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

Options

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

Answer

To determine which of the given compounds 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 and then the attack of the nucleophile.

### Step-by-Step Solution:

1. **Understanding the \( S_N^1 \) Mechanism**:
- The \( S_N^1 \) mechanism involves two steps:
1. The leaving group departs, forming a carbocation.
2. A nucleophile attacks the carbocation to form the final product.
- The stability of the carbocation is crucial; more stable carbocations favor the \( S_N^1 \) pathway.

2. **Analyzing Each Compound**:
- **Benzyl Chloride (C6H5CH2Cl)**:
- When Cl leaves, it forms a benzyl carbocation (C6H5CH2+).
- This carbocation is stabilized by resonance with the benzene ring, making it highly stable.
- Therefore, benzyl chloride can undergo nucleophilic substitution via \( S_N^1 \) mechanism.

- **Isopropyl Chloride ((CH3)2CHCl)**:
- When Cl leaves, it forms a secondary carbocation ((CH3)2C+H).
- This carbocation is stable due to hyperconjugation but is less stable than the benzyl carbocation.
- It can undergo \( S_N^1 \), but it is not exclusive.

- **Chlorobenzene (C6H5Cl)**:
- When Cl leaves, it forms a phenyl carbocation (C6H5+).
- This carbocation is very unstable due to lack of resonance stabilization.
- Chlorobenzene does not favor the \( S_N^1 \) mechanism.

- **Ethyl Chloride (C2H5Cl)**:
- When Cl leaves, it forms a primary carbocation (C2H5+).
- Primary carbocations are very unstable and do not favor \( S_N^1 \) mechanism.
- Ethyl chloride does not undergo \( S_N^1 \).

3. **Conclusion**:
- Among the options analyzed, benzyl chloride is the only compound that undergoes nucleophilic substitution exclusively by the \( S_N^1 \) mechanism due to the stability of the benzyl carbocation formed.

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