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Question #44477

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The Question

Compound $\mathbf{X}$ , \ce{1-(but-2-yn-1-yl)-4-isopropylbenzene}, is treated with a large excess of sodium metal in liquid ammonia containing ethanol at $195\text{ K}$ until complete reduction of all reducible functional groups occurs. The reaction yields $\mathbf{Y}$ as the major organic product. Which of the following represents the correct IUPAC name and stereochemistry of $\mathbf{Y}$ ?

Options

A.
3-((E)-but-2-en-1-yl)-6-isopropylcyclohexa-1,4-diene
B.
1-((E)-but-2-en-1-yl)-4-isopropylcyclohexa-1,4-diene
C.
1-((Z)-but-2-en-1-yl)-4-isopropylcyclohexa-1,4-diene
D.
3-((Z)-but-2-en-1-yl)-6-isopropylcyclohexa-1,4-diene

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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

Lindlar vs. Birch Reduction|Hyperconjugation Logic|Reaction Intermediates Stability

Why This Question Is Tricky

Common Mistakes

Students overlook the steric override of electronic effects in the Birch reduction of bulky alkylbenzenes.

While standard rules dictate that EDGs (like alkyls) stabilize the double bonds and end up on $sp^2$ carbons (1,4-dialkylcyclohexa-1,4-diene), bulky groups like isopropyl suffer severe allylic strain in this geometry.

This forces protonation at the electronically destabilized ipso-carbons, yielding the anomalous 3,6-dialkylcyclohexa-1,4-diene.

The inclusion of the alkyne tests the simultaneous application of standard stereoselective Birch reduction (yielding trans-alkenes) against this anomalous aromatic regioselectivity.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Hydrocarbons - Class 11 Part 2
Concept	Lindlar vs. Birch Reduction
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free