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MCQMediumJEE 2023Electronic Effects (Inductive, Resonance, Hyperconjugation)

JEE Chemistry 2023 Question with Solution

The number of hyperconjugation structures involved to stabilize carbocation formed in the below reaction is _____

Fused cyclohexane and cyclobutane ring system bearing a methyl group and an OH group, reacting with HBr to give product A as major product.
  • A

    66

  • B

    77

  • C

    88

  • D

    99

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: A carbocation is formed in the reaction shown, and we need the number of hyperconjugation structures that stabilize it.

Find: The total number of hyperconjugation structures.

Hyperconjugation involves delocalization of electrons from adjacent C-H\text{C-H} or C-C\text{C-C} bonds to stabilize a carbocation.

From the given solution, after formation of the carbocation and rearrangement, the carbocation is stabilized by adjacent bonds through hyperconjugation. Counting all such hyperconjugative structures, including the shifted carbocation form, gives a total of 77 structures.

Therefore, the correct option is B.

Common mistakes

  • Counting only the initially formed carbocation is incorrect because the solution indicates that carbocation shifting must also be considered. Include the rearranged carbocation before counting hyperconjugative structures.

  • Treating hyperconjugation as involving only one adjacent bond is incorrect. All adjacent eligible C-H\text{C-H} bonds that can donate electron density to the carbocation center must be counted.

  • Confusing inductive effect with hyperconjugation is incorrect. Here the count is of no-bond resonance-like hyperconjugative structures, not the general electron-releasing effect of alkyl groups.

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