Question

Which of the following is correct order of stability of carbocation?

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

Answer 1

Answer: (D)

I > III > II > IV

Answer 2

The stability of carbocations is primarily influenced by resonance, hyperconjugation, and inductive effects. In the given structures, we have carbocations stabilized by cyclopropyl groups ($\sigma$-conjugation) and phenyl groups (resonance).

• Carbocation I is tricyclopropylmethyl carbocation. It is a tertiary carbocation stabilized by $\sigma$-conjugation from three cyclopropyl rings.
• Carbocation II is dicyclopropylmethyl carbocation. It is a secondary carbocation stabilized by $\sigma$-conjugation from two cyclopropyl rings.
• Carbocation III is triphenylmethyl carbocation. It is a tertiary carbocation stabilized by resonance from three phenyl rings.
• Carbocation IV is diphenylmethyl carbocation. It is a secondary carbocation stabilized by resonance from two phenyl rings.

Comparing tertiary and secondary carbocations with the same type of stabilizing group, tertiary is more stable than secondary. Thus, I > II and III > IV.

It is known that cyclopropylmethyl carbocation is more stable than benzyl carbocation. This suggests that $\sigma$-conjugation by a cyclopropyl group is more effective than resonance by a phenyl group.

Combining these, the correct order of stability is I > III > II > IV.