Which is least reactive towards nucleophilic substitution `(S_(N^(2)))`

Options

A.
B. `Me_(3)C - Cl`
C. `PhCl`
D. `MeCH(Cl)Me`

Explanation

To determine which compound is least reactive towards nucleophilic substitution (SN2), we need to analyze the reactivity of the given alkyl halides based on their structure and steric hindrance. Here’s a step-by-step solution:

### Step 1: Understand the SN2 Mechanism
- The SN2 reaction is a bimolecular nucleophilic substitution that occurs in a single step. The nucleophile attacks the carbon atom from the opposite side of the leaving group (halogen), resulting in the inversion of configuration at that carbon.

**Hint:** Remember that steric hindrance affects the rate of SN2 reactions; less steric hindrance leads to higher reactivity.

### Step 2: Analyze the Alkyl Halides
- We have four alkyl halides to consider. The reactivity of these compounds in SN2 reactions is influenced by their structure (primary, secondary, tertiary) and whether they are allylic or benzylic.

**Hint:** Identify the type of each alkyl halide (primary, secondary, tertiary, allylic, or benzylic) to assess their reactivity.

### Step 3: Identify the Most Reactive Compounds
- **Allylic Halide:** This type of halide is typically more reactive in SN2 reactions due to less steric hindrance and resonance stabilization.
- **Tertiary Alkyl Halide:** Tertiary halides are the least reactive in SN2 reactions due to high steric hindrance.
- **Secondary Alkyl Halide:** Secondary halides can undergo SN2 but are less reactive than primary and allylic halides.
- **Aryl Halide (Chlorobenzene):** Aryl halides have a partial double bond character in the carbon-halogen bond, making them less reactive towards SN2.

**Hint:** Compare the steric hindrance and bond characteristics of each compound to determine their reactivity.

### Step 4: Determine the Least Reactive Compound
- Among the compounds, the aryl halide (chlorobenzene) is the least reactive towards SN2. This is because the carbon-chlorine bond has partial double bond character, making it more stable and difficult to break compared to single bonds in alkyl halides.

**Hint:** Focus on the bond characteristics; bonds with partial double bond character are more stable and less reactive in nucleophilic substitutions.

### Conclusion
- The least reactive compound towards nucleophilic substitution (SN2) is **chlorobenzene** (option C) due to the stability of the carbon-chlorine bond arising from its partial double bond character.

**Final Answer:** Chlorobenzene (option C) is the least reactive towards SN2.