Which of the following has highest melting point?

Options

A. Chlorobenzene
B. O - dichlorobenzene
C. M - dichlorobenzene
D. P - dichlorobenzene

Answer

To determine which of the given compounds has the highest melting point, we need to analyze the molecular structure and packing efficiency of each compound. The compounds mentioned are chlorobenzene, ortho-chloro-dibenzene, meta-dichlorobenzene, and para-dichlorobenzene.

### Step-by-Step Solution:

1. **Understanding Melting Point**:
- The melting point is the temperature at which a solid becomes a liquid. It is influenced by the molecular structure and the packing of the molecules in the solid state.

2. **Analyzing the Compounds**:
- **Chlorobenzene**: This compound consists of a benzene ring with one chlorine atom attached. Its packing is relatively less efficient due to the presence of only one chlorine atom.
- **Ortho-Chloro-Dibenzene**: This compound has two chlorine atoms on adjacent carbon atoms of the benzene ring. The proximity of the chlorine atoms can lead to some steric hindrance, which may affect packing.
- **Meta-Dichloro-Benzene**: In this compound, the two chlorine atoms are on non-adjacent carbon atoms. This arrangement also leads to less efficient packing compared to the para isomer.
- **Para-Dichloro-Benzene**: This compound has two chlorine atoms on opposite sides of the benzene ring. This arrangement allows for better symmetry and packing efficiency.

3. **Comparing the Packing Efficiency**:
- The melting point is generally higher in compounds with better packing efficiency. The para isomer is known to have the best packing due to its symmetrical structure, which allows for closer molecular interactions.

4. **Conclusion**:
- Among the compounds listed, para-dichlorobenzene will have the highest melting point due to its superior packing efficiency compared to the other isomers.

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