Decarboxylation

Decarboxylation is a chemical reaction that 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 carbonation, 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).

In organic chemistry
Decarboxylation is one of the oldest organic reactions, since it often entails simple pyrolysis, and volatile products distill from the reactor. Heating is required because the reaction is less favorable at low temperatures. Yields are highly sensitive to conditions. In retrosynthesis, decarboxylation reactions can be considered the opposite of homologation reactions, in that the chain length becomes one carbon shorter. Metals, especially copper compounds, are usually required. Such reactions proceed 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.

Alkylcarboxylic acids and their salts do not always undergo decarboxylation readily. Exceptions are the decarboxylation of beta-keto acids, α,β-unsaturated acids, and α-phenyl, α-nitro, and α-cyanoacids. Such reactions are accelerated due to the stabilization of the cyclic transition state. To generate the carboxylate salts for decarboxylation, the parent carboxylic acid is heated strongly with soda lime. Typically fatty acids do not decarboxylate readily.

Named decarboxylation reactions
Many reactions have been named after early workers in organic chemistry. The Barton decarboxylation, Kolbe electrolysis and Hunsdiecker reaction are radical reactions. The Krapcho decarboxylation is a related decarboxylation of an ester.

Protodecarboxylation
Protodecarboxylations involve the conversion of a carboxylic acid to the corresponding hydrocarbon. This reaction is especially common in conjunction with the malonic ester synthesis and Knoevenagel condensations. Many such reactions entail heating the carboxylic acid with concentrated hydrochloric acid.

Common biochemical examples
Common biosynthetic decarboxylations of amino acids to amines are:
 * tryptophan to tryptamine
 * phenylalanine to phenylethylamine
 * tyrosine to tyramine
 * histidine to histamine
 * serine to ethanolamine
 * glutamic acid to GABA
 * lysine to cadaverine
 * arginine to agmatine
 * ornithine to putrescine
 * melatonin to 5-methoxytryptamine
 * 5-HTP to serotonin
 * L-DOPA to dopamine

Other decarboxylation reactions from the citric acid cycle include:
 * pyruvate to acetyl-CoA
 * oxalosuccinate to α-ketoglutarate
 * α-ketoglutarate to succinyl-CoA.

Case studies
Upon heating, Δ9-Tetrahydrocannabinolic acid decarboxylates to give the psychoactive compound Δ9-Tetrahydrocannabinol.

In beverages stored for long periods, very small amounts of benzene may form from benzoic acid by decarboxylation catalyzed by the presence of vitamin C.