Which of these carbocation will undergo favourable rearrangement?

Options

A.
B.
C.
D.

Answer

To determine which carbocation will undergo favorable rearrangement, we need to analyze the stability of the carbocations involved and see if rearrangement leads to a more stable structure. Here’s a step-by-step solution:

### Step 1: Understand Carbocation Stability
Carbocations are classified based on the number of alkyl groups attached to the positively charged carbon:
- **Tertiary (3°) carbocation**: More stable due to hyperconjugation and inductive effects from three alkyl groups.
- **Secondary (2°) carbocation**: Less stable than tertiary but more stable than primary.
- **Primary (1°) carbocation**: Least stable.

### Step 2: Identify the Carbocations
Assume we have several carbocations to analyze:
1. **Tertiary Carbocation (3°)**
2. **Secondary Carbocation (2°) with one methyl and one ethyl group**
3. **Tertiary Carbocation (3°)**
4. **Secondary Carbocation (2°)**

### Step 3: Analyze Each Carbocation for Rearrangement
1. **First Carbocation (Tertiary)**:
- Already a tertiary carbocation.
- Rearrangement will not lead to increased stability (remains tertiary).
- **Conclusion**: No favorable rearrangement.

2. **Second Carbocation (Secondary with Methyl and Ethyl)**:
- This is a secondary carbocation.
- If a methyl or ethyl group migrates, it can become a tertiary carbocation.
- **Conclusion**: This carbocation will undergo favorable rearrangement due to increased stability.

3. **Third Carbocation (Tertiary)**:
- Similar to the first, it is already a tertiary carbocation.
- Rearrangement will not lead to increased stability.
- **Conclusion**: No favorable rearrangement.

4. **Fourth Carbocation (Secondary)**:
- This is also a secondary carbocation.
- Rearrangement does not lead to a more stable structure (remains secondary).
- **Conclusion**: No favorable rearrangement.

### Step 4: Final Conclusion
The only carbocation that will undergo favorable rearrangement is the **second carbocation**, as it can rearrange to form a more stable tertiary carbocation.

### Summary
The carbocation that will undergo favorable rearrangement is the **second one**, which is a secondary carbocation that can rearrange to a tertiary carbocation.

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