Which of the following will give `SN^(2)` mechanism?

Options

A.
B.
C.
D.

Explanation

To determine which of the given compounds will undergo an SN2 mechanism, we need to analyze the nature of each substituent on the carbon atom where the nucleophilic substitution is taking place. The SN2 mechanism is characterized by a single-step reaction where a nucleophile attacks the electrophilic carbon from the opposite side of the leaving group, leading to a transition state where both the nucleophile and the leaving group are partially bonded to the carbon.

### Step-by-Step Solution:

1. **Identify the Nature of Substituents**:
- Examine each compound and identify the substituents attached to the carbon that will undergo substitution.
- Look for electron-withdrawing groups (EWGs) and electron-donating groups (EDGs).

2. **Evaluate Electron-Withdrawing and Electron-Donating Effects**:
- **Methoxy Group (−OCH3)**: This group is an electron-donating group due to its +M (mesomeric) effect. It increases electron density on the carbon, making it less favorable for nucleophilic attack. Therefore, this compound will not undergo SN2.
- **Nitro Group (−NO2)**: This is a strong electron-withdrawing group (−M effect). It decreases electron density on the carbon, making it more electrophilic and favorable for nucleophilic attack. This compound is likely to undergo SN2.
- **Methyl Group (−CH3)**: The methyl group is an electron-donating group due to its +I (inductive) effect. It also increases electron density, which is not favorable for SN2. Thus, this compound will not undergo SN2.
- **Phenyl Group (−C6H5)**: The phenyl group exhibits a −I effect, but it also introduces significant steric hindrance. The presence of two bulky phenyl groups will hinder the approach of the nucleophile, making SN2 less favorable.

3. **Assess Steric Hindrance**:
- In SN2 reactions, steric hindrance is a critical factor. The more crowded the carbon atom is, the less likely it is for the nucleophile to successfully attack.
- The compound with the nitro group has less steric hindrance compared to the others, making it the most favorable for SN2.

4. **Conclusion**:
- Based on the analysis, the only compound that will undergo an SN2 mechanism is the one with the nitro group (−NO2).

### Final Answer:
The compound that will give an SN2 mechanism is the one with the nitro group (−NO2).