The compound which is least reactive among the following in a nucleophilic substitution reaction is

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 in a nucleophilic substitution reaction, we need to analyze the structures of the given compounds, particularly focusing on their ability to undergo SN2 reactions.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We need to look at the compounds provided in the question. Although they are not listed here, we will assume one of them is a vinyl halide (like CH2=CHCl) and others are typical alkyl halides.

2. **Understand Nucleophilic Substitution (SN2) Reactions**:
In an SN2 reaction, the nucleophile attacks the carbon atom bonded to the leaving group (in this case, a halogen), leading to the substitution of the halogen by the nucleophile. The reactivity of the carbon-halogen bond is crucial.

3. **Analyze the Vinyl Halide**:
The vinyl halide (e.g., CH2=CHCl) has a carbon-carbon double bond. The presence of resonance in vinyl halides means that the C-Cl bond has partial double bond characteristics due to electron delocalization. This makes the bond stronger and harder to break, resulting in lower reactivity in nucleophilic substitution reactions.

4. **Compare with Other Compounds**:
- **Tertiary Alkyl Halides**: These are generally very reactive in SN1 reactions due to the stability of the tertiary carbocation formed after the leaving group departs. However, they can also react via SN2 mechanisms, but steric hindrance can slow this down.
- **Primary and Secondary Alkyl Halides**: These compounds typically react more readily in SN2 reactions because they do not have the same steric hindrance as tertiary halides and do not have resonance stabilization like vinyl halides.

5. **Conclusion**:
Given the analysis, the vinyl halide is the least reactive in nucleophilic substitution reactions due to the resonance stabilization of the C-Cl bond, making it harder to break compared to the other alkyl halides.

### Final Answer:
The compound which is least reactive among the following in a nucleophilic substitution reaction is the vinyl halide (e.g., CH2=CHCl).