Aliphatic and aromatic amines are basic in nature. The following amin…

The correct answer is Option (2) → (B), (D), (C), (A)

An amine is basic because the lone pair on nitrogen can accept a proton. In water, the basic strength of aliphatic amines is decided by three factors acting together:

• Inductive effect (+I): alkyl groups push electrons towards nitrogen and make the lone pair more available. More alkyl groups mean a stronger effect: 3° > 2° > 1°.

• Solvation (hydration): the protonated amine (substituted ammonium ion) is stabilised by hydrogen bonding with water. The more N–H hydrogens it has, the better it is stabilised: 1° > 2° > 3°.

• Steric hindrance: bulky alkyl groups crowd the nitrogen and hinder this hydrogen bonding. The hindrance is least for 1° amines and greatest for 3° amines, so on this count: 1° > 2° > 3°.

For ethyl-substituted amines, the net result of these opposing factors gives the observed order in water:

    \( \displaystyle 2^\circ\ \text{amine} > 3^\circ\ \text{amine} > 1^\circ\ \text{amine} > \mathrm{NH_3} \)

Matching the given compounds:

• (B) \( \ce{(C2H5)2NH} \): diethylamine (2°)

• (D) \( \ce{(C2H5)3N} \): triethylamine (3°)

• (C) \( \ce{C2H5NH2} \): ethylamine (1°)

• (A) \( \ce{NH3} \): ammonia

Shortcut: The secondary amine (B) is the strongest and \( \ce{NH3} \) (A) is the weakest. Only Option (2) starts with (B) and ends with (A).

NCERT (2026-27 edition):
Alkanamines versus Ammonia – …