The reaction of amines with mineral acids to form ammonium salts shows that these are basic in nature. Amines have an unshared pair of electrons on the nitrogen atoms due to which they behave as Lewis bases. The basicity of amines is related to their structure. The basic character of an amine depends upon the ease of formation of a cation by accepting a proton from the acid. The more stable the cation is relative to the amine, the more basic the amine.
Among the following, which one has the highest Kb value?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C2H5)2NH
The larger the Kb (the smaller the pKb), the stronger the base.
Diethylamine combines the +I effect of two ethyl groups with good solvation of its cation, so it has the lowest pKb (3.00) and the highest Kb.
Why the other options are incorrect
Option (1) C2H5NH2 — Incorrect. Only one ethyl group; pKb 3.29, so a smaller Kb than diethylamine.
Option (2) C6H5N(CH3)2 — Incorrect. An aryl amine whose lone pair is delocalised into the ring; pKb 8.92, a very small Kb.
Option (4) CH3NH2 — Incorrect. One methyl group; pKb 3.38, so a smaller Kb than diethylamine.
NCERT (2026-27 edition):
Chemical Reactions – 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 |