Which one is the most reactive towards `S_(N)1` reaction ?

Options

A. `Ph- CH_(2) -Br`
B. `Ph- underset(Ph)underset(|) (CH) -Br`
C. `Ph - underset(CH_(3))underset(|)(CH) - Br`
D. `Ph- overset(CH)overset(|)underset(Ph)underset(|)(C )-Br`

Answer

To determine which compound is the most reactive towards an SN1 reaction, we need to analyze the stability of the carbocations formed during the reaction. The stability of carbocations is crucial because the SN1 mechanism involves the formation of a carbocation intermediate.

### Step-by-Step Solution:

1. **Understand the SN1 Mechanism**:
- The SN1 reaction is a two-step process. The first step involves the formation of a carbocation, and the second step involves the nucleophile attacking the carbocation.
- The stability of the carbocation formed is the key factor in determining the reactivity of the compound towards the SN1 reaction.

2. **Identify Carbocation Types**:
- Carbocations can be classified as primary, secondary, or tertiary based on the number of alkyl groups attached to the positively charged carbon:
- **Primary Carbocation**: One alkyl group attached (least stable).
- **Secondary Carbocation**: Two alkyl groups attached (more stable than primary).
- **Tertiary Carbocation**: Three alkyl groups attached (most stable).

3. **Analyze the Given Compounds**:
- Convert each compound into its corresponding carbocation:
- If a compound forms a primary carbocation, it will be the least reactive.
- If it forms a secondary carbocation, it will be more reactive than primary but less than tertiary.
- If it forms a tertiary carbocation, it will be the most reactive.

4. **Determine the Most Reactive Compound**:
- After analyzing the compounds, identify which one forms a tertiary carbocation.
- The compound that leads to the formation of a tertiary carbocation will be the most reactive towards the SN1 reaction.

5. **Conclusion**:
- The fourth compound, which forms a tertiary carbocation, is the most reactive towards the SN1 reaction due to the stability of the tertiary carbocation.

### Final Answer:
The fourth compound is the most reactive towards the SN1 reaction.