For the same alkyl group, all the three primary, secondary and tertiary amines are more basic than ammonia in aqueous solutions. Their relative basicity, however, depends upon a combination of three factors: +I-effect of alkyl groups, the stability of the ammonium cation (formed after accepting a proton) due to H-bonding with water molecules and the steric effect of the alkyl groups which tend to reduce the extent of H-bonding. All these three factors are favourable for secondary amines thereby making them the strongest bases. If, however, the alkyl group is bigger than the methyl group, +I-effect of the alkyl group outweighs stability of the ammonium cation due to H-bonding thereby making tertiary amines more basic than the primary amines. Due to delocalization of lone pair of electrons of the nitrogen atom on the benzene ring, aniline is a weaker base than ammonia. The basic strength of the substituted anilines, however, depends upon the nature of the substituent. Whereas electron-donating groups tend to increase, electron-withdrawing groups tend to decrease the basic strength.
Match List-I (base) with List-II (pKb value in aqueous phase).
| List-I (Base) | List-II (pKb value) |
|---|---|
| (A) Methanamine | (I) 4.75 |
| (B) N,N-Dimethylmethanamine | (II) 9.38 |
| (C) Ammonia | (III) 4.22 |
| (D) Benzenamine | (IV) 3.38 |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A-IV, B-III, C-I, D-II
pKb = −log Kb, so a stronger base has a smaller pKb.
In water, the methyl-substituted amines and ammonia follow the order (CH3)2NH > CH3NH2 > (CH3)3N > NH3, and aniline is a much weaker base than ammonia.
So the pKb values rise in the order CH3NH2 < (CH3)3N < NH3 < C6H5NH2.
(A) Methanamine — This matches (IV) 3.38. It is the strongest base of the four, so it has the smallest pKb.
(B) N,N-Dimethylmethanamine — This matches (III) 4.22. In water, trimethylamine is a weaker base than methanamine (its cation is less stabilised by H-bonding with water), but it is still stronger than ammonia.
(C) Ammonia — This matches (I) 4.75. Ammonia has no alkyl group to push electrons towards nitrogen, so it is weaker than the aliphatic amines.
(D) Benzenamine — This matches (II) 9.38. In aniline the unshared pair on nitrogen is in conjugation with the benzene ring, so it is much less available for a proton; aniline is the weakest base here.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “pKb = –log Kb. Larger the value of Kb or smaller the value of pKb, stronger is the base. The pKb values of few amines are given in Table 9.3.”
“pKb value of ammonia is 4.75. … On the other hand, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group.”
“The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: … (CH3)2NH > CH3NH2 > (CH3)3N > NH3”
Table 9.3: pKb Values of Amines in Aqueous Phase
