Which of the following explanations accounts for o-nitrophenol to be more volatile than p-nitrophenol.(a) Intermolecular hydrogen bonding(b) Resonance(c) Intramolecular hydrogen bonding(d) Inductive effect

The correct explanation for why o-nitrophenol is more volatile than p-nitrophenol is (c) Intramolecular hydrogen bonding.

Here’s the detailed reasoning:

Intramolecular Hydrogen Bonding in o-Nitrophenol
o-Nitrophenol: In the ortho isomer (o-nitrophenol), the hydroxyl group (-OH) and the nitro group (-NO2) are adjacent to each other on the benzene ring. This proximity allows the formation of an intramolecular hydrogen bond, where the hydrogen atom of the hydroxyl group forms a hydrogen bond with one of the oxygen atoms of the nitro group.

Intramolecular Hydrogen Bonding: This type of hydrogen bonding occurs within a single molecule, making the molecule more stable internally and reducing the molecule’s ability to engage in intermolecular hydrogen bonding with other molecules.
p-Nitrophenol: In the para isomer (p-nitrophenol), the hydroxyl group and the nitro group are opposite each other on the benzene ring. This spatial arrangement does not favor the formation of an intramolecular hydrogen bond, so the hydroxyl group is free to form intermolecular hydrogen bonds with other molecules.

Intermolecular Hydrogen Bonding: This type of hydrogen bonding occurs between different molecules, leading to higher boiling points because more energy is required to break these intermolecular interactions.
Volatility Comparison
o-Nitrophenol: Due to intramolecular hydrogen bonding, o-nitrophenol does not form strong intermolecular hydrogen bonds with other molecules. This results in lower boiling points and higher volatility since less energy is required to separate the molecules during the phase transition from liquid to gas.

p-Nitrophenol: The absence of intramolecular hydrogen bonding means p-nitrophenol forms strong intermolecular hydrogen bonds with other molecules. This leads to higher boiling points and lower volatility as more energy is needed to overcome these intermolecular forces.

Therefore, the correct explanation is (c) Intramolecular hydrogen bonding. This intramolecular interaction in o-nitrophenol reduces its intermolecular interactions, making it more volatile compared to p-nitrophenol.