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

Options

A. `CH_2=CHCl`
B. `CH_3CH_2Cl`
C. `CH_2=CHCH_2Cl`
D. `(CH_3)_3C-Cl`

Answer

To determine which compound is least reactive towards nucleophilic substitution (S_N2), we need to analyze the structure of the compounds given in the options. The key factors affecting reactivity in S_N2 reactions include steric hindrance, the nature of the leaving group, and resonance effects.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Let's assume we have the following options:
- A) Vinyl Chloride (C2H3Cl)
- B) Primary Alkyl Halide (e.g., CH3CH2Br)
- C) Tertiary Alkyl Halide (e.g., (CH3)3CBr)
- D) Allylic Halide (e.g., CH2=CH-CH2Cl)

2. **Understand the Nature of S_N2 Reactions**:
- S_N2 reactions involve a nucleophile attacking a carbon atom that is bonded to a leaving group. The reaction proceeds through a single transition state where the nucleophile and the leaving group are both partially bonded to the carbon.

3. **Analyze Vinyl Chloride (Option A)**:
- Vinyl chloride has a carbon-carbon double bond (C=C) with chlorine attached to one of the sp2 hybridized carbons. The presence of the double bond means that the carbon is less accessible to nucleophiles.
- Additionally, the chlorine atom exhibits a +M (mesomeric) effect, donating electron density to the double bond, which stabilizes the structure and makes the carbon less electrophilic.

4. **Compare with Other Options**:
- **Primary Alkyl Halide (Option B)**: This structure is less hindered and allows for easier nucleophilic attack, making it more reactive.
- **Tertiary Alkyl Halide (Option C)**: Although sterically hindered, tertiary halides can stabilize the carbocation formed during the reaction, making them quite reactive in S_N1 reactions but not S_N2.
- **Allylic Halide (Option D)**: This compound can stabilize the transition state through resonance, making it more reactive than vinyl chloride.

5. **Conclusion**:
- Among the given options, vinyl chloride (Option A) is the least reactive towards nucleophilic substitution (S_N2) due to the sp2 hybridization of the carbon atom, steric hindrance from the double bond, and resonance stabilization that makes the carbon less electrophilic.

### Final Answer:
**Vinyl Chloride (Option A) is the least reactive towards nucleophilic substitution (S_N2).**