For an `SN^(2)` reaction of `CH_(3)-overset(Me)overset(|)CH-CH_(2)-X` the most effective nucleophile will be

Options

A. `MeO^(-)`
B.
C. `Me_(2)CHO^(-)`
D. `MeCH_(2)O^(-)`

Answer

To determine the most effective nucleophile for the given \( SN^2 \) reaction of \( CH_3-CH(CH_3)-CH_2-X \), we need to analyze the nucleophiles provided and their characteristics.

### Step-by-Step Solution:

1. **Identify the Structure**:
The compound \( CH_3-CH(CH_3)-CH_2-X \) is a primary alkyl halide. In \( SN^2 \) reactions, the nucleophile attacks the carbon atom that is bonded to the leaving group (X).

2. **List Possible Nucleophiles**:
The potential nucleophiles mentioned are:
- Methoxide ion (\( CH_3O^- \))
- Tertiary cyclopentoxide ion
- Dimethylcarbinol ion (\( CH_3)_2CHO^- \))
- Isopropoxide ion (\( CH_3CH(OH)CH_3^- \))
- Ethoxide ion (\( CH_3CH_2O^- \))

3. **Analyze Nucleophilicity**:
In \( SN^2 \) reactions, the effectiveness of a nucleophile is influenced by its size and steric hindrance. Smaller, less bulky nucleophiles are generally more effective because they can approach the electrophilic carbon more easily.

4. **Compare the Nucleophiles**:
- **Methoxide ion**: Small and less bulky.
- **Tertiary cyclopentoxide ion**: Larger and more sterically hindered.
- **Dimethylcarbinol ion**: Also bulky due to the two methyl groups.
- **Isopropoxide ion**: Bulky due to the isopropyl group.
- **Ethoxide ion**: Less bulky than isopropoxide but bulkier than methoxide.

5. **Determine the Most Effective Nucleophile**:
Among the listed nucleophiles, the methoxide ion (\( CH_3O^- \)) is the least bulky and therefore the most effective nucleophile for the \( SN^2 \) reaction.

### Conclusion:
The most effective nucleophile for the \( SN^2 \) reaction of \( CH_3-CH(CH_3)-CH_2-X \) is the **methoxide ion** (\( CH_3O^- \)).