Arrange the following amines in increasing order of basic strength in gaseous phase
(A) \( (CH_3)_3N \)
(B) \( CH_3NH_2 \)
(C) \( C_6H_5NH_2 \)
(D) \( C_6H_5 CH_2NH_2 \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (C), (D), (B), (A)
The basic strength of an amine is the readiness of the lone pair on nitrogen to accept a proton. Two factors decide it in the gaseous phase:
Inductive effect: alkyl groups push electron density towards nitrogen (+I effect) and make the lone pair more available. In the gaseous phase there is no solvation by water to disturb this, so basicity rises with the number of alkyl groups: tertiary > secondary > primary > \( \ce{NH3} \). A phenyl group attached through a \( \ce{CH2} \) group withdraws electrons (−I effect), so it makes the amine less basic than a simple alkylamine.
Resonance: when nitrogen is attached directly to a benzene ring, its lone pair is delocalised into the ring and is much less available. Arylamines are therefore the weakest bases.
(A) \( \ce{(CH3)3N} \): trimethylamine, a tertiary amine with three electron-releasing methyl groups: the strongest base.
(B) \( \ce{CH3NH2} \): methylamine, a primary amine with one methyl group.
(C) \( \ce{C6H5NH2} \): aniline; the lone pair is in resonance with the ring: the weakest base.
(D) \( \ce{C6H5CH2NH2} \): benzylamine; the \( \ce{CH2} \) group prevents resonance, so it is far more basic than aniline, but the phenyl group reduces electron density on nitrogen compared with methylamine.
In increasing order of basic strength:
\( \displaystyle \ce{C6H5NH2} \lt \ce{C6H5CH2NH2} \lt \ce{CH3NH2} \lt \ce{(CH3)3N} \)
Shortcut: Aniline must come first and trimethylamine last. Only Options (1) and (2) start with (C) and end with (A); benzylamine, weaker than methylamine, then puts (D) before (B), which leaves Option (1).
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.”