Reactivity in Chemistry

Substitution at Carboxyloids

CX7b.  Enolates: Decarboxylation

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

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.

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. 

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.

 

Problem CX7b.1.

Draw a mechanism 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 is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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

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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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