Out of the following compounds, which is the most basic in aqueous solution?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \((CH_3)_2NH\)
An amine is basic because the lone pair on nitrogen accepts a proton. In water, three factors decide the basic strength of alkyl amines:
Inductive effect (+I): alkyl groups push electrons towards N; more alkyl groups mean a more available lone pair (3° > 2° > 1°).
Solvation: the protonated amine is stabilised by hydrogen bonding with water; more N–H hydrogens mean better stabilisation (1° > 2° > 3°).
Steric hindrance: bulky groups hinder this hydrogen bonding.
For methyl amines the balance gives \( \ce{(CH3)2NH} \) as the strongest base. Aniline is much weaker than all of them, because its lone pair is delocalised into the benzene ring.
Why the other options are incorrect
Option (1) \(CH_3—NH_2\) — Incorrect. A primary amine is well solvated but has only one electron-releasing alkyl group; it is weaker than dimethylamine.
Option (3) \((CH_3)_3N\) — Incorrect. It has the greatest +I effect, but its protonated form has only one N–H for hydrogen bonding with water, so it is poorly solvated.
Option (4) \(C_6H_5NH_2\) — Incorrect. An arylamine; the lone pair takes part in resonance with the ring, so aniline is weaker even than ammonia.
NCERT (2026-27 edition):
Chemical Reactions – “… Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be: primary > secondary > tertiary, which is opposite to the inductive effect based order. Secondly, when the alkyl group is small, like –CH3 group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than CH3 group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from –CH3 to –C2H5 results in change of the order of basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state. The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows:
\( \ce{(C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3} \)
\( \ce{(CH3)2NH > CH3NH2 > (CH3)3N > NH3} \)”