Racemisation occurs in:

Options

A. `S_(N)2`
B. `S_(N)1`
C. Neither `S_(N)2 "nor" S_(N)1"`.
D. `S_(N)2` reaction as well as ` S_(N)1` reaction

Explanation

To determine where racemization occurs, we need to analyze the two types of nucleophilic substitution reactions: SN1 and SN2.

### Step-by-Step Solution:

1. **Understanding Racemization**:
- Racemization refers to the process where a chiral compound is converted into a mixture of its enantiomers (equal amounts of R and S forms). This typically occurs when a reaction allows for the formation of a planar intermediate, which can be attacked from either side.

2. **Analyzing SN1 Mechanism**:
- In an SN1 reaction, the mechanism involves two steps:
- **Step 1**: The leaving group (e.g., Br) departs, forming a carbocation intermediate. This carbocation is planar and can be attacked by a nucleophile from either side.
- **Step 2**: The nucleophile attacks the carbocation, leading to the formation of both R and S products, resulting in racemization.
- Therefore, SN1 reactions can lead to racemization due to the formation of a planar carbocation.

3. **Analyzing SN2 Mechanism**:
- In an SN2 reaction, the mechanism is a single concerted step:
- The nucleophile attacks the substrate from the opposite side of the leaving group, leading to a direct displacement without forming a stable intermediate.
- Since there is no planar intermediate and the attack occurs from one side only, racemization does not occur in SN2 reactions.

4. **Conclusion**:
- Based on the analysis, racemization occurs in SN1 reactions due to the formation of a planar carbocation intermediate that allows for attack from both sides. In contrast, SN2 reactions do not allow for racemization because they proceed through a single transition state without forming a stable intermediate.

### Final Answer:
Racemization occurs in **SN1 reactions**.