Q.67. - Organic Chemistry

Question

Question: Propose a stepwise mechanism for the following reaction of a  $\mathit{\alpha }\mathbf{,}\mathit{\beta }$-keto ester. Suggest a reason why this rearrangement reaction occurs.

Step-by-Step Solution

Answer

Answer

Mechanism of reaction with base

 

Rearrangement occurs because it produces a highly stabilized enolate ion.

Step 1: β -keto esters

Compounds containing the ester group and the ketone group in beta position are called beta keto esters.

They are mainly formed by Claisen condensation in which esters undergo condensation in the presence of a strong base.

These molecules are strong acids and undergo many reactions such as alkylation, hydration, and decarboxylation. 

Step 2: Reaction of β -keto esters with bases

When beta keto esters are treated with bases, the base abstracts hydrogen from the alpha position of carbonyl of the beta keto ester.

The hydrogen is abstracted from the ketone group. This is followed by rearrangement reactions in the molecule for stabilization.

Step 3: Rearrangement in β -keto esters

Abstraction of alpha hydrogen by ethoxide ion causes ring opening and formation of carbanion. The anion abstracts proton from ethanol molecule for stabilization.

Another attack of base produces another carbanion which reacts with the carbonyl group of ester forming a six-membered ring.

Removal of alkoxy group followed by removal of proton produces a resonance stabilized enolate which undergoes hydrolysis.

Rearrangement occurs because it produces a highly stabilized enolate between two carbonyl groups with three resonance structures.

Mechanism of reaction with base