Basicity: Aliphatic Vs Aromatic

Question

Aniline is a markedly weaker base than cyclohexylamine in aqueous solution, and the most fundamental reason for this large difference is best described as which effect?

RankGuru · Accepted Answer

The basicity of an amine is determined by how readily the nitrogen lone pair can accept a proton. In cyclohexylamine the lone pair is fully localised on nitrogen and freely available, making it a strong aliphatic base. In aniline the nitrogen lone pair conjugates with the π system of the benzene ring through resonance, spreading electron density into the ring and substantially lowering its availability for protonation. This resonance stabilisation of the free base, combined with the loss of that stabilisation upon protonation, makes aniline a far weaker base. The option citing molecular mass is irrelevant because basicity is an electronic property, not a mass property. Hydrogen bonding with water affects solubility, not the intrinsic ability to donate the lone pair, so it cannot explain the difference. Boiling point is a physical property unrelated to proton affinity. This resonance argument is the central NCERT explanation for the weak basicity of aromatic amines. As a sanity check, any electron-withdrawing group on the ring further lowers basicity while electron-donating groups raise it, consistent with a resonance-controlled lone pair.

Basicity: Aliphatic Vs Aromatic

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