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Question #44485

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The Question

When 3-(2-chloropropan-2-yl)cycloprop-1-ene is dissolved in a superacidic medium ( $\ce{SbCl5}$ in liquid $\ce{SO2}$ at $223\text{ K}$ ), a highly stable carbocation $\mathbf{X}$ is observed via NMR spectroscopy. Which of the following best describes the structure of $\mathbf{X}$ and the dominant thermodynamic driving force for its formation?

Options

A.
1-isopropylcyclopropenyl cation; driven by the generation of a $2\pi$ Hückel aromatic system, overriding the initial tertiary carbocation stability.
B.
4,4-dimethylcyclobut-2-en-1-yl cation; driven by the relief of severe angular ring strain via a 1,2-alkyl shift (ring expansion).
C.
2-(cycloprop-2-en-1-yl)propan-2-yl cation; driven by extensive hyperconjugation ( $6\alpha$ -H) and preservation of the isolated alkene.
D.
4-methylpenta-1,3-dien-1-yl cation; driven by a symmetry-allowed electrocyclic ring opening to completely relieve ring strain.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Aromaticity (Huckel’s Rule)|Hyperconjugation Logic|Cycloalkane Ring Strain

Why This Question Is Tricky

Common Mistakes

The question tests the student's ability to weigh competing thermodynamic driving forces: hyperconjugation vs. ring strain vs. aromaticity.

A standard tertiary carbocation is highly stable due to hyperconjugation.

Furthermore, small rings like cyclopropene possess massive angular strain, making ring expansion a typical pathway.

However, the 'hidden variable' is Hückel's Rule.

A 1,2-hydride shift generates a cyclopropenyl cation, which is a $2\pi$ aromatic system.

The resonance energy of this aromatic stabilization completely overrides both the loss of hyperconjugation (tertiary to secondary-like) and the retention of the strained three-membered ring.

Students who blindly follow 'tertiary is most stable' or 'always relieve ring strain' will fall into the traps.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Hydrocarbons - Class 11 Part 2
Concept	Aromaticity (Huckel’s Rule)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free