See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: The stability of the enol form of a carbonyl compound depends on whether enolization leads to a product that is stabilized (e.g., by aromaticity, conjugation, intramolecular hydrogen bonding) or destabilized (e.g., by loss of conjugation, ring strain, anti-aromaticity). Step 1: Analyze option (a) - cyclobutane-1,2,3-trione. This compound has three carbonyl groups on a four-membered ring. Its enol form would be stabilized by extensive conjugation and possibly by aromaticity-like delocalization (squaric acid derivatives are known to have stable enol/enolate forms due to aromatic stabilization of the dianion). The enol form is relatively stable. Step 2: Analyze option (b) - cyclopentane-1,2-dione. 1,2-diketones in a five-membered ring readily enolize, and the resulting enol is stabilized by intramolecular hydrogen bonding between the enol OH and the adjacent carbonyl. This makes the enol form quite stable. Step 3: Analyze option (c) - the structure shown is a cyclobutenone with an exocyclic or endocyclic double bond adjacent to the carbonyl (a four-membered ring with one C=C and one C=O, resembling cyclobutenone). For the enol form to form, a proton alpha to the carbonyl must be removed and an OH placed on the carbonyl carbon. However, the resulting enol would place a double bond in a four-membered ring in a way that creates a cross-conjugated or anti-aromatic system. A four-membered ring with two double bonds (cyclobutadiene-like system) is anti-aromatic (4 pi electrons), making the enol form highly unstable. Therefore, the enol form of this compound is unstable. Step 4: Analyze option (d) - 2-cyclohexen-1-one (cyclohexenone). This is an alpha,beta-unsaturated ketone. Its enol form would involve the dienol, which can be stabilized by extended conjugation. The enol form is reasonably stable due to conjugation. Step 5: Conclusion. Option (c) has an unstable enol form because enolization of the cyclobutenone would generate a cyclobutadiene-like anti-aromatic intermediate/product with 4 pi electrons in a four-membered ring, making it energetically highly unfavorable. The other options all have enol forms stabilized by aromaticity, conjugation, or intramolecular hydrogen bonding. Therefore, the correct answer is C.