Which of the following carbocation can not undergo rearrangement ?

Options

A.
B.
C. `CH_3-underset(CH_3)underset(|)overset(o+)(C )-CH-CH_3`
D. `C_6H_5-CH-overset(o+)(C )H_2`

Answer

To determine which carbocation cannot undergo rearrangement, we need to analyze the stability of the carbocations presented in the options. Carbocations tend to rearrange to achieve a more stable state, typically moving from a less stable to a more stable form (e.g., from primary to secondary to tertiary).

### Step-by-Step Solution:

1. **Identify the Carbocations**:
- We have several carbocations to analyze. Let's denote them as:
- Option 1: Tertiary Carbocation (3°)
- Option 2: Primary Carbocation (1°)
- Option 3: Tertiary Carbocation (3°)
- Option 4: Primary Carbocation (1°)

2. **Understanding Carbocation Stability**:
- Carbocations are classified based on their degree:
- Tertiary (3°) > Secondary (2°) > Primary (1°)
- The most stable carbocation is tertiary, followed by secondary, and the least stable is primary.

3. **Analyze Each Option**:
- **Option 1 (3°)**:
- This is a tertiary carbocation. If a hydrogen shifts to form a new carbocation, it would still be a tertiary carbocation. Thus, rearrangement is possible.

- **Option 2 (1°)**:
- This is a primary carbocation. If a hydrogen shifts, it can form a more stable tertiary carbocation. Hence, rearrangement is possible.

- **Option 3 (3°)**:
- This is also a tertiary carbocation. Any rearrangement would lead to a less stable primary carbocation. Therefore, rearrangement will not occur because it would decrease stability.

- **Option 4 (1°)**:
- This is a primary carbocation. If a hydrogen shifts, it can form a more stable secondary or tertiary carbocation. Thus, rearrangement is possible.

4. **Conclusion**:
- The only carbocation that cannot undergo rearrangement is **Option 3**, which is a tertiary carbocation that would lead to a less stable primary carbocation if rearranged.

### Final Answer:
**Option 3** is the carbocation that cannot undergo rearrangement.