Animated Solution for Chemistry - Organic Chemistry: Assertion (A) Enol form of acetone $[C{H}_{3}COC{H}_3]$ exists in $<0.1\%$ quantity. However, the enol form of acetyl acetone $[C{H}_{3}COC{H}_{2}COC{H}_3]$ exists in approximately $15\%$ quantity. Reason (R) Enol form of acetyl acetone is stabilised by intramolecular hydrogen bonding, which is not possible in enol form of acetone. Choose the correct statement.

Why Acetylacetone Defies the Norm

When we dive into the world of carbonyl compounds, one of the most fascinating phenomena we encounter is keto-enol tautomerism. This is a chemical equilibrium between a keto form (a ketone or an aldehyde) and an enol form (an alcohol attached to a carbon-carbon double bond).

For simple monocarbonyl compounds like acetone ($C{H}_{3}COC{H}_3$), the equilibrium heavily favors the keto form. Why? Because the carbon-oxygen double bond ($C=O$) is thermodynamically much stronger and more stable than the carbon-carbon double bond ($C=C$) found in the enol form. As a result, the enol form of acetone exists in a minuscule quantity, typically less than $0.1\%$.

However, the rules of the game change dramatically when we introduce a second carbonyl group into the molecule, specifically at the beta position. Enter acetylacetone ($C{H}_{3}COC{H}_{2}COC{H}_3$), a classic $\beta$-diketone.

When acetylacetone undergoes tautomerization, it forms an enol structure: $C{H}_3-C(OH)=CH-C(=O)-C{H}_3$. In this specific geometry, something magical happens. The hydrogen atom of the newly formed hydroxyl ($-OH$) group finds itself in perfect proximity to the oxygen atom of the adjacent carbonyl ($C=O$) group.

This perfect spatial arrangement allows for the formation of a strong intramolecular hydrogen bond. This interaction effectively locks the molecule into a highly stable, six-membered pseudo-ring structure (a process often referred to as chelation).

Because six-membered rings are exceptionally stable (due to minimal angle strain), this hydrogen bonding provides a massive thermodynamic boost to the enol form. Consequently, the enol form of acetylacetone is stabilized to such an extent that it exists in approximately $15\%$ quantity at room temperature—a massive leap compared to the $<0.1\%$ seen in acetone!

Returning to our problem, the assertion correctly states the stark difference in enol quantities between acetone and acetylacetone. The reason accurately identifies intramolecular hydrogen bonding as the driving force behind the stabilization of acetylacetone's enol form—a stabilization that is structurally impossible in simple acetone. Therefore, both the assertion and the reason are true, and the reason is the perfect explanation for the assertion.