Which of the following is least reactive towards `S_(N)1`?

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds is least reactive towards the \( S_N1 \) reaction, we need to analyze the stability of the carbocations that would form during the reaction. The \( S_N1 \) reaction involves two main steps: the formation of a carbocation intermediate and the subsequent attack by a nucleophile.

### Step-by-Step Solution:

1. **Understanding \( S_N1 \) Mechanism**:
- The \( S_N1 \) mechanism is a two-step process where the first step involves the formation of a carbocation. The stability of this carbocation is crucial because more stable carbocations will form more readily and thus lead to faster reactions.

2. **Identify the Compounds**:
- We need to analyze the given compounds (let's assume they are labeled A, B, C, and D) for their ability to form stable carbocations.

3. **Carbocation Stability**:
- Carbocation stability increases in the following order:
- **Tertiary > Secondary > Primary > Methyl**
- Additionally, resonance and inductive effects can stabilize carbocations. Compounds that can delocalize the positive charge through resonance will be more stable.

4. **Analyzing Each Compound**:
- **Compound A**: If it has resonance stabilization (like an allylic or benzylic carbocation), it will be more stable.
- **Compound B**: If it is a tertiary carbocation, it will be stable due to hyperconjugation and inductive effects.
- **Compound C**: If it is a secondary carbocation, it will be less stable than tertiary but more stable than primary.
- **Compound D**: If it is an anti-aromatic compound, it will be least stable due to the presence of 4n π electrons, making it anti-aromatic and thus less reactive.

5. **Conclusion**:
- Among the compounds analyzed, the one that forms an anti-aromatic carbocation (Compound D) will be the least reactive towards \( S_N1 \) reactions due to its instability.

### Final Answer:
- The compound that is least reactive towards \( S_N1 \) is **Compound D** (the anti-aromatic compound).