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Question #44455
The Question
Consider the stereoisomer (2S, 3R)-2-bromo-3-phenylbutane. When treated with \ce{NaOEt} in \ce{EtOH} at 350 K, it undergoes a stereospecific E2 elimination. Which of the following correctly identifies the major product and the stereoelectronic rationale governing the transition state?
Options
- A.
(E)-2-phenylbut-2-ene; the reactive anti-periplanar conformer possesses severe \ce{CH3-CH3} and \ce{Ph-Br} gauche interactions.
- B.
(Z)-2-phenylbut-2-ene; the reactive anti-periplanar conformer is the global minimum possessing a \ce{Ph-Br} anti relationship.
- C.
(E)-2-phenylbut-2-ene; the reactive anti-periplanar conformer is the global minimum possessing a \ce{Ph-Br} anti relationship.
- D.
(Z)-2-phenylbut-2-ene; the reaction proceeds via syn-elimination from the global minimum conformer to avoid steric 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
Alkane Conformation Energy Barriers|E1/E2 Solvent and Base Effects
Why This Question Is Tricky
Common Mistakes
This question tests the Curtin-Hammett principle and the strict anti-periplanar stereoelectronic requirement of the E2 mechanism.
Students often fall into the trap of assuming that elimination occurs from the most stable ground-state conformer.
To react, the molecule must rotate into a highly sterically hindered conformer.
Failing to draw the correct reactive Newman projection leads to incorrect E/Z assignment or false mechanistic assumptions.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Hydrocarbons - Class 11 Part 2 |
| Concept | Alkane Conformation Energy Barriers |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |