CX7b.  Enolates: Decarboxylation

The formation of a beta-ketoester from two esters is called a "Claisen condensation". 

Figure CX8.1. A Claisen condensation.

It is often followed by another important reaction: decarboxylation.  If a beta-ketoester is treated with aqueous acid and heated, a couple of reactions take place.  First, the ester portion of the molecule is converted into a carboxylic acid. 

Figure CX8.2. Ester hydrolysis on a Claisen condensation product.

Second, the carboxylic acid is decarboxylated.  Carbon dioxide is formed, and the organic molecule becomes a ketone.  The carboxyl group is lost completely from the original molecule, and is converted into CO₂.

Figure CX8.3. Decarboxylation of a β-keto acid.

Decarboxylation is related to the retro-aldol reaction; formally, it can be thought of as leading to an enolate leaving group.  Decarboxylation most commonly occurs in beta-ketoacids, rather than in other carboxylic acids.  Otherwise, that leaving group could not occur.  The ease of decarboxylation in beta-ketoacids is related to the stability of the enolate anion. 

Figure CX8.4. A possible enolate leaving group in decarboxylation.

Under acidic conditions, of course, an enolate anion does not occur; instead, an enol is formed.  However, enols are rapidly converted into the keto tautomers.

Figure CX8.5. The enol product of decarboxylation.

Problem CX7b.1.

Draw a mechanism under acidic conditions for:

a) Conversion of ethyl-3-oxyhexanoate into 3-oxyhexanoic acid. (Oxy is a prefix meaning a ketone or aldehyde is foundalong the chain).

b) Decarboxylation of the resulting 3-oxyhexanoic acid.

 

Problem CX7b.2.

Fill in the products of the following reactions.

This site was written by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (retired) with other authors as noted on individual pages.  It is freely available for educational use.

Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. 

Send corrections to cschaller@csbsju.edu

This material is based upon work supported by the National Science Foundation under Grant No. 1043566.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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