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

The correct answer is Option (2) → Dimethylamine

An amine is basic because the lone pair on nitrogen can accept a proton. Anything that makes this lone pair more available increases basic strength; anything that pulls it away decreases it:

• Alkyl groups (+I effect) push electrons towards nitrogen and increase basicity. In water, solvation of the protonated amine also matters, and for methyl amines the observed order is \( \ce{(CH3)2NH} \) > \( \ce{CH3NH2} \) > \( \ce{(CH3)3N} \) > \( \ce{NH3} \).

• Solvation and steric hindrance: the protonated amine is stabilised by hydrogen bonding with water. A tertiary amine has the fewest N–H hydrogens and the bulkiest groups, so its cation is stabilised the least.

• In aryl amines the lone pair is delocalised into the benzene ring, so it is much less available.

So the strongest base in aqueous solution is dimethylamine, \( \ce{(CH3)2NH} \).

Why the other options are incorrect

• Option (1) Methylamine — Incorrect. It has only one +I methyl group, so it is a little weaker than dimethylamine.

• Option (3) Trimethylamine — Incorrect. Its three methyl groups give the largest +I effect, but its cation has only one N–H hydrogen for hydrogen bonding with water and the bulky groups hinder solvation. So in water it is weaker than both dimethylamine and methylamine.

• Option (4) Aniline — Incorrect. Resonance with the benzene ring takes the lone pair away from nitrogen, so it is weaker than any of the aliphatic amines, and even weaker than ammonia.

NCERT (2026-27 edition):
Chemical Reactions – …

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