Read the passage carefully and answer the Questions.
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 molecules. Aliphatic amines are a stronger bases due to +I effect of an alkyl groups. Intermolecular hydrogen bonding in primary and secondary amines affects the boiling points of amines. The intermolecular association is more in primary amines than in secondary amines as there are more number of hydrogen atoms available for hydrogen bond formation. Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in an acidic medium. Gabriel synthesis is used for the preparation of primary amines. Primary amines react with nitrous acid to give unstable diazonium-salts which can be used to synthesize a variety of compounds.
The compound having the highest boiling point is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → n-Butanol
The four compounds have almost the same molar mass (pentane 72, the two amines 73, butanol 74 g mol−1), so their boiling points are decided by the intermolecular forces, mainly hydrogen bonding:
Oxygen is more electronegative than nitrogen, so the O–H bond is more polar than the N–H bond and alcohols form stronger hydrogen bonds than amines.
A primary amine has two N–H hydrogens and a secondary amine only one, so association is greater in primary amines.
An alkane has no hydrogen bonding at all, only weak van der Waals forces.
\( \displaystyle \text{n-butanol} \gt \text{n-butylamine} \gt \text{diethylamine} \gt \text{n-pentane} \)
So the highest boiling point is that of n-butanol.
Why the other options are incorrect
Option (1) n-pentane — Incorrect. It has no hydrogen bonding and boils lowest of the four.
Option (2) Diethylamine — Incorrect. A secondary amine has only one N–H hydrogen, so its hydrogen bonding is weaker than in n-butylamine.
Option (4) n-Butylamine — Incorrect. It forms hydrogen bonds, but the N–H bond is less polar than O–H, so it boils below n-butanol.
NCERT (2026-27 edition):
Physical Properties – …
