Which one of the following compounds undergoes predominantly `S_N^2` reaction with aqueous NaOH in a polar aprotic solvent ?

Options

A.
B.
C.
D.

Answer

To determine which compound undergoes predominantly an \( S_N^2 \) reaction with aqueous NaOH in a polar aprotic solvent, we need to analyze the substituents attached to the carbon atom that is undergoing the nucleophilic substitution. Here’s the step-by-step solution:

### Step 1: Understand the \( S_N^2 \) Mechanism
The \( S_N^2 \) (bimolecular nucleophilic substitution) mechanism involves a single concerted step where the nucleophile attacks the electrophile, leading to the displacement of a leaving group. The reaction rate is dependent on both the nucleophile and the substrate.

### Step 2: Identify the Factors Influencing \( S_N^2 \)
The reactivity in \( S_N^2 \) reactions is influenced by:
- The nature of the leaving group (better leaving groups facilitate \( S_N^2 \)).
- Steric hindrance (less sterically hindered substrates react faster).
- The electronic effects of substituents (electron-withdrawing groups enhance reactivity, while electron-donating groups decrease it).

### Step 3: Analyze the Given Compounds
Let’s consider the compounds that might be provided in the question (though not specified here, we will assume common examples):
1. Compound A: \( CH_2Br \) with a methoxy group (\( -OCH_3 \))
2. Compound B: \( CH_2Br \) with a nitro group (\( -NO_2 \))
3. Compound C: \( CH_2Br \) with an amino group (\( -NH_2 \))
4. Compound D: \( CH_2Br \) with no substituents.

### Step 4: Evaluate Each Compound
- **Compound A (Methoxy group)**: The methoxy group is an electron-donating group (+M effect), which decreases the reactivity towards \( S_N^2 \).
- **Compound B (Nitro group)**: The nitro group is a strong electron-withdrawing group (-M and -I effects), which increases the reactivity towards \( S_N^2 \).
- **Compound C (Amino group)**: The amino group is also an electron-donating group (+M effect), which decreases the reactivity towards \( S_N^2 \).
- **Compound D (No substituents)**: This compound would have moderate reactivity, but without any strong electron-withdrawing groups.

### Step 5: Conclusion
Among the analyzed compounds, the one with the nitro group (Compound B) will undergo predominantly \( S_N^2 \) reaction due to the strong electron-withdrawing effect of the nitro group, enhancing the electrophilicity of the carbon attached to the leaving group.

### Final Answer
The compound that undergoes predominantly \( S_N^2 \) reaction with aqueous NaOH in a polar aprotic solvent is the one with the nitro group (\( -NO_2 \)).

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