Correct order of boiling points in the following is:
A. \(CH_3CHO\)
B. \(CH_3COOH\)
C. \(CH_3CH_2OH\)
D. \(CH_3CH_3\)
E. \(CH_3CH_2Cl\)
Choose the correct answers from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → B, C, A, E, D
The molecular masses are similar (30–64.5 g mol−1), so the boiling points follow the strength of the intermolecular forces: hydrogen bonding > dipole-dipole > dispersion.
B. CH3COOH, 391 K (first, highest) — extensive intermolecular hydrogen bonding; acid molecules associate strongly, even as dimers in the vapour.
C. CH3CH2OH, 351 K (second) — hydrogen bonding through the –OH group, weaker than in the acid.
A. CH3CHO, 294 K (third) — no hydrogen bonding, but strong dipole-dipole attraction from the polar C=O group.
E. CH3CH2Cl, 285 K (fourth) — dipole-dipole attraction from the less polar C–Cl bond.
D. CH3CH3, 184 K (last, lowest) — non-polar, with only weak dispersion forces.
NCERT (2026-27 edition):
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.”
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. Also, their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding.”