Arrange the following amines in the order of decreasing basic character in gaseous phase:
(A) \(NH_3\)
(B) \((CH_3)_3N\)
(C) \((CH_3)_2NH\)
(D) \(CH_3NH_2\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \((CH_3)_3N, (CH_3)_2NH, CH_3NH_2, NH_3\)
The basic character of an amine is the availability of the lone pair on nitrogen. Each methyl group has a +I effect that pushes electron density onto N and stabilises the ammonium ion formed by accepting a proton.
In the gaseous phase there is no solvent, so there is no solvation (hydrogen bonding) of the ammonium ion to consider; the inductive effect alone decides the order. The more methyl groups, the stronger the base:
tertiary > secondary > primary > \( \ce{NH3} \)
\( \ce{(CH3)3N > (CH3)2NH > CH3NH2 > NH3} \)
(A) \( \ce{NH3} \): ammonia, no alkyl group
(B) \( \ce{(CH3)3N} \): trimethylamine (tertiary)
(C) \( \ce{(CH3)2NH} \): dimethylamine (secondary)
(D) \( \ce{CH3NH2} \): methylamine (primary)
Shortcut: in the gas phase the tertiary amine comes first; only Options (2) and (4) start with \( \ce{(CH3)3N} \), and only Option (2) then has \( \ce{(CH3)2NH} \) second.
NCERT (2026-27 edition):
Alkanamines versus Ammonia – “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.”