Which of the following is the most reactive towards \( S_N1 \) mechanism?
Show Hint
Reactivity for \( S_N1 \propto \text{Stability of Carbocation} \). Look for:
1. Resonance (Benzylic/Allylic)
2. Degree ($3^\circ \textgreater 2^\circ \textgreater 1^\circ$)
More phenyl groups attached to the cationic carbon usually mean higher stability.
The reactivity of alkyl halides towards SN1 reactions depends on the stability of the intermediate carbocation formed.
The more stable the carbocation, the faster the reaction.
Stability order: 3° > 2° > 1°
Also, resonance stabilization such as benzylic and allylic stabilization increases carbocation stability significantly.
Step 2: Key Formula or Approach:
In an SN1 reaction, the leaving group first departs to form a carbocation.
So, the compound that forms the most stable carbocation will show the highest reactivity.
Step 3: Detailed Explanation:
Let us analyze the carbocations formed after removal of Br-:
(A) Ph-CH2+
This is a primary benzylic carbocation.
It is stabilized by resonance with one phenyl ring.
(B) Ph-CH+-CH3
This is a secondary benzylic carbocation.
It is stabilized by resonance with one phenyl ring and also by the +I effect of the methyl group.
So, it is more stable than (A).
(C) Ph-CH+-Ph
This is a benzylic carbocation stabilized by resonance with two phenyl rings.
So, it is very stable.
(D) Ph-C+(CH3)-Ph
This carbocation is stabilized by resonance with two phenyl rings and also by the +I effect of the methyl group.
Hence, this is the most stable carbocation among all the options.
Since compound (D) forms the most stable carbocation, it will be the most reactive towards the SN1 reaction.
