Which of the following amines has a lower pKb value than ammonia?

The correct answer is Option (4) → Benzylamine

• The smaller the pK_b, the stronger the base. Ammonia has pK_b = 4.75, so the answer must be a stronger base than ammonia.

• In benzylamine, \( \ce{C6H5CH2NH2} \) (phenylmethanamine), the –NH₂ group is on a \( \ce{CH2} \) group, not on the ring. Its lone pair is not in conjugation with the ring, so it behaves like an alkylamine: pK_b = 4.70, just below 4.75.

• N,N-Dimethylbenzenamine, N-methylbenzenamine and benzenamine are arylamines: the nitrogen is directly on the benzene ring, its lone pair is in conjugation with the ring and less available for a proton, so each is a weaker base than ammonia.

Why the other options are incorrect

Option (1) N,N-Dimethylbenzenamine — Incorrect. pK_b = 8.92 (N,N-dimethylaniline), higher than 4.75.

Option (2) N-Methylbenzenamine — Incorrect. pK_b = 9.30 (N-methylaniline), higher than 4.75.

Option (3) Benzenamine — Incorrect. pK_b = 9.38 (aniline), higher than 4.75.

NCERT (2026-27 edition):
9.6 Chemical Reactions – “… Larger the value of K_b or smaller the value of pK_b, stronger is the base. The pK_b values of few amines are given in Table 9.3.”

Table 9.3: pK_b Values of Amines in Aqueous Phase

“pK_b value of ammonia is 4.75. … On the other hand, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group.”