Ketonic decarboxylation

Ketonic decarboxylation (also known as decarboxylative ketonization) is a type of organic reaction involving decarboxylation, converting two equivalents of a carboxylic acid (R−C(=O)OH) to a symmetric ketone (R₂C=O). The reaction typically requires heat and a metal catalyst, and generally proceeds in low yields. It can be thought of as a decarboxylative Claisen condensation. Water and carbon dioxide are byproducts:

2 RCO₂H → R₂CO + CO₂ + H₂O

Bases promote this reaction. The reaction mechanism is proposed to involve nucleophilic attack of the alpha-carbon of one acid group on the other carboxylic acid group, possibly as a concerted reaction with the decarboxylation. The initial formation of an intermediate carbanion via decarboxylation of one of the acid groups prior to the nucleophilic attack is unlikely since the byproduct resulting from the carbanion's protonation by the acid has not been reported. This reaction is different from oxidative decarboxylation, which proceeds through a radical mechanism and is characterised by a different product distribution in isotopic labeling experiments with two different carboxylic acids.

With two different carboxylic acids the reaction has poor selectivity for the mixed product, unless one of the acids (for example, a small, volatile one) is used in large excess to minimise the loss of the other acid.