Which of the following pairs of structures pepresent conformational isomers?

Options

A.
B.
C.
D.

Answer

To determine which pairs of structures represent conformational isomers, we need to understand the concept of conformational isomerism. Conformational isomers are stereoisomers that can be interconverted by rotation around single (sigma) bonds.

### Step-by-Step Solution:

1. **Identify the Structures**: Examine the pairs of structures given in the question. We need to analyze each pair to see if they can be interconverted by rotation around a sigma bond.

2. **Check for Sigma and Pi Bonds**: Identify the types of bonds present in each structure. Conformational isomers can only be formed by rotating around sigma bonds. If a structure contains a pi bond, it cannot be rotated freely, which means the structures cannot be conformational isomers.

3. **Analyze Each Pair**:
- **Pair 1**: If the first structure has a pi bond and the second structure also has a pi bond, then they cannot be conformational isomers because rotation around a pi bond is not possible.
- **Pair 2**: If both structures in this pair consist of only sigma bonds, then we can rotate around the bonds to interconvert them, making them conformational isomers.
- **Pair 3**: Similar to Pair 1, if there is a pi bond present in either structure, they cannot be conformational isomers.
- **Pair 4**: Again, check for the presence of pi bonds. If both structures have sigma bonds only, then they are conformational isomers.

4. **Conclusion**: After analyzing all pairs, identify which pair(s) can be interconverted by rotation around sigma bonds. The correct answer will be the pair that allows for such rotation.

### Final Answer:
Based on the analysis, the pair that represents conformational isomers is **Pair B**.