See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Keto-enol tautomerism and factors that stabilize the enol form. Step 1: Identify the three equilibria. - x%: Cyclobutanone ⇌ its enolate anion (deprotonated enol, i.e., the 2+ charged enolate of cyclobutenone with O- charge). This represents the ionized/enolate form percentage, which reflects enol content under basic conditions. The enolate is stabilized by charge delocalization and the ring system. - y%: Cyclobutanone ⇌ cyclobutenol with an endocyclic double bond (four-membered ring with C=C inside the ring and OH). This is enol tautomer with endocyclic double bond in a four-membered ring. - z%: Cyclobutanone ⇌ cyclobutenol with an exocyclic arrangement (OH on the four-membered ring with the double bond exocyclic or a different connectivity). This is the standard enol tautomer. Step 2: Analyze stabilization. - For x: The enolate (O-) form is highly stabilized because the negative charge is delocalized. In cyclobutanone systems, the enolate anion is significantly stabilized. The 2+ label suggests a dication or a specific resonance stabilization. The enolate percentage x reflects that under the conditions shown, significant enolate formation occurs. More importantly, cyclobutenyl systems with charge delocalization (aromatic-like 2pi electrons in the anion - cyclobutadienyl dianion is antiaromatic, but the enolate here may benefit from other stabilization) make x large. - For y: The endocyclic enol of cyclobutanone places the C=C inside the four-membered ring alongside the OH. Four-membered ring enols with endocyclic double bonds have significant angle strain relief considerations and conjugation with OH. - For z: The exocyclic enol (standard cyclobutenol from cyclobutanone) is a simpler enol without additional conjugation benefits. Step 3: Key reasoning for x > y > z. - x is largest because the enolate anion (O-) is much more stabilized than neutral enol forms. The cyclobutenyl anion with 2 pi electrons in the ring may gain some aromatic stabilization (cyclopropyl analog thinking) or simply the anion is stabilized by the ring and charge delocalization, making x the highest enol/enolate content. - y > z because the endocyclic double bond in the four-membered ring enol (cyclobutenol with ring C=C) provides additional conjugation between the C=C and C-OH, making the enol more stable compared to the simpler exocyclic enol arrangement z. - z is the smallest because it is the least stabilized enol form. Step 4: Why other options fail. - (b) z > y > x: Incorrect because the enolate (x) should be most favored, not least. - (c) y > x > z: Incorrect because x (enolate) should be greater than y (neutral enol). - (d) x > z > y: Incorrect because y (endocyclic enol with more conjugation) should be greater than z. Therefore, the correct answer is A.