Aliphatic amines are stronger bases than ammonia. Which of the following is the reason?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The alkyl groups push electrons towards nitrogen, which raises the electron density on nitrogen.
The basic strength of an amine depends on how easily it accepts a proton and how stable the cation formed is.
Alkyl groups are electron releasing (+I effect). They make the unshared pair on nitrogen more available for the proton, and they spread out the positive charge of the substituted ammonium ion, which stabilises it.
So aliphatic amines have pKb values (3 to 4.22) below that of ammonia (4.75).
Option (1) The alkyl groups withdraw electrons from nitrogen, which makes the N–H bonds more polar. — Incorrect. Alkyl groups release electrons; they do not withdraw them. An electron withdrawing group, such as the aryl group, makes an amine a weaker base than ammonia.
Option (3) The substituted ammonium ions are stabilised by solvation with water more than the ammonium ion. — Incorrect.
The greater the size of the ion, the less it is solvated.
Substituted ammonium ions are larger than NH4+, so solvation does not favour them; this is why the order in water is not regular.
Option (4) The alkyl groups increase the number of hydrogen atoms on nitrogen that can form hydrogen bonds. — Incorrect. Each alkyl group takes the place of a hydrogen on nitrogen, so alkylamines have fewer N–H hydrogens than ammonia, not more.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “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.”