Q32. - Organic Chemistry

Question

Question: Both pentane-2,4-dione and ethyl acetoacetate have two carbonyl groups separated by a single carbon atom. Although an equilibrium mixture of pentane-2,4-dione tautomers contains 76% of the enol forms, an equilibrium mixture of ethyl acetoacetate tautomers contains only 8% of the enol forms. Suggest a reason for this difference.

Step-by-Step Solution

Answer

Answer

 

An equilibrium mixture of pentane-2,4-dione tautomers contains 76% of the enol forms, while an equilibrium mixture of ethyl acetoacetate tautomers contains only 8% of the enol forms. It is because pentane-2,4-dione is stabilized by the intramolecular H-bonding in an enol form and exists in a six-membered ring, which is not possible for ethyl acetoacetate.

Step 1: Definition of tautomers

Tautomers are structural isomers that interconvert in an acidic or a basic medium in the presence of water. Keto-enol tautomers fall into such category of tautomerization.

Step 2: Stability of the keto and enol tautomers

Generally, the keto forms are more stable than the enol forms due to the presence of a stronger C=O bond. In an enol form, a less strong C=C bond is present; hence, its percentage in that state decreases. But exceptions are seen where the enol form is more stable than the keto form.

Step 3: Features that make enol form more stable

The enol form exists in a higher percentage in cases where the structure contains a $\beta$ -dicarbonyl feature. So, when it interconverts into an enol form, it undergoes an intramolecular H-bonding and exists as a six-membered ring that increases its stability.

Representation of keto-enol tautomerism for pentane-2,4-dione

 

Hence, based on the explanations, an equilibrium mixture of pentane-2,4-dione tautomers contains 76% of the enol forms, and an equilibrium mixture of ethyl acetoacetate tautomers contains only 8% of the enol forms. It is because pentane-2,4-dione is stabilized by intramolecular H-bonding in an enol form and exists in a six-membered ring, which is not possible for ethyl acetoacetate.

Representation of an intermolecular H-bonding in an enol state for pentane-2,4-dione