Which one of the following compounds most readily undergoes substitution by `S_(N^(2))` mechanism?

Options

A.
B.
C.
D.

Answer

To determine which compound most readily undergoes substitution by the \( S_N2 \) mechanism, we need to analyze the stability of the carbocations formed during the reaction and the steric hindrance around them. Here’s a step-by-step solution:

### Step 1: Understand the \( S_N2 \) Mechanism
The \( S_N2 \) mechanism involves a nucleophile attacking the electrophilic carbon atom from the opposite side of the leaving group, leading to a simultaneous bond formation and bond breaking. The rate of this reaction is influenced by the steric hindrance around the carbon atom that is attached to the leaving group.

### Step 2: Identify the Stability of Carbocations
The stability of carbocations is crucial in determining the reactivity in \( S_N2 \) reactions. The order of stability for carbocations is:
- Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl

Since \( S_N2 \) reactions are inversely proportional to the stability of the carbocation, we want to favor the least stable carbocation.

### Step 3: Analyze the Given Compounds
Let’s assume we have four options (A, B, C, D) with different alkyl halides. We need to analyze each compound to determine the type of carbocation formed upon the departure of the leaving group (halide).

1. **Option A**: Forms a tertiary carbocation.
2. **Option B**: Forms a primary carbocation.
3. **Option C**: Forms a secondary carbocation.
4. **Option D**: Forms a methyl carbocation.

### Step 4: Evaluate Steric Hindrance
In addition to carbocation stability, steric hindrance plays a significant role in \( S_N2 \) reactions. More bulky groups around the electrophilic carbon will hinder the nucleophile's approach.

- **Option A**: Tertiary carbocation with significant steric hindrance due to bulky groups.
- **Option B**: Primary carbocation with minimal steric hindrance.
- **Option C**: Secondary carbocation with moderate steric hindrance.
- **Option D**: Methyl carbocation with no steric hindrance.

### Step 5: Conclusion
Since \( S_N2 \) reactions are favored by less stable carbocations and minimal steric hindrance, the compound that most readily undergoes substitution by the \( S_N2 \) mechanism will be the one that forms a primary carbocation with the least steric hindrance.

Thus, **Option B** is the correct answer as it forms a primary carbocation and has the least steric hindrance.