Decarboxylation

Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is the first chemical step in photosynthesis, is called carboxylation, the addition of CO2 to a compound. Enzymes that catalyze decarboxylations are called decarboxylases or, the more formal term, carboxy-lyases (EC number 4.1.1). The term "decarboxylation" usually means replacement of a carboxyl group () with a hydrogen atom: RCO2H -> RH + CO2 Decarboxylation is one of the oldest known organic reactions. It is one of the processes assumed to accompany pyrolysis and destructive distillation. Metal salts, especially copper compounds, facilitate the reaction via the intermediacy of metal carboxylate complexes. Decarboxylation of aryl carboxylates can generate the equivalent of the corresponding aryl anion, which in turn can undergo cross coupling reactions. Decarboxylation of alkanoic acids is often slow. Thus, typical fatty acids do not decarboxylate readily. Overall, the facility of decarboxylation depends upon stability of the carbanion intermediate R-. Important exceptions are the decarboxylation of beta-keto acids, β,γ-unsaturated acids, and α-phenyl, α-nitro, and α-cyanoacids. Such reactions are accelerated due to the formation of a zwitterionic tautomer in which the carbonyl is protonated and the carboxyl group is deprotonated. Decarboxylations are the bases of many named reactions. These include Barton decarboxylation, Kolbe electrolysis, Kochi reaction, and Hunsdiecker reaction. All are radical reactions. The Krapcho decarboxylation is a related decarboxylation of an ester. The Tsuji–Trost reaction involves the intermediacy of an allyl complex. In ketonic decarboxylation a carboxylic acid is converted to a ketone. Hydrodecarboxylations involve the conversion of a carboxylic acid to the corresponding hydrocarbon.