot! But reaction with salicilic acid has some problems. The newest procedure to obtain your target compound in reaction of acid and diamine is in poliphosporic acid to obtain about 40% yield (European Journal of Medicinal Chemistry, 2014 , vol. 84, p. 698 - 707). But it is better to use saliylic aldehyde and iodine to have 90% yield (Tetrahedron Letters, 2013 , vol. 54, # 40 p. 5505 - 5509)