Assertion (A): The boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of comparable molecular masses.
Reason (R): Aldehydes and ketones undergo weak intermolecular association due to dipole-dipole interactions.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Boiling point rises with the strength of the forces between molecules. The polar C=O group gives aldehydes and ketones a substantial dipole moment.
Assertion (A): The boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of comparable molecular masses. — True. For example, propanal (322 K) and acetone (329 K) boil higher than n-butane (273 K) and methoxyethane (281 K) of similar molecular mass.
Reason (R): Aldehydes and ketones undergo weak intermolecular association due to dipole-dipole interactions. — True. The Cδ+=Oδ− dipoles of neighbouring molecules attract one another.
Why Reason (R) explains Assertion (A)?
Hydrocarbons are almost non-polar and ethers are only weakly polar, so their molecules are held less firmly.
The extra dipole-dipole attraction in aldehydes and ketones needs more energy to overcome, so they boil at higher temperatures.
NCERT (2026-27 edition):
8.3 Physical Properties – “… The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions.”