Application of Linear Free Energy Relationships to Log \(K_{\text{aH}^{+}}\) Data of Benzimidazolium Cations: Chemical Education Perspective

In this study, the application of linear free energy relationships (LFER) to log \(K_{\text{aH}^{+}}\) $$K_{{\text{aH}}^{+}}$$ data in water at 25°C of deprotonation of protonated fused ring systems like benzimidazolium cations is carried out. With a good comparison of the sites of substituents with reference to a functional group in the benzene ring and the imidazolium ring, an excellent Hammett correlation is observed for the deprotonation of (log \(K_{\text{aH}^{+}}\) $$K_{{\text{aH}}^{+}}$$ ) of protonated fused ring systems like benzimidazolium cations. The three substituents OH, MeO, and Me at position 4 in benzimidazole satisfy the correlation with σ_I values. A positive Hammet ρ value of 1.93 indicates that electron-withdrawing substituents facilitate the deprotonation. Under the same conditions, a Taft ρ* value of 1.11 is obtained for the deprotonation of 2-substituted benzimidazolium cations. The available log \(K_{\text{aH}^{+}}\) $$K_{{\text{aH}}^{+}}$$ data in 5% aqueous ethanol at 30° C of 2-methyl benzimidazolium cations and 2-(hydroxyethyl) benzimidazolium cations also follow Hammett correlation. The lower Hammett ρ value of 0.89 for 2-(hydroxyethyl) benzimidazolium cation series than that of 1.78 for 2-methyl benzimidazolium cation series is explained in terms of strong intramolecular hydrogen bonding in 2-(hydroxyethyl) benzimidazolium cations, which resists easy deprotonation. Deprotonation of 1-substituted benzimidazolium cations does not follow the Hammett relationship, for which possible explanations are given.