The correct increasing order of basicity of following amines in water is
(A) methanamine
(B) N-methylmethanamine
(C) N, N-dimethylmethanamine
(D) benzenamine
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (D), (C), (A), (B)
The basic strength of an amine depends on how easily the lone pair on nitrogen can accept a proton. Two separate ideas decide this order:
Aliphatic vs aromatic: in an arylamine the lone pair on nitrogen is delocalised into the benzene ring by resonance, so it is much less available. Arylamines are weaker bases than ammonia and far weaker than alkylamines.
Among methyl amines in water, the +I effect (3° > 2° > 1°) competes with solvation of the protonated amine and steric hindrance (both favour 1° > 2° > 3°). The net result is:
\( \ce{(CH3)2NH} \) > \( \ce{CH3NH2} \) > \( \ce{(CH3)3N} \)
Matching the given IUPAC names:
(A) methanamine: \( \ce{CH3NH2} \) (1°)
(B) N-methylmethanamine: \( \ce{(CH3)2NH} \) (2°)
(C) N, N-dimethylmethanamine: \( \ce{(CH3)3N} \) (3°)
(D) benzenamine: \( \ce{C6H5NH2} \) (aniline, an arylamine)
Combined order of basicity:
\( \ce{C6H5NH2} \) < \( \ce{(CH3)3N} \) < \( \ce{CH3NH2} \) < \( \ce{(CH3)2NH} \)
Shortcut: Aniline (D) is the weakest base, so the order must start with (D); Options (2) and (3) do. For methyl amines the 3° amine is weaker than the 1° one, so (C) comes before (A): Option (2).
NCERT (2026-27 edition):
Alkanamines versus Ammonia – …
Arylamines versus Ammonia – “pKb value of aniline is quite high. … It is because 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.”