See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Keto-enol tautomerism stability is governed by resonance stabilization, aromaticity, and the relative energies of keto vs enol forms. Step 1: Analyze Structure (II) - bicyclo[2.2.1]heptan-2-one (norcamphor). This is a simple ketone (keto form). Ketones are generally more stable than their enol tautomers due to the stronger C=O bond compared to C=C + O-H. This is the standard keto form and is typically the most stable tautomer in simple systems. Step 2: Analyze Structure (III) - bicyclo[2.2.1]hepta-2,5-dien-2-ol. This structure has two double bonds in the bicyclic framework with an OH group on one of the double bond carbons. This resembles a phenol-like enol where the OH is conjugated with the diene system. The conjugation and partial aromatic/resonance stabilization makes this enol more stable than a simple enol. The extended conjugation (cross-conjugated diene-enol) provides additional stabilization compared to structure (I). Step 3: Analyze Structure (I) - bicyclo[2.2.1]hept-2-en-2-ol. This is a simple vinyl alcohol (enol) with one double bond and an OH. Simple enols are the least stable tautomers because they lack extended conjugation or aromaticity, and the simple C=C-OH system is higher in energy than the corresponding ketone. Step 4: Establish stability order. - Structure (II) (ketone) > Structure (III) (conjugated/diene enol) > Structure (I) (simple enol) - The ketone (II) is most stable due to the inherent stability of the C=O bond. - Structure (III) is more stable than (I) because of the extra double bond providing conjugation with the OH, giving partial resonance/aromatic character to the enol. - Structure (I) is the least stable as a simple unconjugated enol. Why other options fail: - (a) I > II > III: Incorrect; simple enol (I) cannot be more stable than ketone (II). - (b) III > II > I: Incorrect; the conjugated enol (III) cannot surpass the ketone (II) in stability in this non-aromatic bicyclic system. - (c) II > I > III: Incorrect; (I) being more stable than (III) ignores the conjugation advantage of (III). Therefore, the correct answer is D.