Arrange the following amines in increasing order of their basic strength in the gaseous phase
\( (A)\text{C}_6\text{H}_5\text{NH}_2 \)
\( (B)\text{C}_6\text{H}_5\text{CH}_2\text{NH}_2 \)
\( (C)\text{(CH}_3)_3\text{N} \)
\( (D)\text{CH}_3\text{NH}_2 \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (B), (D), (C)
An amine is basic because the lone pair on nitrogen accepts a proton. The more available this lone pair, the stronger the base. Three factors decide the order here:
+I effect of alkyl groups: alkyl groups push electron density onto nitrogen and also stabilise the substituted ammonium ion formed. In the gaseous phase there is no solvation by water, so this effect alone decides the order: tertiary > secondary > primary > \( \ce{NH3} \).
Resonance in aryl amines: in aniline the lone pair on nitrogen is delocalised into the benzene ring, so it is much less available; aniline is a far weaker base than any alkylamine.
Benzyl group: in benzylamine a \( \ce{CH2} \) group separates nitrogen from the ring, so there is no resonance and it behaves as a primary alkylamine. The phenyl group, however, withdraws electrons slightly (−I) where the methyl group releases them, so benzylamine is a weaker base than methylamine (in water their pKb values are 4.70 and 3.38).
Applying these to the four amines:
(A) \( \ce{C6H5NH2} \): aniline (aryl amine, lone pair in resonance): the weakest base.
(B) \( \ce{C6H5CH2NH2} \): benzylamine (primary amine, slightly −I benzyl group).
(C) \( \ce{(CH3)3N} \): trimethylamine (tertiary amine, three +I methyl groups): the strongest base in the gas phase.
(D) \( \ce{CH3NH2} \): methylamine (primary amine, one +I methyl group).
In increasing order of basic strength in the gaseous phase:
\( \displaystyle \ce{C6H5NH2} \lt \ce{C6H5CH2NH2} \lt \ce{CH3NH2} \lt \ce{(CH3)3N} \)
Shortcut: Aniline is the weakest base and, in the gas phase, the tertiary amine is the strongest, so the order starts with (A) and ends with (C). Only Option (2) does both.
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. … 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.”