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.
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.
Fill in the products of the following reactions.
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