Which of the following is optical active subtance ?

Options

A.
B.
C.
D. Both (A) and ( B)

Answer

To determine which of the given substances is optically active, we need to analyze each compound based on their structural characteristics and the presence of chiral centers or restricted rotation.

### Step-by-Step Solution:

1. **Understanding Optical Activity**:
- Optical activity refers to the ability of a substance to rotate plane-polarized light. A compound is optically active if it lacks an internal plane of symmetry and has a chiral center or if it has restricted rotation due to steric hindrance.

2. **Analyzing the First Substance (Biphenyl)**:
- Biphenyl can be optically active if there is restricted rotation around the single bond connecting the two phenyl rings.
- If large groups are present at the ortho positions, they hinder rotation, making the compound optically inactive.
- In this case, if small groups (like deuterium and fluorine) are present at the ortho positions, rotation is possible, making this biphenyl optically active.

3. **Analyzing the Second Substance (Allene)**:
- The structure of allene is CH3-C=C-CH3, which has three double bonds.
- A rule of thumb is that if the number of double bonds is odd, the compound is typically optically inactive.
- Since allene has three double bonds (an odd number), it is optically inactive.

4. **Conclusion**:
- From the analysis, the first biphenyl compound with small groups at the ortho position is optically active, while the allene is optically inactive.
- Therefore, the optically active substance among the given options is the first biphenyl compound.

### Final Answer:
The optically active substance is the biphenyl compound with small groups at the ortho position.

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