Why is o-nitrophenol steam volatile whereas p-nitrophenol has a higher boiling point. Explain.

The difference in the steam volatility and boiling points of o-nitrophenol and p-nitrophenol can be attributed to their structural differences and the influence of intramolecular hydrogen bonding.

Positional Isomerism:

In o-nitrophenol (ortho isomer), the nitro group (-NO2) is attached to the ortho positions (adjacent carbon atoms) of the phenolic ring.
In p-nitrophenol (para isomer), the nitro group is attached to the para positions (opposite carbon atoms) of the phenolic ring.
Intramolecular Hydrogen Bonding:

In o-nitrophenol, the nitro group is positioned in such a way that it can form intramolecular hydrogen bonding with the hydroxyl (OH) group on the phenolic ring. This hydrogen bonding stabilizes the molecule.
In p-nitrophenol, due to the para position of the nitro group, there is less opportunity for intramolecular hydrogen bonding between the nitro and hydroxyl groups.
Effect on Boiling Point:

The presence of intramolecular hydrogen bonding in o-nitrophenol makes the molecule more stable and reduces its vapor pressure. As a result, it requires less energy to convert o-nitrophenol into the vapor phase, making it more steam volatile. The lower boiling point is indicative of this easier transition into the vapor phase.

In contrast, the absence of significant intramolecular hydrogen bonding in p-nitrophenol leads to a higher vapor pressure, requiring more energy to convert it into the vapor phase. This results in a higher boiling point compared to o-nitrophenol.

In summary, the difference in the boiling points and steam volatility between o-nitrophenol and p-nitrophenol is primarily due to the presence of intramolecular hydrogen bonding in o-nitrophenol, which enhances its stability and lowers its boiling point.