The boiling point of `p-`nitrophenol is higher than that of `o-`nitrophenol because.

Options

A. `NO_2` group at p-position behave in a different way from that at o-position
B. intramolecular hydrogen bonding exists in p-nitrophenol
C. there is intermolecular hydrogen bonding in p-nitrophenol
D. p-nitrophenol has a higher molecular weight than o-nitrophenol

Explanation

To understand why the boiling point of para-nitrophenol is higher than that of ortho-nitrophenol, we can analyze the molecular structures and the types of hydrogen bonding present in each compound.

### Step-by-Step Solution:

1. **Draw the Structures**:
- **Para-nitrophenol**: The nitro group (-NO2) is positioned at the para position (1,4) relative to the hydroxyl group (-OH) on the benzene ring.
- **Ortho-nitrophenol**: The nitro group is positioned at the ortho position (1,2) relative to the hydroxyl group on the benzene ring.

2. **Identify Hydrogen Bonding**:
- In **ortho-nitrophenol**, the hydroxyl hydrogen can form an intramolecular hydrogen bond with the nitro group. This means that the hydrogen from the -OH group interacts with the oxygen of the -NO2 group within the same molecule, stabilizing it but not contributing to interactions between different molecules.
- In **para-nitrophenol**, there is no possibility for intramolecular hydrogen bonding since the -OH and -NO2 groups are on opposite sides of the benzene ring. Instead, para-nitrophenol can form intermolecular hydrogen bonds between different molecules.

3. **Analyze the Effects of Hydrogen Bonding**:
- The presence of **intramolecular hydrogen bonding** in ortho-nitrophenol limits the interactions between molecules. This means that while the molecule is stable, it does not have strong interactions with other molecules, leading to a lower boiling point.
- In contrast, **intermolecular hydrogen bonding** in para-nitrophenol results in stronger attractions between different molecules. This increased attraction requires more energy to break, leading to a higher boiling point.

4. **Conclusion**:
- Therefore, the boiling point of para-nitrophenol is higher than that of ortho-nitrophenol due to the presence of stronger intermolecular hydrogen bonding in para-nitrophenol compared to the intramolecular hydrogen bonding in ortho-nitrophenol.

### Final Answer:
The boiling point of para-nitrophenol is higher than that of ortho-nitrophenol because para-nitrophenol can form stronger intermolecular hydrogen bonds, while ortho-nitrophenol primarily exhibits intramolecular hydrogen bonding.