Question

Pentane-2,4-dione (acetylacetone) exists as a tautomeric mixture of 8% keto and 92% enol forms. Draw the stable enol tautomer, and explain its unusual stability.

Accepted Answer

Understand the concept of tautomerism: Tautomerism is a chemical equilibrium between two structural isomers (tautomers) that differ in the position of a proton and the double bond. In this case, pentane-2,4-dione (acetylacetone) exists in keto and enol forms. Draw the keto form of pentane-2,4-dione: The keto form has two carbonyl groups (C=O) at positions 2 and 4 of the pentane chain. Represent this structure as CH3-CO-CH2-CO-CH3. Draw the enol form of pentane-2,4-dione: The enol form is created by transferring a proton from the central methylene group (-CH2-) to one of the oxygen atoms of the carbonyl group, forming a hydroxyl group (-OH) and a double bond between the central carbon and the adjacent carbon. Represent this structure as CH3-C(OH)=CH-C(O)-CH3. Explain the unusual stability of the enol form: The enol form of acetylacetone is stabilized by intramolecular hydrogen bonding. The hydroxyl group (-OH) forms a hydrogen bond with the oxygen atom of the adjacent carbonyl group, creating a six-membered ring structure. This hydrogen bonding significantly lowers the energy of the enol form, making it more stable than expected. Discuss the dominance of the enol form: Due to the stabilization provided by intramolecular hydrogen bonding, the enol form is the predominant tautomer in the equilibrium mixture, accounting for 92% of the total, compared to only 8% for the keto form.

Pentane-2,4-dione (acetylacetone) exists as a tautomeric mixture of 8% keto and 92% enol forms. Draw the stable enol tautomer, and explain its unusual stability.

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Concetti chiave

Tautomerism

Enol Stability

Keto-Enol Equilibrium