Intramolecular hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom (like O, N, F) and also forms a hydrogen bond with another electronegative atom within the same molecule. This typically leads to the formation of a stable five- or six-membered ring structure.

Why o-Nitrophenol is correct:

In o-Nitrophenol, the hydroxyl group (-OH) and the nitro group (-NO₂) are present on adjacent carbon atoms of the benzene ring (ortho position). The hydrogen atom of the -OH group forms a hydrogen bond with one of the oxygen atoms of the -NO₂ group within the same molecule. This forms a stable six-membered ring, characteristic of intramolecular H-bonding.

The structure of o-Nitrophenol showing intramolecular H-bonding is:

$$\text{o-Nitrophenol: }$$

$${\text{Benzene ring with -OH and -NO}}_2\text{ at ortho positions, forming H-bond between them.}$$

Option Analysis:

• A) ${\mathrm{H}}_2\mathrm{O}$: Water molecules exhibit extensive intermolecular hydrogen bonding, where hydrogen bonds form between different water molecules, not within a single molecule.
• B) $\mathrm{HF}$: Hydrogen fluoride molecules also exhibit strong intermolecular hydrogen bonding, forming zig-zag chains of HF molecules, not intramolecular H-bonding.
• D) ${\mathrm{CH}}_3\mathrm{OH}$: Methanol molecules exhibit intermolecular hydrogen bonding between the hydroxyl groups of different methanol molecules, not intramolecular H-bonding.

Correct Answer: (C)