See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the structures. X is resorcinol (1,3-dihydroxybenzene), an aromatic compound with two -OH groups on a benzene ring. Y is a non-aromatic six-membered ring (cyclohex-2,4-diene-1,4-dione or similar diketone structure) containing two C=O (carbonyl/ketone) groups and a C=C double bond — the keto form of the same molecule. Step 2 - Determine the molecular formula of each. X (resorcinol, 1,3-dihydroxybenzene): C6H6O2. Y (the diketone/cyclohexadienedione): C6H6O2. Both have the same molecular formula, so they are isomers of some type. Step 3 - Determine the relationship. X is the enol form: the two -OH groups on the aromatic ring represent the di-enol tautomer. Y is the diketo form: the two C=O groups represent the keto tautomer. The interconversion between the enol form (aromatic phenol/diol, X) and the keto form (diketone, Y) via proton transfer is the classic keto-enol tautomerism. Tautomers are constitutional isomers that interconvert rapidly (usually via proton transfer) and exist in equilibrium. Step 4 - Why the other options fail. (a) Resonance structures: Resonance structures differ only in electron placement, not in the position of atoms or hydrogen atoms. X and Y differ in the position of hydrogen atoms (O-H in X vs. C-H alpha in Y), so they are NOT resonance structures. (c) Functional isomers: Functional isomers have the same molecular formula but different functional groups that are NOT interconvertible by simple proton transfer under normal conditions. Although X has -OH and Y has C=O, their specific relationship (rapid interconversion via proton shift) classifies them as tautomers, not merely functional isomers. (d) Positional isomers: Positional isomers have the same functional groups but differ in the position of substitution on the carbon skeleton. X and Y differ in functional group type (enol vs. keto), not just position, so they are not positional isomers. Step 5 - Conclusion. X (the phenol/enol form) and Y (the keto form) are keto-enol tautomers — they have the same molecular formula C6H6O2 and interconvert by proton transfer between oxygen and carbon. Therefore, the correct answer is B.