Alkyl halide undergoes nucleophilic substitution reaction in which halogen atom is replaced by other atom.
`RX+ Nu:^(-) rarr Nu- R + X^(-)`
Which of the following is least reactive towards `SN_(2)` mechanism?

Options

A. `(CH_(3))_(3)C CH_(2)Br`
B. `(CH_(3))_(2) CHCH_(2)Br`
C. `CH_(3)CH_(2)Br`
D. `(CH_(3))_(2)CHBr`

Explanation

To determine which alkyl halide is least reactive towards the SN2 mechanism, we need to analyze the structure of the given alkyl halides and their degree of substitution.

### Step-by-Step Solution:

1. **Understand the SN2 Mechanism**:
- The SN2 (Substitution Nucleophilic Bimolecular) mechanism involves a nucleophile attacking the carbon atom that is bonded to the leaving group (halogen) from the opposite side, leading to a simultaneous bond formation and bond breaking. This results in an inversion of configuration at the carbon center.

2. **Identify the Degree of Alkyl Halides**:
- The reactivity of alkyl halides in SN2 reactions decreases with increasing steric hindrance. The order of reactivity is:
- 1° (primary) > 2° (secondary) > 3° (tertiary).

3. **Analyze the Given Options**:
- Let's denote the options:
- Option 1: \( \text{CH}_3\text{CH}(\text{CH}_3)_2\text{Br} \) (This is a 2° alkyl halide)
- Option 2: \( \text{C}(\text{CH}_3)_3\text{Br} \) (This is a 3° alkyl halide)
- Option 3: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{Br} \) (This is a 1° alkyl halide)

4. **Determine the Least Reactive Alkyl Halide**:
- Since 3° alkyl halides are the least reactive in SN2 reactions due to steric hindrance, option 2 (\( \text{C}(\text{CH}_3)_3\text{Br} \)) is the least reactive towards the SN2 mechanism.

5. **Conclusion**:
- The alkyl halide that is least reactive towards the SN2 mechanism is option 2: \( \text{C}(\text{CH}_3)_3\text{Br} \).