Aniline’s higher pKb value is attributed to the -NH₂ group being directly attached to the benzene ring, causing the unshared electron pair on nitrogen to be in conjugation with the benzene ring, making it less available for protonation.
Why does aniline exhibit a higher pKb value compared to ammonia, and what structural feature contributes to this difference in basicity?
Share
Aniline exhibits a higher pKb value compared to ammonia due to the resonance stabilization of its conjugate acid, anilinium ion (C₆H₅NH₃⁺). In aniline, the lone pair on nitrogen can delocalize into the aromatic ring through resonance, stabilizing the positive charge on nitrogen in the conjugate acid. This resonance stabilization is absent in ammonia, making anilinium ion less acidic. The presence of the aromatic ring in aniline, allowing for resonance, enhances the stability of the conjugate acid and contributes to its lower basicity, resulting in a higher pKb compared to ammonia.