Which of the following is the weakest base?

The correct answer is Option (1) → Aniline, \( \ce{C6H5NH2} \)

• An amine is basic because of the unshared electron pair on nitrogen; the more available this pair is for a proton, the stronger the base.

• In aniline the –NH₂ group is attached directly to the benzene ring, so the lone pair is in conjugation with the ring and less available for protonation.

• The other three are aliphatic amines, in which alkyl groups push electrons towards nitrogen (+I effect).

Why the other options are incorrect

Option (2) Piperidine — Incorrect. Nitrogen carries two alkyl carbons of a saturated ring: an aliphatic secondary amine, a stronger base than ammonia.

Option (3) Cyclohexylamine — Incorrect. The –NH₂ group is on a saturated (alkyl) ring, not on a benzene ring, so there is no conjugation; it is an aliphatic primary amine.

Option (4) \( \ce{CH3NH2} \) — Incorrect. Methylamine is an aliphatic amine (pK_b 3.38), a much stronger base than aniline (pK_b 9.38).

NCERT (2026-27 edition):
9.6 Chemical Reactions – “pK_b value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom. … On the other hand, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group.”