See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Base-catalyzed H/D exchange at the alpha-carbon requires (1) removal of an alpha-hydrogen by base to form an enolate, and (2) reprotonation (here deuteration) by D2O. For this exchange to occur, the enolate must be accessible and stable. A key steric requirement is that the alpha-hydrogen must be removable and the resulting enolate must be able to form — this requires reasonable orbital overlap between the alpha C-H bond and the carbonyl pi system, and the carbanion/enolate must not be excessively hindered. Step 1 — Analyze option (a), cyclohexane-1,3-dione: This 1,3-diketone has alpha-hydrogens at C2 (between the two carbonyls). These hydrogens are highly acidic (pKa ~5–6) due to stabilization of the enolate by two flanking carbonyl groups. Base-catalyzed exchange proceeds readily. So (a) DOES undergo exchange. Step 2 — Analyze option (b), 2-norbornanecarboxaldehyde (bridgehead aldehyde): The aldehyde CHO is at a bridgehead of the norbornane skeleton. The alpha-carbon to the aldehyde is the bridgehead carbon itself. However, looking more carefully at the structure, the CHO is attached to the bridgehead. The alpha-hydrogens are on the carbons adjacent to the carbonyl. In norbornane systems, the alpha-hydrogens on the exo/endo positions can still be abstracted. This compound CAN undergo base-catalyzed exchange because the alpha-hydrogens (on C1 or C3 adjacent carbons) are accessible and the enolate can form. So (b) does undergo exchange (it is not the answer by itself). Step 3 — Analyze option (c), camphor (1,7,7-trimethylbicyclo[2.2.1]heptan-2-one): Camphor has a ketone at C2 of the norbornane skeleton, with gem-dimethyl groups at C1 (the bridgehead, C7,7-dimethyl). The alpha-carbons are C1 (bridgehead, quaternary — no H) and C3. At C3, there are alpha-hydrogens (the exo-H at C3). However, the critical issue is that camphor is extremely resistant to base-catalyzed H/D exchange despite having alpha-hydrogens at C3. The reason is steric: the gem-dimethyl groups (the two methyl groups at C7) create severe steric hindrance around the carbonyl face, making it very difficult for base to approach and abstract the alpha-hydrogen. Furthermore, even if the enolate forms, reprotonation/deuteration is hindered. Camphor is a classic example in textbooks of a compound with alpha-hydrogens that does NOT undergo base-catalyzed exchange in D2O due to steric inaccessibility — the exo face is blocked by the gem-dimethyl bridge, and overall the rigid bicyclic framework with bulky methyls prevents the reaction. This is a well-known fact used in total synthesis (camphor-derived auxiliaries are stable under basic conditions). Step 4 — Why not (d) both (b) and (c): Option (b), 2-norbornanecarboxaldehyde, does undergo base-catalyzed exchange because the aldehyde alpha-hydrogens are accessible (less sterically hindered than camphor). Only camphor uniquely fails to exchange despite having alpha-hydrogens. Step 5 — Conclusion: Only camphor (option c) fails to undergo base-catalyzed H/D exchange in D2O despite possessing alpha-hydrogens, due to severe steric hindrance from the gem-dimethyl groups blocking base access to the alpha-hydrogens. Therefore, the correct answer is C.