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.
The most basic amine in gaseous medium is :
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (CH3)3N
In the gaseous phase there is no water, so solvation plays no part, and basic strength depends only on the +I effect of the alkyl groups: the more alkyl groups push electrons towards nitrogen, the more available its lone pair is and the more stable the cation formed. So in the gas phase:
\( \displaystyle \ce{(CH3)3N > (CH3)2NH > CH3NH2 > NH3} \)
So the most basic amine in the gaseous medium is trimethylamine.
Why the other options are incorrect
Option (1) (CH3)2NH — Incorrect. It is the strongest base in water, where solvation also counts, but in the gas phase two methyl groups push less than three.
Option (2) CH3NH2 — Incorrect. One methyl group only.
Option (4) NH3 — Incorrect. No alkyl group; the weakest base in both phases.
NCERT (2026-27 edition):
Alkanamines versus Ammonia – “… Thus, the basic nature of aliphatic amines should increase with increase in the number of alkyl groups. This trend is followed in the gaseous phase. The order of basicity of amines in the gaseous phase follows the expected order: tertiary amine > secondary amine > primary amine > NH3.”