Which among the following is the strongest base in aqueous solution?

Dimethylamine

The correct answer is Option (2) → Dimethylamine

Basic strength of amines depends on:

1. Availability of lone pair on nitrogen

2. +I effect of alkyl groups

3. Solvation in aqueous medium

In aqueous solution, solvation plays a very important role along with inductive effect.

Option 1: Methylamine (CH₃NH₂)

It is a primary amine. Alkyl group shows +I effect, which increases electron density on nitrogen and makes lone pair more available for protonation. It is fairly basic. However, only one alkyl group is donating electron density. Hence it is less basic than secondary amines.

Option 2: Dimethylamine ((CH₃)₂NH)

It is a secondary amine. Two alkyl groups donate electron density through +I effect. This increases electron density on nitrogen and enhances availability of lone pair. Also, in aqueous solution, secondary amines are well solvated through hydrogen bonding. So they get:

Good electron donation

Good solvation

This makes secondary amines the strongest bases in aqueous solution.

Option 3: Trimethylamine ((CH₃)₃N)

It is a tertiary amine. Three alkyl groups donate electron density, which should increase basicity. But in aqueous solution: Bulky structure causes steric hindrance. This reduces effective solvation of the conjugate acid formed after protonation. Poor solvation reduces stability of ammonium ion. Hence, tertiary amines become less basic than secondary amines in water.

Option 4: Aniline (C₆H₅NH₂)

In aniline, lone pair on nitrogen is delocalized into benzene ring by resonance. This reduces availability of lone pair for protonation. Therefore, aniline is much less basic than aliphatic amines.

Order of basic strength in aqueous solution:

Dimethylamine > Methylamine > Trimethylamine > Aniline

Strongest base = Dimethylamine

Order of basic strength in water:

Secondary amine > Primary amine > Tertiary amine > Aniline