Which of the following amines has highest pKb in aqueous phase?

The correct answer is Option (1) → Benzenamine

A weaker base has a higher pK_b, because pK_b = −log K_b. Alkyl groups push electrons towards nitrogen (+I effect), so aliphatic amines are stronger bases than ammonia. In aryl amines the lone pair on nitrogen is delocalised into the benzene ring, so they are weaker bases than ammonia.

Of the four amines, benzenamine (aniline) is the weakest base, so it has the highest pK_b (9.38).

Why the other options are incorrect

• Option (2) N-Ethylethanamine — Incorrect. A secondary aliphatic amine; the +I effect of the two ethyl groups makes it the strongest base of the four, so it has the lowest pK_b (3.00).

• Option (3) N-Methylaniline — Incorrect. It is also an aryl amine with its lone pair delocalised into the ring, but the +I effect of the methyl group on nitrogen makes it slightly more basic than aniline (pK_b 9.30).

• Option (4) Phenylmethanamine — Incorrect. The –NH₂ group is on a CH₂ group, not on the ring, so its lone pair is not delocalised into the ring. It behaves like an aliphatic amine (pK_b 4.70).

NCERT (2026-27 edition):
Chemical Reactions – …

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