Step 1: Understanding the Concept:
Nucleophilic substitution (\( \text{S}_{\text{N}}1 \) or \( \text{S}_{\text{N}}2 \)) reactivity depends on the nature of the C-X bond and the stability of the intermediate carbocation (for \( \text{S}_{\text{N}}1 \)) or the steric hindrance and transition state (for \( \text{S}_{\text{N}}2 \)).
Step 2: Detailed Explanation:
(A) Vinyl Chloride (\( \text{CH}_{2}=\text{CH-Cl} \)): The lone pair of Chlorine is in resonance with the double bond, giving the C-Cl bond a partial double bond character. It is very hard to break.
(B) Chlorobenzene (\( \text{C}_{6}\text{H}_{5}\text{Cl} \)): Similar to vinyl chloride, resonance with the benzene ring gives the C-Cl bond partial double bond character. It is extremely inert to nucleophilic substitution.
(C) 1-Chloroprop-1-ene (\( \text{CH}_{3}\text{CH=CH-Cl} \)): This is also a vinyl halide. The C-Cl bond is stabilized by resonance with the adjacent double bond.
(D) Allyl Chloride (\( \text{ClCH}_{2}-\text{CH}=\text{CH}_{2} \)): In this molecule, the Chlorine is attached to an \( sp^{3} \) hybridized carbon adjacent to a double bond.
- In \( \text{S}_{\text{N}}1 \), it forms an Allyl carbocation (\( \text{CH}_{2}^{+}-\text{CH}=\text{CH}_{2} \)), which is highly stable due to resonance.
- In \( \text{S}_{\text{N}}2 \), the \( \pi \)-electron cloud of the double bond stabilizes the transition state.
Thus, Allyl halides are much more reactive than normal alkyl halides and significantly more reactive than vinyl or aryl halides.
Step 3: Final Answer:
The most reactive compound is Allyl chloride (\( \text{ClCH}_{2} - \text{CH} = \text{CH}_{2} \)).