Read the following passage and answer questions
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 hinderance 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 medium.
Which base among the following is least basic in aqueous medium ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → NH3
Alkyl groups push electrons towards nitrogen (+I effect) and stabilise the protonated amine, so every alkylamine is a stronger base than ammonia. For methyl amines in water, where the small methyl groups hinder solvation only a little, the order is:
\( \displaystyle \ce{(CH3)2NH > CH3NH2 > (CH3)3N > NH3} \)
So the least basic of the four in aqueous medium is ammonia.
Why the other options are incorrect
Option (1) (CH3)2NH — Incorrect. Dimethylamine is the strongest base of the four in water.
Option (2) CH3NH2 — Incorrect. Methylamine is stronger than ammonia because of the +I effect of its methyl group.
Option (3) (CH3)3N — Incorrect. Trimethylamine is the weakest of the three amines in water, because its cation has only one N–H for hydrogen bonding, but it is still stronger than ammonia.
NCERT (2026-27 edition):
Alkanamines versus Ammonia – “… The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3 (CH3)2NH > CH3NH2 > (CH3)3N > NH3”