Which of the following has lowest boililng point?

Options

A. p-nitrophenol
B. m-nitrophenol
C. o-nitrophenol
D. Phenol

Explanation

To determine which of the given compounds has the lowest boiling point, we need to analyze the intermolecular forces present in each compound. The boiling point of a substance is influenced significantly by the strength of these intermolecular forces, particularly hydrogen bonding in the case of alcohols and phenols.

### Step-by-Step Solution:

1. **Identify the Compounds**: We have four compounds to consider:
- Para-nitrophenol
- Meta-nitrophenol
- Ortho-nitrophenol
- Phenol

2. **Draw the Structures**:
- **Para-nitrophenol**: The -NO2 group is at the para position relative to the -OH group.
- **Meta-nitrophenol**: The -NO2 group is at the meta position relative to the -OH group.
- **Ortho-nitrophenol**: The -NO2 group is at the ortho position relative to the -OH group.
- **Phenol**: A simple benzene ring with a -OH group.

3. **Analyze Hydrogen Bonding**:
- **Para-nitrophenol**: Can form hydrogen bonds with other para-nitrophenol molecules through the -OH group.
- **Meta-nitrophenol**: Also forms hydrogen bonds, but the -NO2 group is positioned such that it may not participate in stabilizing hydrogen bonding as effectively as in para-nitrophenol.
- **Ortho-nitrophenol**: This compound can exhibit intramolecular hydrogen bonding due to the proximity of the -OH and -NO2 groups, which can significantly weaken its ability to form intermolecular hydrogen bonds.
- **Phenol**: Forms hydrogen bonds with other phenol molecules through the -OH group.

4. **Compare the Strength of Hydrogen Bonding**:
- **Ortho-nitrophenol**: The presence of intramolecular hydrogen bonding means that it cannot effectively engage in intermolecular hydrogen bonding with other ortho-nitrophenol molecules. This leads to a lower boiling point.
- **Para-nitrophenol and Meta-nitrophenol**: Both can form strong intermolecular hydrogen bonds, leading to higher boiling points compared to ortho-nitrophenol.
- **Phenol**: Also forms strong intermolecular hydrogen bonds, but not as strong as para-nitrophenol due to the absence of the nitro group.

5. **Conclusion**: Based on the analysis, **ortho-nitrophenol** has the lowest boiling point due to its intramolecular hydrogen bonding, which reduces the overall strength of hydrogen bonding with other molecules.

### Final Answer:
**Ortho-nitrophenol has the lowest boiling point.**