Assertion (A): Aliphatic amines are stronger bases than ammonia.
Reason (R): Like ammonia, amines have an unshared pair of electrons on the nitrogen atom, due to which they behave as Lewis bases.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): Aliphatic amines are stronger bases than ammonia. — True. The pKb of ammonia is 4.75, while the pKb values of aliphatic amines lie between 3 and 4.22.
A smaller pKb means a stronger base.
Reason (R): Like ammonia, amines have an unshared pair of electrons on the nitrogen atom, due to which they behave as Lewis bases. — True. The nitrogen atom can share this pair with a proton, so amines form salts with acids:
\( \displaystyle \ce{R-NH2 + H^+ -> R-NH3^+} \)
Why R does not explain A?
R names a feature that amines share with ammonia, so it tells why amines are bases at all, not why they are stronger bases than ammonia.
The correct reason is the +I effect of the alkyl groups: they push electrons towards nitrogen, making the unshared pair more available, and they disperse the positive charge of the substituted ammonium ion.
NCERT (2026-27 edition):
9.1 Structure of Amines – “Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons.”
9.6 Chemical Reactions – “Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base.”
“pKb value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom. Their pKb values lie in the range of 3 to 4.22.”
“Due to the electron releasing nature of alkyl group, it 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, alkylamines are stronger bases than ammonia.”