Which one of the following compounds is most reactive towards $SN^{1}$ reaction?

Question

Accepted Answer

Option 3, $C_{6}H_{5}-C(CH_3)(Br)-C_{6}H_{5}$, is the most reactive towards an $SN^{1}$ reaction due to the stability of the carbocation formed during the reaction process. Here's a detailed explanation:

1. **Stability of Carbocation**: The $SN^{1}$ reaction mechanism involves the formation of a carbocation intermediate. The stability of this carbocation is crucial for the reaction rate. In Option 3, the compound is $C_{6}H_{5}-C(CH_3)(Br)-C_{6}H_{5}$, which, upon losing the bromide ion, forms a tertiary carbocation. This carbocation is highly stabilized by resonance with two phenyl groups. The phenyl groups can delocalize the positive charge through resonance, making the carbocation more stable compared to other options.

2. **Comparison with Other Options**:
   - **Option 1: $C_{6}H_{5}CH(Br)-C_{6}H_{5}$**: This compound forms a secondary carbocation upon losing the bromide ion. Although it is stabilized by resonance with one phenyl group, it is less stable than the tertiary carbocation in Option 3.
   - **Option 2: $C_{6}H_{5}CH(Br)-CH_3$**: This compound also forms a secondary carbocation, which is less stable than the tertiary carbocation in Option 3. It has only one phenyl group for resonance stabilization.
   - **Option 4: $(C_{6}H_{5})CH_{2}Br$**: This compound forms a primary carbocation, which is the least stable among all options. It has minimal resonance stabilization compared to the tertiary carbocation in Option 3.

3. **Conclusion**: The tertiary carbocation formed in Option 3 is the most stable due to the resonance stabilization provided by two phenyl groups. This increased stability leads to a faster $SN^{1}$ reaction rate, making Option 3 the most reactive compound towards the $SN^{1}$ reaction. Therefore, Option 3 is correctly identified as the most reactive compound for an $SN^{1}$ reaction.