Arrange the following in increasing order of basic strength in aqueous solution:
(A) \( C_6H_5NH_2 \)
(B) \( C_2H_5NH_2 \)
(C) \( (C_2H_5)_2NH \)
(D) \( NH_3 \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (D), (B), (C)
An amine is basic because the lone pair on nitrogen can accept a proton. In water, three factors decide the basic strength of aliphatic amines:
Inductive effect (+I): alkyl groups push electrons towards nitrogen: 3° > 2° > 1°.
Solvation: the protonated amine is stabilised by hydrogen bonding with water; more N–H hydrogens give better stabilisation: 1° > 2° > 3°.
Steric hindrance: bulky groups hinder this hydrogen bonding, which again favours 1° > 2° > 3°.
In aromatic amines, the lone pair on nitrogen is delocalised into the benzene ring by resonance. It is much less available for a proton, so aniline is a weaker base than ammonia.
For the compounds given here, the basic strength in water increases as:
\( \displaystyle \text{aryl amine} < \ce{NH3} < 1^\circ\text{ amine} < 2^\circ\text{ amine} \)
\( \displaystyle \ce{C6H5NH2} < \ce{NH3} < \ce{C2H5NH2} < \ce{(C2H5)2NH} \)
\( \displaystyle \mathrm{p}K_b:\ 9.38 > 4.75 > 3.29 > 3.00 \)
Matching the given compounds:
(A) \( \ce{C6H5NH2} \): aniline (aryl amine), the weakest base
(D) \( \ce{NH3} \): ammonia
(B) \( \ce{C2H5NH2} \): ethylamine (1°)
(C) \( \ce{(C2H5)2NH} \): diethylamine (2°), the strongest base
NCERT (2026-27 edition):
Basic Character of Amines – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 22-05 S2 11-05 S1 | 21-05 S1 16-05 S1 | – | – | – |