Arrange the following in decreasing order of basic strength.
(A) \(C_6H_5NH_2\)
(B) \(C_6H_5N(CH_3)_2\)
(C) \((C_2H_5)_2NH\)
(D) \(CH_3NH_2\)
Choose the correct answers from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → C, D, B, A
The basic strength of an amine depends on how freely its nitrogen lone pair can accept a proton (a smaller pKb means a stronger base):
Alkyl groups push electrons to N by their +I effect, so aliphatic amines are stronger bases than ammonia; in water, solvation and steric effects also count.
In arylamines the lone pair is delocalised into the benzene ring by resonance, so they are much weaker bases. Methyl groups on N raise the basicity a little.
(C) \( \ce{(C2H5)2NH} \), N-ethylethanamine — pKb 3.00; two ethyl groups (+I), secondary amine.
(D) \( \ce{CH3NH2} \), methanamine — pKb 3.38; one methyl group.
(B) \( \ce{C6H5N(CH3)2} \), N,N-dimethylaniline — pKb 8.92; arylamine, but two \( \ce{CH3} \) groups on N.
(A) \( \ce{C6H5NH2} \), aniline — pKb 9.38; the weakest base.
Shortcut: All options start with C; only Option (2) has D second and A last.
NCERT (2026-27 edition):
Basic Character of Amines – Table 9.3: pKb Values of Amines in Aqueous Phase
| Name of amine | pKb |
|---|---|
| Methanamine | 3.38 |
| N-Methylmethanamine | 3.27 |
| N,N-Dimethylmethanamine | 4.22 |
| Ethanamine | 3.29 |
| N-Ethylethanamine | 3.00 |
| N,N-Diethylethanamine | 3.25 |
| Benzenamine | 9.38 |
| Phenylmethanamine | 4.70 |
| N-Methylaniline | 9.30 |
| N,N-Dimethylaniline | 8.92 |