Amongst the following, the strongest base in aqueous medium is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (CH3)2NH
An amine acts as a base by giving the lone pair on nitrogen to a proton. Groups that push electrons towards nitrogen (+I effect of alkyl groups) increase basic strength; groups that pull electrons away decrease it. In water, solvation of the cation by hydrogen bonding and steric hindrance also count.
For the methyl amines in aqueous solution, these effects together make dimethylamine the strongest base: its two methyl groups push electron density onto nitrogen, and the cation \( \ce{(CH3)2NH2^+} \) still has two N–H hydrogens for hydrogen bonding with water.
Why the other options are incorrect
Option (1) CH3NH2 — Incorrect. Methylamine has only one methyl group; in aqueous solution it is a weaker base than dimethylamine.
Option (2) N≡CCH2NH2 — Incorrect. The cyano group is electron withdrawing; it pulls electron density away from nitrogen and makes the lone pair less available for a proton.
Option (4) C6H5NHCH3 — Incorrect. Here nitrogen is attached to the benzene ring, so its lone pair is in conjugation with the ring and less available for protonation; arylamines are weaker bases even than ammonia.
NCERT (2026-27 edition):
(a) 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:”
\( \displaystyle \ce{(C2H5)2NH} \gt \ce{(C2H5)3N} \gt \ce{C2H5NH2} \gt \ce{NH3} \)
\( \displaystyle \ce{(CH3)2NH} \gt \ce{CH3NH2} \gt \ce{(CH3)3N} \gt \ce{NH3} \)