See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Enol content depends on the stability of the enol form relative to the keto form. Key factors include: (1) aromaticity or conjugation stabilizing the enol, (2) intramolecular hydrogen bonding, and (3) the acidity of the alpha-hydrogens (more acidic alpha-H means easier enolization). Step 1 - Analyze compound II (2(3H)-furanone / gamma-crotonolactone): This is an alpha,beta-unsaturated lactone with a C=C double bond already present in the ring. The enol form of this compound would be the vinylogous/conjugated form where the enol tautomer is stabilized by conjugation with both the C=C bond and the carbonyl. The enol of compound II would be fully conjugated (aromatic-like furanone system) — effectively, the enol tautomer here gives a hydroxyfuran (2-hydroxyfuran) system which has significant aromatic stabilization. This makes the enol form exceptionally stable, so compound II has the highest enol content. Step 2 - Analyze compound I (cyclopentanone): Cyclopentanone is a simple cyclic ketone. It has alpha-CH2 groups on both sides of the carbonyl. The enol form is stabilized by the C=C conjugation with the carbonyl. It is a typical ketone with moderate enol content. There is no extra stabilization from oxygen lone pairs in the ring or aromaticity, but the beta-keto-like situation does not apply here. Cyclopentanone has a typical enol content for a simple ketone — higher than a simple lactone because the carbonyl of a ketone is less electrophilic/less stabilized than a lactone, meaning the keto form is less strongly favored compared to a lactone. In other words, alpha-H in a ketone is more acidic relative to ester/lactone alpha-H, so ketones enolize more readily than esters/lactones. Step 3 - Analyze compound III (gamma-butyrolactone): This is a saturated five-membered lactone. Lactones (cyclic esters) have their carbonyl stabilized by resonance donation from the ring oxygen, making the carbonyl less reactive and the alpha-C-H less acidic compared to a ketone. Therefore, the enol content of gamma-butyrolactone is the lowest of the three. Step 4 - Order: II (aromatic/conjugated enol stabilization) > I (simple ketone, moderate enol) > III (lactone, low enol content due to resonance stabilization of carbonyl by oxygen). Why other options fail: (a) I > II > III is wrong because II has higher enol content than I due to aromatic stabilization. (b) III > II > I is wrong; lactones have lower enol content than ketones. (d) II > III > I is wrong; III (lactone) should have lower enol content than I (ketone). Therefore, the correct answer is C.