What will be increasing order of basic strength of the following comp…

The correct answer is Option (2) → C₆H₅NH₂ < C₂H₅NH₂ < (C₂H₅)₃N < (C₂H₅)₂NH

An amine is basic because the lone pair on nitrogen can accept a proton. Three factors decide the basic strength of aliphatic amines in water:

• Inductive effect (+I): alkyl groups push electrons towards nitrogen and make the lone pair more available: 3° > 2° > 1°.

• Solvation: the protonated amine is stabilised by hydrogen bonding with water; the more N–H hydrogens it has, the better: 1° > 2° > 3°.

• Steric hindrance: bulky ethyl groups crowd the nitrogen and hinder this hydrogen bonding, most of all in the 3° amine.

For ethyl amines in water the net result is:

    \( \displaystyle \ce{(C2H5)2NH > (C2H5)3N > C2H5NH2} \)

So among the ethylamines, ethylamine comes second, triethylamine third and diethylamine last.

In aniline, the lone pair on nitrogen is delocalised into the benzene ring by resonance, so it is far less available; aniline is a weaker base than all the alkylamines (and even than ammonia), so it comes first.

Shortcut: Aniline is the weakest base and diethylamine the strongest; only Option (2) starts with aniline and ends with diethylamine.

NCERT (2026-27 edition):
Alkanamines versus Ammonia – “… 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 state. The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃”

NCERT (2026-27 edition):
Arylamines versus Ammonia – “… in aniline or other arylamines, the -NH₂ group is attached directly to the benzene ring. It results in the unshared electron pair on nitrogen atom to be in conjugation with the benzene ring and thus making it less available for protonation.”