Potentiometric Titrations of Some 2-Substituted 5-Nitrobenzimidazole Derivatives in Nonaqueous Solvent

Abstract

The acidity constants of 16 2-substituted 5-nitrobenzimidazole derivatives were determined in dimethyl sulfoxide, 60% (v/v) isopropyl alcohol +40% (v/v) water, and 60% (v/v) dimethyl sulfoxide +40% (v/v) water by the Potentiometric method. Tetrabutylammonium hydroxide was used as the titrant for the measurements done in dimethyl sulfoxide as a solvent, and aqueous KOH was used for the measurements done in mixtures. The pKa values were obtained from the half-neutralization potentials which were obtained from an analysis of the Potentiometrie titrations. The order of the acidic strengths obtained depends on the alkyl, aryl, and heterocyclic substituents on 5-nitrobenzimidazole. The acidic strengths of the alkyl-substituted 5-nitrobenzimidazoles in each solvent were in the following order: 2-(trifluoromethyl) > 2-(difluorochloromethyl) > 2-(chloromethyl) > 3-nitrobenzimidazole > 2-methyl > 2-(hy-droxymethyl)-5-nitrobenzimidazole. The acidic strengths of the aryl-substituted 5-nitrobenzimidazoles in each solvent were in the following order: 2-(4′-chlorophenyl) > 2-(2′-chlorophenyl) > 2-(3′-chlorophenyl) > 2-(2′-hydroxyphenyl) > 2-phenyl > 5-nitrobenzimidazole > 2-(4′-methoxyphenyl) > 5-nitrobenzimidazole. The increasing acidic strengths of the heterocyclic-group-substituted 5-nitrobenzimidazoles for three solvent systems were in the order 2-(2′-thienyl) > 2-(2′-pyridyl) > 2-(3′-pyridyl) > 2-(2′-furyl) > 5-nitrobenzimidazole. © 1995, American Chemical Society. All rights reserved.