Which of the following undergoes nucleophilic substitution exclusively by `S_(N^(1))` mechanism?

Options

A. Ethyl chloride
B. Isopropyl chloride
C. Benzyl chloride
D. Chlorobenzene

Answer

To determine which of the following undergoes nucleophilic substitution exclusively by the \( S_N^1 \) mechanism, we need to analyze the stability of the carbocations formed during the reaction. The \( S_N^1 \) mechanism involves two main steps: the formation of a carbocation intermediate and the subsequent attack of the nucleophile.

### Step-by-Step Solution:

1. **Understanding the \( S_N^1 \) Mechanism**:
- The \( S_N^1 \) (Substitution Nucleophilic Unimolecular) mechanism is a two-step process:
1. Formation of a carbocation by the departure of the leaving group (halide ion).
2. Nucleophilic attack on the carbocation to form the final product.
- The rate of the reaction depends on the stability of the carbocation formed.

2. **Analyzing the Compounds**:
- We need to consider the stability of the carbocations that would form from the given compounds:
- **Benzyl Chloride (C₆H₅CH₂Cl)**: When the chlorine leaves, it forms a benzyl carbocation (C₆H₅CH₂⁺), which is stabilized by resonance with the aromatic ring.
- **Isopropyl Chloride ((CH₃)₂CHOCl)**: This forms a secondary carbocation, which is relatively stable due to hyperconjugation.
- **Ethyl Chloride (C₂H₅Cl)**: This forms a primary carbocation, which is less stable.
- **Chlorobenzene (C₆H₅Cl)**: The formation of a carbocation from chlorobenzene is not favorable due to the strong carbon-chlorine bond and the sp² hybridization of the carbon.

3. **Comparing Carbocation Stability**:
- The stability order of carbocations is:
- Benzyl carbocation > Isopropyl carbocation > Ethyl carbocation > Primary carbocation (from ethyl chloride).
- Benzyl carbocation is particularly stable due to resonance stabilization from the adjacent aromatic ring.

4. **Conclusion**:
- Among the compounds analyzed, **Benzyl Chloride** undergoes nucleophilic substitution exclusively by the \( S_N^1 \) mechanism because it forms a highly stable carbocation that is stabilized by resonance.

### Final Answer:
**Benzyl Chloride (C₆H₅CH₂Cl)** undergoes nucleophilic substitution exclusively by the \( S_N^1 \) mechanism.