Which of the following pinacol - pinacolone type of reaction will involve ring contraction ?

Options

A.
B.
C.
D.

Explanation

To solve the question regarding which pinacol-pinacolone type of reaction will involve ring contraction, we will analyze the options step by step.

### Step-by-Step Solution:

1. **Understanding Pinacol-Pinacolone Reaction**:
- The pinacol-pinacolone rearrangement typically involves the conversion of a pinacol (a diol) into a pinacolone (a ketone) through the formation of a carbocation and subsequent rearrangement.

2. **Analyzing Each Option**:
- We need to evaluate each option to determine whether it leads to ring contraction or expansion.

3. **Option 1**:
- The structure involves a penta ring with two hydroxyl (OH) groups.
- When H⁺ attacks, it leads to the formation of a carbocation after the departure of water (H₂O).
- The carbocation formed will seek stability by expanding the ring.
- **Conclusion**: This option involves ring expansion, not contraction.

4. **Option 2**:
- The structure involves an alcohol with an NH₂ group, treated with NaNO₂ and HCl (which generates HNO₂).
- After H⁺ attack and the departure of water, a carbocation is formed.
- The carbocation will also expand the ring to attain stability.
- **Conclusion**: This option also involves ring expansion.

5. **Option 3**:
- The structure shows a cyclohexane ring with an epoxide.
- When the oxygen attacks the cyclohexane, it leads to the breaking of the epoxide and the formation of a carbocation.
- This carbocation is unstable and will contract the ring to achieve stability.
- **Conclusion**: This option involves ring contraction.

6. **Option 4**:
- Similar to option 1, this structure has a diol.
- H⁺ attack leads to the formation of a carbocation after the departure of water.
- The carbocation will also expand the ring for stability.
- **Conclusion**: This option involves ring expansion.

7. **Final Answer**:
- The only option that involves ring contraction is **Option 3**.