Question

Which of the following options is the correct order of relative stabilities of cations I, II and III as writter below (most stable first)?

$$\text{I } \quad \begin{array}{c} \text{+} \\ \text{CH}_2 \end{array} \text{ (Benzene Ring)}$$ $$\text{II } \quad \text{H}_2\text{C} = \text{CH} - \text{CH}_2 - \overset{\text{+}}{\text{CH}} - \text{CH}_3$$ $$\text{III } \quad \text{H}_3\text{C} - \overset{\text{+}}{\text{C}}\text{H} - \text{CH}_2 - \text{CH}_3$$

• A. I > II > III
• B. II > III > I
• C. III > I > II
• D. I > III > II

Answer 1

Answer: (A)

I > II > III

Answer 2

Cation I is a benzyl carbocation, stabilized by resonance. Cation II is an allylic carbocation, stabilized by resonance. Cation III is a secondary carbocation, stabilized by hyperconjugation and inductive effect. Resonance stabilization is generally stronger than hyperconjugation. Benzyl carbocations are more stable than allylic carbocations due to more extensive resonance. Thus, the order of stability is I > II > III.