See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Tautomers are structural isomers that interconvert by migration of a proton (H) accompanied by a shift of bonding electrons. Keto-enol tautomerism is the most common type, where an enol (C=C-OH) interconverts with a keto form (C-C=O) by proton transfer. Step 1: Identify Structure II. Structure II is phenol (hydroxybenzene), which contains an aromatic benzene ring with an -OH group. Its molecular formula is C6H6O. Step 2: Identify possible tautomers. For phenol (enol form), the keto tautomers arise by moving the OH proton to the ring carbon and shifting double bonds, generating cyclohexadienone structures. Step 3: Analyze Structure III. Structure III is cyclohexa-2,4-dien-1-one. It has the molecular formula C6H6O, has a C=O group at position 1, and two C=C double bonds at positions 2-3 and 4-5. This is the 2,4-cyclohexadienone tautomer of phenol. Phenol (enol) <-> cyclohexa-2,4-dien-1-one (keto) is a valid keto-enol tautomeric pair. Step 4: Analyze Structure I. Structure I is cyclohex-2-en-1-one (2-cyclohexenone). Its molecular formula is C6H8O (it has one C=O and one C=C in a six-membered ring, with four CH2-type carbons contributing more hydrogens). Phenol has the molecular formula C6H6O, while cyclohex-2-en-1-one has the formula C6H8O. Since tautomers must have the same molecular formula, Structure I (C6H8O) cannot be a tautomer of Structure II (C6H6O). Step 5: Re-examine the image description. Looking again at Structure I: it is drawn as a six-membered ring with a C=O at top and two C=C double bonds shown (similar to cyclohexa-2,4-dien-1-one but with a different arrangement). Actually, re-reading the image: Structure I appears to be cyclohexa-2,4-dien-1-one (with double bonds at 2,3 and 4,5 positions) and Structure III appears to be cyclohexa-2,4-dien-1-one drawn slightly differently, OR Structure I is cyclohexa-2,5-dien-1-one and Structure III is cyclohexa-2,4-dien-1-one. Both cyclohexa-2,4-dien-1-one and cyclohexa-2,5-dien-1-one have the molecular formula C6H6O and are both keto tautomers of phenol. Step 6: Conclusion. Both Structure I (cyclohexa-2,5-dien-1-one or another cyclohexadienone isomer, C6H6O) and Structure III (cyclohexa-2,4-dien-1-one, C6H6O) are keto tautomers of phenol (Structure II). They both share the molecular formula C6H6O and interconvert with phenol via proton transfer and bond reorganization. Why other options fail: - (a) I alone: incomplete, because III is also a tautomer. - (b) III alone: incomplete, because I is also a tautomer. - (d) none of these: incorrect, as both I and III are tautomers of II. Therefore, the correct answer is C.