PrayogX › Chemistry › Aldehydes, Ketones and Carboxylic Acids

Carbonyl groups in X and Y: double decarboxylation of a beta-keto diacid and the cyclic anhydride of a cis-1,2-diacid

Two diacids are heated, and every reaction is watched at two levels at once: the flask on the left, the molecules on the right, in step. Flask 1 (2,4-dimethyl-3-oxopentanedioic acid) fizzes CO2 into limewater while curved arrows run each six-membered cyclic decarboxylation and the acid-catalysed keto-enol tautomerism, twice, to pentan-3-one. Flask 2 (cis-4-oxocyclopentane-1,2-dicarboxylic acid) gives off water that turns anhydrous copper sulfate blue while the hydroxyl attacks, a tetrahedral intermediate forms, a proton moves and water leaves to close the cyclic anhydride. Products come from pushing the arrows on the molecule graphs; the C=O groups are then counted. No prediction stage - input, run, watch, result.

Sub-topics covered

• Beta-keto acid decarboxylation through a six-membered cyclic transition state
• Keto-enol tautomerism (acid catalysed)
• Intramolecular nucleophilic acyl substitution
• Cyclic anhydrides from cis-1,2-dicarboxylic acids
• Counting carbonyl groups in ketones, acids and anhydrides

What it teaches

• Reactant 1 is a double beta-keto acid: each CO2H is on a carbon alpha to the ketone
• Heat: three curved arrows around a six-membered ring give CO2 and an enol, which tautomerises to the ketone; this happens twice, giving pentan-3-one (1 C=O)
• Reactant 2 has its CO2H groups on C3 and C4 - not a beta-keto acid, so no CO2 is lost
• The two CO2H are cis (wedged ring bonds): one OH attacks the other C=O, a proton transfers and water leaves, closing a five-membered cyclic anhydride (1 ketone + 2 anhydride C=O = 3)
• Sum = 1 + 3 = 4; the water comes from an OH oxygen, so no C=O is lost in forming Y

Simulation ID

• ADV-2026-P1-CHE-Q11 · revision 1

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