Question

Which of the following order is not correct?

• A. \( {MeBr}>{Me}_2 {CHBr}>{Me}_3 {CBr}>{Et}_3 {CBr} \) (SN2)
• B. \( {PhCH}_2 {Br}>{PhCHBrMe}>{PhCBrMe}_2>{PhCBrMePh} \) (SN1)
• C. \( {MeI}>{MeBr}>{MeCl}>{MeF} \) (SN2)
• D. All are correct

Hint:

In SN1 reactions, carbocation stability governs the reaction rate. More substituted carbocations (such as those with phenyl groups) are more stable.

Answer 1

The Correct Option is B

Step 1: In SN1 reactions, the rate-determining step involves the departure of the leaving group, followed by the formation of a carbocation. The more stable the carbocation, the faster the reaction.
Step 2: The stability of the carbocation depends on the groups attached to the carbon bearing the leaving group. In \( {PhCBrMePh} \), the carbocation would be less stable due to the large steric hindrance from the phenyl groups.
Step 3: Therefore, the order \( {PhCH}_2 {Br}>{PhCHBrMe}>{PhCBrMe}_2>{PhCBrMePh} \) is incorrect as it does not follow the correct trend based on carbocation stability.