NGP (neighouring group of participation) assistance for `S_(N)2` reaction will not be seen in -

Options

A.
B. `CH_3-S-CH_2-CH_2-Cl`
C.
D.

Answer

To determine which reaction will not exhibit neighboring group participation (NGP) assistance in an \( S_N2 \) reaction, we need to analyze the conditions required for NGP to occur.

### Step-by-Step Solution:

1. **Understanding NGP**: Neighboring group participation (NGP) occurs when a nucleophile from a neighboring group attacks the electrophile (the carbon attached to the leaving group) during an \( S_N2 \) reaction. For this to happen, two conditions must be satisfied:
- There must be a good leaving group (e.g., Cl, Br, I, H2O, etc.).
- There must be a neighboring atom or group with a lone pair of electrons that can act as a nucleophile.

2. **Analyzing the Options**:
- **Option A**: Contains a good leaving group (Cl) and a neighboring group with a lone pair (e.g., an alcohol or amine). This can participate in NGP.
- **Option B**: Also has a good leaving group (Cl) and a neighboring sulfur atom with a lone pair. This can also participate in NGP.
- **Option C**: Lacks a good leaving group. The presence of an OH group does not suffice as it is not a good leaving group. Therefore, it cannot participate in NGP.
- **Option D**: Contains two leaving groups (I and Br). Iodine is a good leaving group and can leave, while bromine can act as a nucleophile. This can also participate in NGP.

3. **Conclusion**: Since option C does not have a good leaving group, it will not show NGP assistance for the \( S_N2 \) reaction.

### Final Answer:
The reaction that will not show NGP assistance for the \( S_N2 \) reaction is **Option C**.