O-nitrophenol has lower boiling point than p-nitrophenol. Why?

The difference in boiling points between o-nitrophenol and p-nitrophenol can be attributed to the varying intermolecular forces and molecular structures of the two compounds.

Positional Isomerism: O-nitrophenol and p-nitrophenol are positional isomers, meaning they have the same molecular formula (C6H5NO3) but differ in the arrangement of atoms within the molecule. In o-nitrophenol, the nitro group (-NO2) is attached to an ortho position (adjacent carbons) relative to the hydroxyl group (-OH), while in p-nitrophenol, the nitro group is attached to a para position (opposite carbons) relative to the hydroxyl group.

Hydrogen Bonding: Both compounds can form hydrogen bonds due to the presence of the hydroxyl group (-OH). However, in o-nitrophenol, the hydrogen bonding is more obstructed due to the closer proximity of the nitro group to the hydroxyl group. This can result in weaker intermolecular forces and a lower boiling point compared to p-nitrophenol.

Steric Hindrance: The ortho positioning of the nitro group in o-nitrophenol can lead to steric hindrance, where the bulky nitro group interferes with the formation of intermolecular forces to a greater extent compared to the para position in p-nitrophenol. This interference further weakens the intermolecular attractions in o-nitrophenol, contributing to its lower boiling point.

Overall, the combination of hindered hydrogen bonding and steric effects in o-nitrophenol results in weaker intermolecular forces, leading to a lower boiling point compared to p-nitrophenol.