which of the following statement is correct for the reactivity in `S_(N)2` reaction ?

Options

A.
B.
C.
D. is more reactive than

Answer

To determine the correct statement regarding the reactivity in an SN2 reaction, we need to analyze the nature of the substrates involved in the reaction. Here's a step-by-step breakdown:

### Step 1: Understand SN2 Reaction
- **Definition**: SN2 stands for bimolecular nucleophilic substitution. In this reaction, the rate depends on the concentration of both the substrate (alkyl halide) and the nucleophile.
- **Mechanism**: The nucleophile attacks the carbon atom from the opposite side of the leaving group, resulting in a transition state where both the nucleophile and the leaving group are partially bonded to the carbon.

### Step 2: Analyze the Reactivity of Different Alkyl Halides
- **Primary vs. Secondary Alkyl Halides**:
- Primary alkyl halides are generally more reactive in SN2 reactions compared to secondary alkyl halides. This is due to steric hindrance; primary halides have less crowding around the carbon atom, allowing easier access for the nucleophile.

### Step 3: Evaluate the Given Statements
1. **Statement A**: A secondary alkyl halide is less reactive than a primary alkyl halide.
- **Analysis**: This statement is **correct** because primary alkyl halides are favored in SN2 reactions due to less steric hindrance.

2. **Statement B**: A primary alkyl halide with a double bond is more reactive than a primary alkyl halide without a double bond.
- **Analysis**: This statement is **incorrect**. The presence of a double bond can create electron-withdrawing effects that make the carbon less nucleophilic and thus less reactive in SN2.

3. **Statement C**: A secondary alkyl halide is more reactive than a primary alkyl halide.
- **Analysis**: This statement is **incorrect** for the same reason as above; primary alkyl halides are more reactive than secondary ones in SN2 reactions.

4. **Statement D**: A molecule with chlorine is more reactive than a molecule with iodine.
- **Analysis**: This statement is **incorrect**. Iodine is a better leaving group than chlorine, making the molecule with iodine more reactive in SN2 reactions.

### Conclusion
The only correct statement is **Statement A**: A secondary alkyl halide is less reactive than a primary alkyl halide.