The decreasing order of basic strength of the following amines in aqu…

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

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 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; least for 1°, greatest for 3°, so on this count: 1° > 2° > 3°.

For ethyl-substituted amines, these factors together make the secondary amine the strongest base in water, followed by the tertiary and then the primary amine. Matching the given compounds:

• (C) \( \ce{(Et)2NH} \): diethylamine (2°)

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

• (A) \( \ce{EtNH2} \): ethylamine (1°)

• (B) \( \ce{NH3} \): ammonia

Shortcut: The secondary amine (C) is the strongest and \( \ce{NH3} \) (B) is the weakest. Options (2) and (3) both start with (C) and end with (B); the 3° ethyl amine is stronger than the 1° one, which leaves Option (2).

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