The steam distillation method is used for the separation of 1 : 1 mixture of p-nitrophenol and o-nitrophenols because
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → p-nitrophenol forms an inter-molecular H-bond with the nitro group of the other molecules and o-nitrophenol forms an intramolecular H-bond with the nitro group of the same molecule.
Steam distillation separates a substance that is steam volatile from one that is not. Volatility depends on how strongly the molecules are held to one another.
In o-nitrophenol the –OH and –\( \ce{NO2} \) groups are next to each other, so the hydrogen bond closes a six-membered ring within the molecule. In p-nitrophenol the two groups are on opposite sides of the ring and cannot reach each other.
So the mixture is separated because p-nitrophenol forms intermolecular and o-nitrophenol intramolecular hydrogen bonds.
Why the other options are incorrect
Option (1) p-nitrophenol forms an intramolecular H-bond with the nitro group of the same molecule and o-nitrophenol forms an inter-molecular H-bond with the nitro group of other molecules. — Incorrect. It swaps the two isomers; the groups of the para isomer are too far apart for an intramolecular hydrogen bond.
Option (2) p-nitrophenol and o-nitrophenol form an intramolecular H-bond with the nitro group of the same molecule. — Incorrect. The para isomer cannot form an intramolecular hydrogen bond, and if both did, both would be steam volatile and could not be separated.
Option (4) p-nitrophenol and o-nitrophenol form an inter-molecular H-bond with the nitro group of the other molecules. — Incorrect. If both were associated by intermolecular hydrogen bonds, neither would distil readily with steam.
NCERT (2026-27 edition):
Nitration – …
