Arrange the following amines in decreasing order of \( pK_b \) values in aqueous solution:
(A) Phenylmethanamine
(B) N-Ethylethanamine
(C) N-methylaniline
(D) Benzenamine
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (D), (C), (A), (B)
The basic strength of an amine is measured by \( K_b \), or by \( \mathrm{p}K_b = -\log K_b \): the stronger the base, the smaller its pKb. So decreasing pKb means increasing basic strength. Two factors decide it here:
When nitrogen is attached directly to a benzene ring, its lone pair is delocalised into the ring by resonance and is less available for a proton; such aryl amines are much weaker bases than ammonia.
Alkyl groups push electrons towards nitrogen (+I effect), so aliphatic amines are stronger bases than ammonia.
(D) Benzenamine (aniline), \( \ce{C6H5NH2} \): lone pair delocalised into the ring; the weakest base.
(C) N-Methylaniline, \( \ce{C6H5NHCH3} \): still an aryl amine, but the methyl group adds a little electron density.
(A) Phenylmethanamine (benzylamine), \( \ce{C6H5CH2NH2} \): the \( \ce{CH2} \) group keeps nitrogen away from the ring, so there is no resonance and it behaves as an aliphatic amine.
(B) N-Ethylethanamine, \( \ce{(C2H5)2NH} \): two ethyl groups give a strong +I effect, and the ion formed is well solvated; the strongest base.
Shortcut: The largest pKb belongs to the weakest base, aniline. Only Option (1) starts with (D).
NCERT (2026-27 edition):
Basic Character of Amines – …
