Which of the following reactions cannot proceed by `S_(N)1` mechaism ?

Options

A.
B.
C.
D.

Explanation

To determine which of the given reactions cannot proceed by the \( S_N1 \) mechanism, we need to analyze the stability of the carbocations formed in each reaction. The \( S_N1 \) mechanism involves the formation of a carbocation intermediate, and the stability of this carbocation is crucial for the reaction to proceed.

### Step-by-Step Solution:

1. **Understand the \( S_N1 \) Mechanism**:
- The \( S_N1 \) mechanism involves two steps:
1. Formation of a carbocation by the departure of a leaving group (like Cl).
2. Nucleophilic attack on the carbocation.
- The stability of the carbocation is key; more stable carbocations favor \( S_N1 \) reactions.

2. **Identify the Stability of Carbocations**:
- Carbocations can be classified based on their degree:
- **Tertiary (3°)**: Most stable due to hyperconjugation and inductive effects.
- **Secondary (2°)**: Moderately stable.
- **Primary (1°)**: Least stable and often does not form via \( S_N1 \).
- **Methyl**: Very unstable, does not form via \( S_N1 \).

3. **Analyze Each Option**:
- **Option A**: Forms a carbocation that is stabilized by resonance from a ring structure. This reaction can proceed via \( S_N1 \).
- **Option B**: Forms a tertiary carbocation upon the departure of Cl. This reaction can proceed via \( S_N1 \).
- **Option C**: Forms a secondary carbocation upon the departure of Cl. This reaction can also proceed via \( S_N1 \).
- **Option D**: Forms a primary carbocation upon the departure of Cl. Primary carbocations are very unstable and do not favor \( S_N1 \); they typically undergo \( S_N2 \) reactions instead.

4. **Conclusion**:
- The reaction that cannot proceed by \( S_N1 \) mechanism is the one that forms a primary carbocation, which is found in **Option D**.

### Final Answer:
**Option D** cannot proceed by \( S_N1 \) mechanism.