Arrange the following in the increasing order of pKb in aqueous phase…

The correct answer is Option (3) → (C), (B), (D), (A)

pK_b measures basic strength inversely: the stronger the base, the smaller its pK_b. So the increasing order of pK_b is the same as the decreasing order of basic strength.

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

• Inductive effect (+I): alkyl groups push electrons towards nitrogen: 3° > 2° > 1°.

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

• Steric hindrance: bulky groups hinder this hydrogen bonding, which again favours 1° > 2° > 3°.

In aromatic amines, the lone pair on nitrogen is delocalised into the benzene ring by resonance. It is much less available for a proton, so aniline is a weaker base than ammonia.

For the compounds given here, the order of basic strength in water is:

    \( \displaystyle 2^\circ\text{ amine} > 1^\circ\text{ amine} > \ce{NH3} > \text{aryl amine} \)

    \( \displaystyle \ce{(C2H5)2NH} > \ce{C2H5NH2} > \ce{NH3} > \ce{C6H5NH2} \)

    \( \displaystyle \mathrm{p}K_b:\ 3.00 < 3.29 < 4.75 < 9.38 \)

Matching the given compounds:

• (C) \( \ce{(C2H5)2NH} \): diethylamine (2°)

• (B) \( \ce{C2H5NH2} \): ethylamine (1°)

• (D) \( \ce{NH3} \): ammonia

• (A) \( \ce{C6H5NH2} \): aniline (aryl amine)

NCERT (2026-27 edition):
Basic Character of Amines – …

Note: The same/similar question has been asked in different years by NTA as per the details given below: