Comprehension:
Amines are derivatives of ammonia obtained by replacement of one, two or all the three hydrogen atoms by alkyl and / or aryl groups. Nitrogen atom of amines like ammonia is trivalent and carries an unshared pair of electrons. Orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the composition of the amines. The fourth orbital of nitrogen in all amines contains an unshared pair of electrons. Due to the presence of unshared pair of electrons, the angle C-N-E (where E is C or H) is less than tetrahedral angle 109.5°. Amines are classified as primary (1°), secondary (2°) and tertiary (3°) amines. All amines are basic in nature and basicity differs between primary amines, secondary amines and tertiary amines. In aryl amines, –NH2 group is directly attached to the benzene ring. C6H5NH2 is simplest example of arylamine. Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.
Among the following, which amines is least basic in aqueous phase?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Benzenamine
A weaker base has a higher pKb, because pKb = −log Kb. Alkyl groups push electrons towards nitrogen (+I effect), so aliphatic amines are stronger bases than ammonia. In aryl amines the lone pair on nitrogen is delocalised into the benzene ring, so they are weaker bases than ammonia.
Of the four amines, benzenamine (aniline) is the weakest base, so it is the least basic.
Why the other options are incorrect
Option (1) Methanamine — Incorrect. A primary aliphatic amine; the +I effect of the methyl group makes it a stronger base than ammonia (pKb 3.38).
Option (3) N-Ethylethanamine — Incorrect. It is the strongest base of the four, so it has the lowest pKb (3.00).
Option (4) N,N-Dimethylmethanamine — Incorrect. A tertiary amine; poorer solvation of its cation makes it weaker than the 1° and 2° methyl amines in water, but it is still far more basic than aniline (pKb 4.22).
NCERT (2026-27 edition):
Chemical Reactions – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 31-05 S2 29-05 S2 | – | – | – | – |