Which compound should have zero dipole moment ?

Options

A.
B.
C.
D.

Answer

To determine which compound has a zero dipole moment, we need to analyze the molecular structure and symmetry of each compound provided in the options. A compound will have a zero dipole moment if it is symmetrical and the individual dipole moments cancel each other out.

### Step-by-Step Solution:

1. **Understanding Dipole Moment**:
- A dipole moment occurs due to the difference in electronegativity between atoms in a molecule, leading to a separation of charge. The dipole moment is a vector quantity, having both magnitude and direction.

2. **Analyzing the First Option**:
- The first compound has a structure with C=O (carbonyl) groups and C2H5 (ethyl) groups.
- The presence of lone pairs and the orientation of these groups leads to an asymmetrical distribution of charge.
- **Conclusion**: This compound does not have a zero dipole moment.

3. **Analyzing the Second Option**:
- The second compound also contains C=O groups and OH groups.
- Similar to the first option, the lone pairs and the orientation create an asymmetrical charge distribution.
- **Conclusion**: This compound does not have a zero dipole moment.

4. **Analyzing the Third Option**:
- The third compound features a symmetrical structure with C≡N (nitrile) groups.
- The symmetry of the molecule means that the dipole moments from each end cancel each other out.
- **Conclusion**: This compound has a zero dipole moment.

5. **Analyzing the Fourth Option**:
- The fourth compound has a C≡N group and a NO2 group.
- The different functional groups lead to an asymmetrical charge distribution, and the dipole moments do not cancel out.
- **Conclusion**: This compound does not have a zero dipole moment.

### Final Answer:
The compound with a zero dipole moment is the **third option** (C≡N symmetrical structure).