Read the following passage and answer questions
Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be : primary > secondary > tertiary, which is opposite to the inductive effect based order. Secondly, when the alkyl group is small, like –CH3 group, there is no steric hinderance to H-bonding. In case the alkyl group is bigger than –CH3 group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from –CH3 to –C2H5 results in change of the order of basic strength. 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 medium.
Which among the following amines is most basic in aqueous medium ?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (C2H5)2NH
In aqueous solution the basic strength of an amine is decided by three factors together: the +I effect of the alkyl groups (more alkyl groups push more electron density on N), the solvation (hydrogen bonding) of the substituted ammonium cation by water (more N–H hydrogens give more stabilisation), and the steric hindrance to hydrogen bonding from bulky groups. Their combined effect makes the secondary amine the strongest base among the ethylamines.
Diethylamine (pKb 3.00) is the most basic.
Why the other options are incorrect
Option (2) (C2H5)3N — Incorrect. The largest +I effect, but its cation has only one N–H for hydrogen bonding and the three ethyl groups hinder solvation (pKb 3.25).
Option (3) C2H5NH2 — Incorrect. Only one alkyl group; its cation is well solvated but the +I effect is smaller (pKb 3.29).
Option (4) NH3 — Incorrect. It has no electron releasing alkyl group; it is the weakest base of the four (pKb 4.75).
NCERT (2026-27 edition):
Chemical Reactions – “… Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be: primary > secondary > tertiary, which is opposite to the inductive effect based order. Secondly, when the alkyl group is small, like –CH3 group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than CH3 group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from –CH3 to –C2H5 results in change of the order of basic strength. 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:
\( \ce{(C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3} \)
\( \ce{(CH3)2NH > CH3NH2 > (CH3)3N > NH3} \)”
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | 19-07 S1 | – | 10-08 S1 |