Which of the following explanations accounts for o-nitro-phenol to be more volatile than p-nitrophenol?

Options

A. Resonance
B. Hyperconjugation
C. Hydrogen bonding
D. Steric hindrance

Answer

To determine why o-nitrophenol is more volatile than p-nitrophenol, we need to analyze the molecular structures and the types of hydrogen bonding present in each compound.

### Step-by-Step Solution:

1. **Understand the Structures**:
- **o-Nitrophenol**: In this compound, the nitro group (NO2) is positioned ortho (adjacent) to the hydroxyl group (OH).
- **p-Nitrophenol**: In this compound, the nitro group is positioned para (opposite) to the hydroxyl group.

2. **Identify Hydrogen Bonding**:
- In **o-nitrophenol**, the hydroxyl group can form an **intramolecular hydrogen bond** with the nitro group. This means that the hydrogen atom of the hydroxyl group can bond with the oxygen atom of the nitro group within the same molecule.
- In **p-nitrophenol**, the hydroxyl group can form **intermolecular hydrogen bonds** with other p-nitrophenol molecules. This means that the hydrogen atom of one molecule's hydroxyl group can bond with the oxygen atom of another molecule's nitro group.

3. **Compare the Types of Hydrogen Bonding**:
- **Intramolecular Hydrogen Bonding** (o-nitrophenol): This type of bonding stabilizes the molecule but does not contribute to the overall stability of the substance when considering multiple molecules.
- **Intermolecular Hydrogen Bonding** (p-nitrophenol): This type of bonding involves interactions between multiple molecules, leading to a stronger overall attraction between the molecules.

4. **Analyze Volatility**:
- Volatility is inversely related to boiling point: the lower the boiling point, the more volatile the substance.
- Because o-nitrophenol has intramolecular hydrogen bonding, it does not have as strong interactions with other molecules compared to p-nitrophenol, which has stronger intermolecular hydrogen bonding.
- Therefore, o-nitrophenol has a lower boiling point and is more volatile than p-nitrophenol.

5. **Conclusion**:
- The presence of intramolecular hydrogen bonding in o-nitrophenol makes it less stable compared to the stronger intermolecular hydrogen bonding in p-nitrophenol. This results in o-nitrophenol being more volatile.

### Final Answer:
o-Nitrophenol is more volatile than p-nitrophenol due to the presence of intramolecular hydrogen bonding in o-nitrophenol, which leads to a lower boiling point compared to the stronger intermolecular hydrogen bonding in p-nitrophenol.

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