Read the following passage and answer the next five questions based on it.
Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two large groups in ketones hinders the attack of nucleophile to carbonyl carbon than in aldehydes. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in the former.
The correct decreasing order of basic strength of following amines in aqueous solution is :
CH3NH2, (CH3)2NH, (CH3)3N, NH3
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (CH3)2NH > CH3NH2 > (CH3)3N > NH3
In water, the basic strength of an alkylamine depends on three factors together: the +I effect of the alkyl groups (3° > 2° > 1°), the solvation of the protonated amine by hydrogen bonding with water (1° > 2° > 3°) and the steric hindrance of the alkyl groups to this hydrogen bonding. With small methyl groups, the steric effect is small and solvation decides the place of the tertiary amine:
\( \displaystyle \ce{(CH3)2NH > CH3NH2 > (CH3)3N > NH3} \)
All three amines are stronger bases than ammonia, which has no alkyl group, so ammonia comes last; dimethylamine comes first, methylamine second and trimethylamine third.
Note: This question is from the chapter Amines, while the passage is from the chapter Aldehydes, Ketones and Carboxylic Acids.
NCERT (2026-27 edition):
Alkanamines versus Ammonia – “… The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3 (CH3)2NH > CH3NH2 > (CH3)3N > NH3”