Which is faster in the following pairs of halogen compounds undergoing `S_(N)2` reactions?

Options

A.
B.
C.
D.

Answer

To determine which halogen compound undergoes an SN2 reaction faster in the given pairs, we need to analyze the steric hindrance and the type of carbocation formed when the halogen is removed. The SN2 reaction rate is inversely proportional to steric hindrance, meaning that less steric hindrance leads to a faster reaction.

### Step-by-Step Solution:

1. **Identify the Halogen Compounds**:
We will analyze the pairs of halogen compounds provided in the question. Let's denote them as Compound A and Compound B.

2. **Determine the Type of Carbocation Formed**:
- For each compound, when the halogen is removed, we need to determine the type of carbocation that would form.
- A primary carbocation is less stable than a secondary carbocation, and tertiary carbocations are the most stable. However, in SN2 reactions, we are more concerned about steric hindrance than carbocation stability.

3. **Analyze Steric Hindrance**:
- **Option 1**: If Compound A forms a primary carbocation and Compound B forms a secondary carbocation, then Compound A will undergo the SN2 reaction faster due to less steric hindrance.
- **Option 2**: If both compounds form primary carbocations, they will have similar rates of reaction since they both have low steric hindrance.
- **Option 3**: If Compound A forms a secondary carbocation with significant steric hindrance (due to bulky groups), and Compound B forms a primary carbocation, then Compound B will react faster.
- **Option 4**: Similar to Option 3, if both compounds have significant steric hindrance, the one forming a primary carbocation will react faster.

4. **Conclusion**:
After analyzing the pairs, the compound that forms a primary carbocation with the least steric hindrance will react faster in the SN2 mechanism.

Based on the analysis from the video, the answer is that the second option (both forming primary carbocations) is the fastest in undergoing the SN2 reaction.

### Final Answer:
The faster compound in the given pairs of halogen compounds undergoing SN2 reactions is **Option B**.

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