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. Carbocation rearrangement typically occurs to achieve a more stable carbocation, with tertiary carbocations being the most stable, followed by secondary and then primary.

### Step-by-Step Solution:

1. **Identify the Carbocations**:
- We need to look at the structure of each carbocation provided in the options.

2. **Evaluate Stability**:
- **Tertiary Carbocation**: A carbocation attached to three alkyl groups. It is highly stable due to hyperconjugation and inductive effects.
- **Secondary Carbocation**: A carbocation attached to two alkyl groups. It is less stable than tertiary but more stable than primary.
- **Primary Carbocation**: A carbocation attached to only one alkyl group. It is the least stable.

3. **Analyze Each Option**:
- **Option A**: Tertiary carbocation. It is stable and can rearrange, but it may not need to since it is already stable.
- **Option B**: Primary carbocation. This can rearrange to a more stable tertiary carbocation.
- **Option C**: A tertiary carbocation (CH3)3C+. Since it is already in the most stable form, it cannot undergo rearrangement.
- **Option D**: A primary carbocation that can rearrange to a more stable secondary carbocation.

4. **Conclusion**:
- The carbocation that cannot undergo rearrangement is **Option C** because it is already a tertiary carbocation, which is the most stable form.

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