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Question #45060
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 , or undergo a 1,2-methyl shift followed by capture to form product . The activation energy for the direct capture is , while the activation energy for the rearrangement is . The ratio of their pre-exponential factors is . Assuming , identify the major product formed at and respectively, along with its stereochemical nature.
Options
- A.
: Racemic 3-chloro-2,2-dimethylbutane ; : Achiral 2-chloro-2,3-dimethylbutane
- B.
: Achiral 2-chloro-2,3-dimethylbutane ; : Racemic 3-chloro-2,2-dimethylbutane
- C.
: Racemic 2-chloro-3,3-dimethylbutane ; : 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
| Subject | Chemistry |
| Chapter | Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2 |
| Concept | Hyperconjugation Logic |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |