Which is the best-suited method for the separation of para and ortho-nitrophenols from 1:1 mixture?

The correct option is steam distillation.

To understand why steam distillation is the best method to separate a 1:1 mixture of ortho-nitrophenol and para-nitrophenol, we need to examine their chemical structures and the types of hydrogen bonding they undergo.

1. ortho-Nitrophenol:
In ortho-nitrophenol, the nitro group (–NO₂) and the hydroxyl group (–OH) are situated on adjacent carbon atoms of the benzene ring. Due to their close proximity, they form intramolecular hydrogen bonding (hydrogen bonding within the same molecule). This internal bonding prevents the molecule from associating with neighboring molecules. As a result, the intermolecular forces between ortho-nitrophenol molecules are weak, making it highly volatile and easily vaporized by steam (steam-volatile).

2. para-Nitrophenol:
In para-nitrophenol, the nitro and hydroxyl groups are located on opposite ends of the benzene ring (para position). Because they are far apart, they cannot form internal hydrogen bonds. Instead, they form strong intermolecular hydrogen bonding (hydrogen bonding between different molecules). This links many molecules together in an associated network, significantly raising its boiling point and making it non-volatile in steam.

Separation Mechanism:
When steam is passed through the 1:1 mixture, the steam-volatile ortho-nitrophenol vaporizes and distills over with the steam, which can then be condensed and collected. The non-volatile para-nitrophenol remains behind in the distillation flask, resulting in a clean separation of the two isomers.