Which of the following is the most reactive towards nucleophilic substitution reaction?

Options

A. `ClCH_2-CH=CH_2`
B. `CH_2=CH-Cl`
C. `CH_3CH=CH-Cl`
D. `C_6H_5Cl`

Explanation

To determine which of the given reactants is most reactive towards nucleophilic substitution reactions, we need to analyze the stability of the carbocations formed during the reaction. Here's a step-by-step solution:

### Step 1: Identify the Reactants
The reactants given are:
1. ClCH2-CH=CH2 (Allylic halide)
2. CH2=CH-CN (Vinylic halide)
3. CH3-CH=CH-CN (Allylic halide)
4. Chlorobenzene (C6H5Cl)

### Step 2: Understand Nucleophilic Substitution
Nucleophilic substitution reactions involve the attack of a nucleophile on a carbon atom that is bonded to a leaving group (in this case, a halogen). The stability of the carbocation intermediate formed during this process is crucial for determining the reactivity of the compound.

### Step 3: Analyze the Reactants
- **First Reactant (Allylic Halide)**: ClCH2-CH=CH2
- When this compound undergoes nucleophilic substitution, it forms an allylic carbocation (CH2=CH-CH2+). Allylic carbocations are relatively stable due to resonance stabilization.

- **Second Reactant (Vinylic Halide)**: CH2=CH-CN
- This compound forms a vinylic carbocation (CH2=C+-CN) upon nucleophilic attack. Vinylic carbocations are generally less stable than allylic carbocations due to the lack of resonance stabilization.

- **Third Reactant (Allylic Halide)**: CH3-CH=CH-CN
- Similar to the first reactant, this also forms an allylic carbocation (CH2=CH-CH3+), which is stable due to resonance.

- **Fourth Reactant (Chlorobenzene)**: C6H5Cl
- Chlorobenzene does not readily undergo nucleophilic substitution due to the stability of the aromatic ring and the lack of a suitable carbocation formation.

### Step 4: Compare the Stability of Carbocations
- Allylic carbocations (from the first and third reactants) are more stable than vinylic carbocations (from the second reactant).
- Among the allylic carbocations, the one from the first reactant (ClCH2-CH=CH2) is more stable due to the presence of the chlorine atom, which can stabilize the positive charge through inductive effects.

### Step 5: Conclusion
Based on the stability of the carbocations formed, the most reactive compound towards nucleophilic substitution is the first reactant: **ClCH2-CH=CH2**.

### Final Answer
**The most reactive towards nucleophilic substitution reaction is ClCH2-CH=CH2 (the first reactant).**