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

Options

A. `C_(6)H_(5)CH(C_(6)H_(5))Br`
B. `C_(6)H_(5)CH(CH_(3))Br`
C. `C_(6)H_(5)C(CH_(3))(C_(6)H_(5))Br`
D. `C_(6)H_(5)CH_(2)Br`

Explanation

To determine which compound is most reactive towards SN1 reactions, we need to analyze the stability of the carbocations that would form from each compound. The stability of carbocations follows the order: tertiary > secondary > primary. Thus, the more stable the carbocation, the more reactive the compound will be in an SN1 reaction.

### Step-by-Step Solution:

1. **Identify the Compounds**: We have four options to analyze. Let's denote them as:
- Option A: C6H5-CHBr-C6H5 (a secondary carbocation)
- Option B: C6H5-CH3-CHBr (a secondary carbocation)
- Option C: C6H5-C(CH3)-C6H5 (a tertiary carbocation)
- Option D: Benzyl bromide (C6H5-CH2Br, a primary carbocation)

2. **Determine the Type of Carbocation Formed**:
- **Option A**: Forms a secondary carbocation when the bromine leaves.
- **Option B**: Also forms a secondary carbocation.
- **Option C**: Forms a tertiary carbocation upon the departure of bromine.
- **Option D**: Forms a primary carbocation.

3. **Evaluate Carbocation Stability**:
- **Secondary Carbocations** (Options A and B): These are less stable than tertiary carbocations.
- **Tertiary Carbocation** (Option C): This is the most stable due to the presence of three alkyl groups that can donate electron density.
- **Primary Carbocation** (Option D): This is the least stable as it has only one alkyl group.

4. **Rank the Reactivity**:
- Since the reactivity in SN1 reactions increases with carbocation stability, we can rank the compounds:
- Option C (tertiary) > Options A and B (secondary) > Option D (primary).

5. **Conclusion**: The compound that is most reactive towards SN1 reactions is **Option C**, which forms a tertiary carbocation.

### Final Answer:
**Option C** is the most reactive towards SN1 reactions.

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