In which mechanism the carbocation intermediate is formed ?

Options

A. `S_N1` reaction mechanism
B. `S_N2` reaction mechanism
C. `S_N` reaction mechanism
D. Both (a) and (b)

Answer

To determine in which mechanism the carbocation intermediate is formed, we need to analyze the mechanisms of nucleophilic substitution reactions, specifically SN1 and SN2.

### Step-by-Step Solution:

1. **Understanding Reaction Mechanisms**:
- There are two primary mechanisms for nucleophilic substitution: SN1 and SN2.
- SN1 (Substitution Nucleophilic Unimolecular) involves a two-step process, while SN2 (Substitution Nucleophilic Bimolecular) involves a one-step process.

2. **Identifying the SN1 Mechanism**:
- In the SN1 mechanism, the first step involves the departure of the leaving group (e.g., a halide like Br). This step results in the formation of a carbocation intermediate.
- For example, if we have an alkyl halide like R-Br, the leaving group (Br) departs, resulting in a positively charged carbocation (R+).

3. **Nucleophilic Attack**:
- After the formation of the carbocation, a nucleophile can then attack this intermediate. This attack can occur from either side of the planar carbocation, leading to the possibility of different stereochemical outcomes (inversion or retention).

4. **Conclusion**:
- The formation of a carbocation intermediate is characteristic of the SN1 mechanism. Therefore, the answer to the question is that the carbocation intermediate is formed in the SN1 mechanism.

### Final Answer:
The carbocation intermediate is formed in the SN1 mechanism.