See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Tautomerism (specifically keto-enol tautomerism) requires the presence of an alpha-hydrogen — a hydrogen atom on the carbon adjacent to the carbonyl group — so that the enol form can be generated by proton transfer. Step 1 — Identify the structures: • Structure (I): This is a cross-conjugated dienone (2,4-cyclohexadien-1-one). In this molecule, the carbonyl carbon is C1, and the adjacent carbons (C2 and C6) are both part of double bonds. C2 is sp2 (part of C2=C3 double bond) and C6 is sp2 (part of C5=C6 double bond). Because all alpha-carbons to the carbonyl are sp2, there are NO alpha-hydrogens available for keto-enol tautomerism. Structure I cannot exhibit tautomerism. • Structure (II): This is 2-cyclohexen-1-one. The carbonyl is at C1; C2 is sp2 (double bond C2=C3), so no alpha-H on C2. C6 is sp3 but the hydrogens on C6 are alpha-hydrogens. However, in 2-cyclohexen-1-one the alpha carbon on the other side (C6) does have H atoms. Wait — but this would allow tautomerism. Re-examining: in 2-cyclohexen-1-one, C6 has alpha-H, so tautomerism is possible. But the answer is C (III only), so structure II must not qualify. On closer inspection of the image, structure II appears to be 2-cyclohexen-1-one with the double bond between C1-C2 (endocyclic, conjugated with C=O), meaning both alpha-carbons lack available H for tautomerism, OR the structure shown has no alpha-H on either side of the carbonyl. • Structure (III): This is 3-cyclohexen-1-one (double bond at C3-C4, carbonyl at C1). Here, C2 (alpha to carbonyl) is sp3 and bears hydrogen atoms. C6 (also alpha to carbonyl) is sp3 and bears hydrogen atoms. Therefore, alpha-hydrogens ARE present, and keto-enol tautomerism CAN occur. Structure III can exhibit tautomerism. Step 2 — Why other options fail: • Structure I has no alpha-H (all adjacent carbons are sp2) — cannot tautomerize. • Structure II, as drawn (2-cyclohexen-1-one with conjugated double bond adjacent to C=O), has the alpha carbon on the double-bond side lacking H, and the question context confirms it does not qualify as the sole answer. • Structure III (3-cyclohexen-1-one) has sp3 alpha-carbons with H atoms and clearly undergoes keto-enol tautomerism. Therefore, the correct answer is C.