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 pKb value?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C6H5NH2
The larger the pKb, the weaker the base. In aniline the lone pair on nitrogen is delocalised into the benzene ring by resonance, so it is much less available for a proton; aryl amines are therefore far weaker bases than alkyl amines. So aniline, with the least available lone pair, has the highest pKb (9.38).
Why the other options are incorrect
Option (1) C2H5NH2 — Incorrect. An alkyl amine with a localised lone pair; a strong base with a low pKb (3.29).
Option (2) C6H5NHCH3 — Incorrect. An aryl amine, but the +I effect of the methyl group makes it slightly more basic than aniline (pKb 9.30).
Option (3) (C2H5)2NH — Incorrect. The strongest base of the four, so it has the lowest pKb (3.00).
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 |