See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Enol content in keto-enol tautomerism is governed by the stability of the enol form relative to the keto form. Key factors that increase enol content include: (1) conjugation of the enol double bond with adjacent pi systems, (2) intramolecular hydrogen bonding in the enol form, and (3) aromaticity gained upon enolization. Step 1 - Analyze Compound (I): This is a 7-membered ring 1,2-diketone (two adjacent C=O groups). When one carbonyl enolizes, the resulting enol OH can form an intramolecular hydrogen bond with the adjacent carbonyl. Additionally, the enol form of a 1,2-diketone (an alpha-diketone) is strongly stabilized by conjugation between the enol double bond and the remaining carbonyl. The enol of a vicinal diketone benefits from very strong intramolecular H-bonding and conjugation, giving very high enol content. Step 2 - Analyze Compound (II): This is a 7-membered ring 1,3-diketone (two C=O groups separated by one CH2, with additional unsaturation). A 1,3-diketone (beta-diketone) normally has high enol content due to: conjugation of the enol with the second C=O forming a stable conjugated enol system, and intramolecular hydrogen bonding. However, compound II is a cyclic 1,3-diketone with additional double bonds. The enol form is stabilized by extended conjugation across the ring. This gives significant enol content, but less than compound I because the 1,2-diketone enol stabilization (both conjugation and H-bonding in a strained-geometry-favorable arrangement) is stronger in the 7-membered ring context. Step 3 - Analyze Compound (III): This is a 7-membered ring with two carbonyls that are farther apart (1,6-relationship in a 7-membered ring, making them essentially a non-conjugated diketone with only one intervening double bond but no 1,3-relationship). The enol of compound III lacks the strong intramolecular H-bonding and conjugation stabilization present in I and II. Its enol content is the lowest of the three. Step 4 - Order: The enol content decreases as: I (1,2-diketone with strong H-bond and conjugation) > II (1,3-diketone with conjugation and H-bond but slightly less effective) > III (non-conjugated, non-1,3 diketone, least enol stabilization). Why other options fail: - Option (b) III > II > I: Incorrect; III has the least enol stabilization. - Option (c) II > I > III: Incorrect; I (vicinal diketone) has stronger enol stabilization than II. - Option (d) II > III > I: Incorrect; both the relative ordering of II vs I and III vs I are wrong. Therefore, the correct answer is A.