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Question #45108

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The Question

The position of keto-enol equilibrium in cyclic dicarbonyl compounds is dictated by a delicate thermodynamic balance between intramolecular hydrogen bonding, dipole-dipole repulsions, and Baeyer angle strain. Consider the tautomerization of the following compounds in a non-polar solvent ( $CCl_4$ ) at $298\text{ K}$ :

(I) 1,3-Cyclobutanedione (II) 1,2-Cyclopentanedione (III) 1,3-Cyclohexanedione

Which of the following statements correctly identifies the dominant tautomeric form for (I) and (II) along with the primary thermodynamic rationale?

Options

A.
(I) Diketone, as enolization drastically increases Baeyer strain; (II) Enol, to relieve severe dipole-dipole repulsion of adjacent coplanar carbonyls
B.
(I) Enol, to relieve transannular steric strain; (II) Diketone, to minimize Baeyer angle strain
C.
Both (I) and (II) are Enols, driven by the thermodynamic formation of a resonance-stabilized intramolecular hydrogen bond
D.
(I) Diketone, to avoid the antiaromaticity of the enol form; (II) Diketone, as introducing a third $sp^2$ carbon drastically increases 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

Bond Angle and Strain|Tautomerism Conditions

Why This Question Is Tricky

Common Mistakes

The question tests a 'Chemical Exception' where the standard heuristic '1,3-dicarbonyls exhibit high enol content due to resonance and H-bonding' fails.

Students often overlook that in small rings (like cyclobutane), geometric constraints (Baeyer strain) override electronic stabilization.

Forcing an sp2 carbon (120 degree ideal) into a 90 degree ring creates prohibitive strain.

Conversely, in 1,2-cyclopentanedione, the hidden variable is the severe dipole-dipole repulsion of adjacent coplanar carbonyls, which forces enolization despite it not being a 1,3-dicarbonyl system.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Bond Angle and Strain
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free