Which of the following isomer has the highest melting point:

Options

A. 1,2-dichlorobenzene
B. 1,3-dichlorobenzene
C. 1,4-dichlorobenzene
D. all isomers have same melting points

Answer

To determine which isomer has the highest melting point among the given dichlorobenzenes, we will analyze the structures and their properties step by step.

### Step 1: Identify the Isomers
We have three isomers of dichlorobenzene:
1. **1,2-Dichlorobenzene (Ortho)**
2. **1,3-Dichlorobenzene (Meta)**
3. **1,4-Dichlorobenzene (Para)**

### Step 2: Analyze the Structures
- **1,2-Dichlorobenzene (Ortho)**: The two chlorine atoms are adjacent to each other on the benzene ring.
- **1,3-Dichlorobenzene (Meta)**: The two chlorine atoms are separated by one carbon atom on the benzene ring.
- **1,4-Dichlorobenzene (Para)**: The two chlorine atoms are opposite each other on the benzene ring.

### Step 3: Consider Symmetry and Stability
- **Symmetry**: The para isomer (1,4-dichlorobenzene) is more symmetric compared to the ortho and meta isomers. Symmetry in molecular structure often leads to greater stability.
- **Stability**: More stable molecules typically have higher melting points because they require more energy to disrupt their intermolecular forces.

### Step 4: Compare Melting Points
- **Ortho Isomer**: Due to steric hindrance (the proximity of the two chlorine atoms), 1,2-dichlorobenzene is less stable and has a lower melting point.
- **Meta Isomer**: 1,3-dichlorobenzene has moderate stability and melting point.
- **Para Isomer**: 1,4-dichlorobenzene, being the most symmetric, is the most stable and thus has the highest melting point among the three.

### Conclusion
Based on the analysis, the isomer with the highest melting point is **1,4-dichlorobenzene**.

### Final Answer
**1,4-Dichlorobenzene has the highest melting point.**

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