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.




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