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Question #45060

Difficulty: 8/10 | 🟢 Free

The Question

The electrophilic addition of \ce{HCl(g)} to 3,3-dimethylbut-1-ene in anhydrous \ce{CH2Cl2} proceeds via a secondary carbocation intermediate. This intermediate faces a kinetic competition: it can either undergo direct nucleophilic capture by \ce{Cl^-} to form product P1P_1, or undergo a 1,2-methyl shift followed by capture to form product P2P_2. The activation energy for the direct capture is Ea,cap=15.0 kJ/molE_{a,cap} = 15.0 \text{ kJ/mol}, while the activation energy for the rearrangement is Ea,rearr=27.45 kJ/molE_{a,rearr} = 27.45 \text{ kJ/mol}. The ratio of their pre-exponential factors is ArearrAcap=e10\frac{A_{rearr}}{A_{cap}} = e^{10}. Assuming R=8.3 J K−1mol−1R = 8.3 \text{ J K}^{-1} \text{mol}^{-1}, identify the major product formed at 100 K100 \text{ K} and 300 K300 \text{ K} respectively, along with its stereochemical nature.

Options

  • A.

    100 K100 \text{ K}: Racemic 3-chloro-2,2-dimethylbutane ; 300 K300 \text{ K}: Achiral 2-chloro-2,3-dimethylbutane

  • B.

    100 K100 \text{ K}: Achiral 2-chloro-2,3-dimethylbutane ; 300 K300 \text{ K}: Racemic 3-chloro-2,2-dimethylbutane

  • C.

    100 K100 \text{ K}: Racemic 2-chloro-3,3-dimethylbutane ; 300 K300 \text{ K}: Achiral 2-chloro-2,3-dimethylbutane

  • D.

    Racemic 3-chloro-2,2-dimethylbutane is the major product at both temperatures due to the lower activation energy of capture.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Hyperconjugation Logic|Graphical Slopes Analysis

Why This Question Is Tricky

Common Mistakes

The problem targets the 'Kinetic vs Thermodynamic Control' exception in carbocation rearrangements.

Students often rote-memorize that a secondary carbocation adjacent to a quaternary center will always rearrange to a tertiary carbocation due to hyperconjugative stabilization.

However, rearrangement has an activation barrier.

By providing Arrhenius parameters, the question forces the student to quantitatively evaluate the transition state accessibility.

At low temperatures, the lower activation energy of direct nucleophilic capture outcompetes the higher barrier of the 1,2-methyl shift, leading to the kinetically favored unrearranged product.

At higher temperatures, the higher pre-exponential factor of the unimolecular rearrangement dominates, yielding the thermodynamically favored rearranged product.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterOrganic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
ConceptHyperconjugation Logic
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree