DETERMINATION OF THERMODYNAMIC AQUEOUS ACID-BASE STABILITY-CONSTANTS FOR SEVERAL BENZIMIDAZOLE DERIVATIVES

In conjunction with a study of compounds for laboratory instruction in nonaqueous quantitative analysis, we became interested in using a series of substituted benzimidazoles as weak bases for titration by perchloric acid in glacial acetic acid. For this purpose the acid-base stability constants of several commercially available derivatives were needed. Acid dissociation constants for several protonated benzimidazoles have been reported, some measured in water and others in 50 by volume aqueous ethanol, by Davies and co-workers (1), but the data were incomplete. We report here the determination of thermodynamic pK(b) values in water for benzimidazole and its 2-phenyl, 5-nitro, 2-methyl, and 5,6-dimethyl derivatives.