Read the passage given below and answer the questions based on it.
Amines constitute an important class of organic compounds derived by replacing one or more hydrogen atoms of ammonia molecule by alkyl/aryl group(s). Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore sp3 hybridised and the geometry of amines is pyramidal. Amines are classified as primary (1°), secondary (2°) and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. Due to the electron releasing nature of alkyl group, it (R) pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton of the acid. Moreover, the substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +I effect of the alkyl group. Hence, alkyl amines are stronger bases than ammonia. Thus, the basic nature of aliphatic amines should increase with increase in the number of alkyl groups. The order of basicity of amines in the gaseous phase follows the expected order: 3° > 2° > 1° > NH3. Adrenaline and ephedrine, both containing a secondary (2°) amino group, are used to increase blood pressure. The well-known antihistaminic drug Benadryl contains a tertiary (3°) amino group.
Which one amongst the following will have the lowest \(pK_b\)?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Ethanamine
The lower the pKb, the stronger the base. Alkyl groups increase basic strength by their +I effect; a phenyl group on the carbon next to nitrogen withdraws electrons and lowers it.
So ethanamine has the lowest pKb.
Why the other options are incorrect
Option (2) Methanamine — Incorrect. Its methyl group has a slightly weaker +I effect than ethyl, so pKb is a little higher (3.38).
Option (3) N, N-dimethylmethanamine — Incorrect. A tertiary amine; poor solvation of its protonated form makes it weaker in water (4.22).
Option (4) Phenyl Methanamine — Incorrect. The phenyl group withdraws electron density, so it is the weakest of the four (4.70).
NCERT (2026-27 edition):
Chemical Reactions – Table 9.3: pKb Values of Amines in Aqueous Phase
| Name of amine | pKb |
|---|---|
| Methanamine | 3.38 |
| N-Methylmethanamine | 3.27 |
| N,N-Dimethylmethanamine | 4.22 |
| Ethanamine | 3.29 |
| N-Ethylethanamine | 3.00 |
| N,N-Diethylethanamine | 3.25 |
| Benzenamine | 9.38 |
| Phenylmethanamine | 4.70 |
| N-Methylaniline | 9.30 |
| N,N-Dimethylaniline | 8.92 |