See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Under basic conditions (OH-) in D2O, alpha-hydrogen atoms adjacent to carbonyl groups undergo base-catalyzed H/D exchange. The mechanism involves: OH- abstracts an alpha-H to form an enolate, which is then reprotonated by D2O to incorporate D. Over a long reaction time, all exchangeable alpha-H atoms adjacent to the carbonyls are replaced by D. Additionally, the hydroxyl group (OH) on the cyclopentane ring will also exchange its proton with D2O under these conditions, converting OH to OD, because O-H protons are acidic and exchange rapidly with D2O in the presence of base. Step 1 - Identify exchangeable positions: - The 1,3-diketone (cyclohexane-1,3-dione) ring system has alpha carbons at C2 (between the two carbonyls) and the carbons adjacent to each carbonyl on the other side. - The alpha-H atoms at all alpha positions to both ketones will be exchanged to D. - The OH on the cyclopentane ring: the O-H proton exchanges to O-D under D2O/OH- conditions. Step 2 - Count D incorporations: - Alpha carbons to both ketones get their H replaced by D (giving D,D patterns at the relevant CH2 groups). - The hydroxyl proton is replaced: OH becomes OD. Step 3 - Match to options: - Option (a): OH retained (not OD), so the hydroxyl proton exchange is not shown. This is incorrect because OH would exchange to OD in D2O/OH-. - Option (b): OD shown on cyclopentane ring (correct exchange of OH to OD), and D atoms at all alpha positions to both carbonyls. This correctly represents full H/D exchange at all alpha carbons AND exchange of the hydroxyl proton. - Option (c): OH retained (not OD) and one position retains H - incomplete exchange, incorrect. - Option (d): None of these - incorrect since (b) is correct. Why (b) is correct: Long reaction time in D2O/OH- ensures complete exchange of all alpha-H atoms adjacent to carbonyls AND the OH proton (to OD). Option (b) shows OD (exchanged hydroxyl) and D at all alpha positions to both ketones, consistent with exhaustive deuterium exchange. Therefore, the correct answer is B.