LINEAR FREE-ENERGY RELATIONS (LFER) FOR AMPHOTERIC 5-(OR 6-) SUBSTITUTED BENZIMIDAZOLES

Aqueous thermodynamic pKa values at 25° have been obtained spectrophotometrieally for a series (Br, CH3, CF3, Cl, F, H, NH2, and NO2) of 5- (or 6-) substituted benzimidazoles. ΔpKa (acidity) values are excellently correlated with corresponding ΔpK+ (basicity) values with ΔpK+ = (1.16 ± 0.05)ΔpKa - 0.02 (±0.04) with standard error in ΔpK+ of 0.09 and correlation coefficient 0.995. ΔpK+ and ΔpKa give good correlations with Taft's σI and σR° and Lupton and Swain's F and R. The relative importance of resonance and nonresonance effects is near the same for ΔpK+ and ΔpKa. The composite substituent constant (σI + 0.8 σR°) or (F + R) lies midway between σm and σp° in the importance of resonance. Both the ΔpKa and ΔpK+ series show a sensitivity to the polar (i.e., nonresonance) effects of substituents that is characteristic of benzene derivatives undergoing a change in formal charge of one on the first atom of a side-chain reaction site. Hammett indicator plots of log [(anion)/(molecule)] vs. log COH give slopes of unity for the title compounds. © 1969, American Chemical Society. All rights reserved.