Vinyl chloride does not give `S_(N)` reaction but allyl chloride gives. Explain.

Answer

To explain why vinyl chloride does not undergo SN (substitution nucleophilic) reactions while allyl chloride does, we can analyze the structures and stability of the carbocations formed during these reactions.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **Vinyl Chloride**: The structure of vinyl chloride is CH₂=CHCl. It has a carbon-carbon double bond and a chlorine atom attached to one of the carbons.
- **Allyl Chloride**: The structure of allyl chloride is CH₂=CH-CH₂Cl. It has a carbon-carbon double bond and a chlorine atom attached to a carbon that is adjacent to the double bond.

2. **Formation of Carbocations**:
- In a nucleophilic substitution reaction (SN1 or SN2), the first step often involves the formation of a carbocation after the halide ion (Cl⁻) leaves.
- For **vinyl chloride**, if we remove the Cl⁻ ion, we form the carbocation CH₂=CH⁺. This carbocation is unstable because it does not have any resonance stabilization.
- For **allyl chloride**, when Cl⁻ is removed, we form the carbocation CH₂=CH-CH₂⁺. This carbocation is stable due to resonance; the positive charge can be delocalized over the adjacent double bond.

3. **Stability of Carbocations**:
- The stability of carbocations is crucial for the likelihood of an SN reaction. Stable carbocations are more likely to form and participate in nucleophilic substitution.
- The carbocation from vinyl chloride (CH₂=CH⁺) is a primary carbocation and lacks resonance stabilization, making it highly unstable.
- The carbocation from allyl chloride (CH₂=CH-CH₂⁺) is stabilized by resonance, as the positive charge can be shared between the carbons involved in the double bond.

4. **Conclusion**:
- Since vinyl chloride forms an unstable carbocation that cannot participate effectively in nucleophilic substitution, it does not undergo SN reactions.
- In contrast, allyl chloride forms a stable carbocation that can undergo nucleophilic substitution reactions due to its resonance stabilization.

### Summary:
- **Vinyl Chloride**: Forms an unstable carbocation (CH₂=CH⁺) → Does not undergo SN reactions.
- **Allyl Chloride**: Forms a stable carbocation (CH₂=CH-CH₂⁺) → Undergoes SN reactions.