Question

Why is aniline less basic than ethylamine ?

Hint:

Key phrase: "Delocalization of lone pair." Whenever an amine's N is attached to a benzene ring, basicity drops drastically.

Answer 1

1. Resonance Effect: In aniline (C$_6$H$_5$NH$_2$), the lone pair of electrons on the nitrogen atom is in conjugation with the benzene ring. It participates in resonance, delocalizing the negative charge over the ortho and para positions of the ring. This makes the lone pair less available for donation to an acid.
2. Inductive Effect: The phenyl group exerts a -I (electron withdrawing) effect, further reducing electron density on nitrogen. In contrast, the ethyl group in ethylamine has a +I (electron releasing) effect, increasing electron density on nitrogen.
3. Stability of Conjugate Acid: The anilinium ion formed by protonation is less stable because resonance is disrupted (to an extent), whereas the ethylammonium ion is stabilized by the +I effect of the ethyl group.