Identify the least basic amine in gaseous phase?

The correct answer is Option (3) → \(C_6H_5-NH_2\)

The basicity of an amine depends on how freely the lone pair on nitrogen is available to accept a proton. In the gaseous phase there is no solvent, so only the electronic effect of the groups on nitrogen counts. Alkyl groups push electrons towards nitrogen (+I effect), so the basicity rises with their number:

    \( \displaystyle \text{tertiary} \gt \text{secondary} \gt \text{primary} \gt \ce{NH3} \)

In aniline the lone pair of nitrogen is delocalised into the benzene ring by resonance, so it is much less available for a proton; the sp² carbon of the ring also withdraws electrons. Aniline is therefore weaker than ammonia and far weaker than any alkylamine, so it is the least basic of the four.

Why the other options are incorrect

• Option (1) \((CH_3)_2NH\) — Incorrect. A secondary amine; two methyl groups push electrons towards nitrogen, making it strongly basic.

• Option (2) \(CH_3CH_2NH_2\) — Incorrect. A primary amine; the +I effect of the ethyl group makes it more basic than ammonia.

• Option (4) \((CH_3)_3N\) — Incorrect. A tertiary amine; with three methyl groups it is the most basic of the four in the gaseous phase.

NCERT (2026-27 edition):
Basic Character of Amines – “… Due to the electron releasing nature of alkyl group, it pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton of the acid. Moreover, the substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +I effect of the alkyl group. Hence, alkylamines are stronger bases than 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.”

Basic Character of Amines – “… We know that greater the number of resonating structures, greater is the stability. Thus you can infer that aniline (five resonating structures) is more stable than anilinium ion. Hence, the proton acceptability or the basic nature of aniline or other aromatic amines would be less than that of ammonia.”