Which of the following compounds would you expect to have a dipole moment ?

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds has a dipole moment, we need to analyze the molecular structure and the electronegativity of the atoms involved in each compound. Here's a step-by-step breakdown of the solution:

### Step 1: Understand Dipole Moment
A dipole moment occurs when there is a separation of charge in a molecule, typically due to differences in electronegativity between bonded atoms. A molecule will have a dipole moment if it has polar bonds that do not cancel each other out.

### Step 2: Analyze Each Compound
1. **1,4-Dinitrobenzene**:
- Structure: A benzene ring with two nitro (NO2) groups at the para position.
- Electronegativity: Nitrogen is more electronegative than carbon, so the dipole moments from each NO2 group point towards the nitro groups.
- Result: The dipole moments cancel each other out because they are equal in magnitude and opposite in direction. **Dipole Moment = 0**.

2. **1,4-Dicyanobenzene**:
- Structure: A benzene ring with two cyano (CN) groups at the para position.
- Electronegativity: Carbon in CN is more electronegative than hydrogen, so the dipole moments point towards the cyano groups.
- Result: Similar to the first compound, the dipole moments cancel each other out. **Dipole Moment = 0**.

3. **1,4-Dichlorobenzene**:
- Structure: A benzene ring with two chloro (Cl) groups at the para position.
- Electronegativity: Chlorine is more electronegative than carbon, so the dipole moments point towards the chlorine atoms.
- Result: The dipole moments cancel each other out. **Dipole Moment = 0**.

4. **Meta-Dichlorobenzene**:
- Structure: A benzene ring with two chloro (Cl) groups at the meta position.
- Electronegativity: The dipole moments from each chlorine atom point towards the chlorine atoms.
- Result: The dipole moments do not cancel out due to their arrangement, leading to a net dipole moment. **Dipole Moment ≠ 0**.

### Step 3: Conclusion
After analyzing all four compounds, we find that only the meta-dichlorobenzene has a non-zero dipole moment due to the arrangement of the chlorine atoms. Therefore, the correct answer is:

**Option 4: Meta-Dichlorobenzene has a dipole moment.**

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