Which of the following compounds will have highest melting point?

Options

A. Chlorobenzene
B. o-Dichlorobenzene
C. m-Dichlorobenzene
D. p-Dichlorobenzene

Explanation

To determine which of the given compounds has the highest melting point, we will analyze the compounds based on their symmetry and molecular packing. The compounds in question are chlorobenzene, ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- The compounds are:
- Chlorobenzene
- Ortho-dichlorobenzene
- Meta-dichlorobenzene
- Para-dichlorobenzene

2. **Understand the Concept of Melting Point**:
- Melting point is influenced by the symmetry of the molecule. Higher symmetry leads to better packing of molecules in the solid state, which typically results in a higher melting point.

3. **Analyze the Symmetry of Each Compound**:
- **Chlorobenzene**: It has a single chlorine substituent, leading to moderate symmetry.
- **Ortho-dichlorobenzene**: The two chlorine atoms are adjacent to each other, which disrupts symmetry due to steric hindrance.
- **Meta-dichlorobenzene**: The chlorine atoms are separated by one carbon, providing some symmetry but not as much as para.
- **Para-dichlorobenzene**: The two chlorine atoms are opposite each other, leading to the highest symmetry among the four compounds.

4. **Evaluate the Packing**:
- **Chlorobenzene**: Moderate packing due to its structure.
- **Ortho-dichlorobenzene**: Poor packing due to steric hindrance from adjacent chlorines.
- **Meta-dichlorobenzene**: Better packing than ortho but not as good as para.
- **Para-dichlorobenzene**: Excellent packing due to high symmetry, allowing for a more compact arrangement in the solid state.

5. **Conclusion**:
- Since para-dichlorobenzene has the highest symmetry and packing efficiency, it will have the highest melting point among the given compounds.

### Final Answer:
**Para-dichlorobenzene has the highest melting point.**

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