Conjugate acid of which of the following amine is most stable?

The correct answer is Option (4) → Diethylamine

When an amine accepts a proton it forms its conjugate acid, the substituted ammonium ion. The more stable this cation, the more readily the amine takes up a proton, so the most stable conjugate acid belongs to the strongest base (lowest pK_b).

In water the ammonium ion is stabilised in two ways: by the +I effect of the alkyl groups and by hydrogen bonding (solvation) through its N–H hydrogens.

In \( \ce{(C2H5)2NH2+} \) (from diethylamine) two ethyl groups give a strong +I effect and two N–H hydrogens still allow good solvation. This is the best balance; diethylamine has pK_b = 3.00, the lowest of the four.

Why the other options are incorrect

• Option (1) Ethyl amine — Incorrect. \( \ce{C2H5NH3+} \) is well solvated, but it has only one electron-releasing ethyl group; pK_b = 3.29.

• Option (2) Triethyl amine — Incorrect. \( \ce{(C2H5)3NH+} \) has three ethyl groups, but only one N–H and a crowded ion, so it is poorly solvated in water; pK_b = 3.25.

• Option (3) Aniline — Incorrect. Aniline has five resonance structures because its lone pair is delocalised into the ring; the anilinium ion keeps only two Kekulé structures, so it is the least stable conjugate acid of the four; pK_b = 9.38.

NCERT (2026-27 edition):
Alkanamines versus Ammonia – “… Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. … The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃”

Basic Character of Amines – Table 9.3: pK_b Values of Amines in Aqueous Phase