Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. Which of the following is the reason for this?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → More extensive association of the acid molecules through intermolecular hydrogen bonding
The –COOH group has both a polar C=O group and an O–H group, so carboxylic acid molecules are held to one another by intermolecular hydrogen bonds, more extensively than alcohol molecules.
These hydrogen bonds are not broken completely even in the vapour phase: most carboxylic acids exist as dimers there.
More energy is needed to separate the molecules, so the boiling point is higher.
Why the other options are incorrect
Option (1) Weak molecular association arising out of dipole–dipole interactions — Incorrect. This is why aldehydes and ketones boil higher than hydrocarbons and ethers; such weak association cannot make an acid boil higher than an alcohol, which is hydrogen bonded.
Option (2) Formation of hydrogen bonds with water molecules — Incorrect. This explains why the lower acids are miscible with water; a pure acid boils without any water present.
Option (4) Increased hydrophobic interaction of the hydrocarbon part — Incorrect. This explains why the higher acids are practically insoluble in water; it is not the reason for the high boiling points.
NCERT (2026-27 edition):
8.8 Physical Properties – “…Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding. The hydrogen bonds are not broken completely even in the vapour phase. In fact, most carboxylic acids exist as dimer in the vapour phase or in the aprotic solvents.”