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Some derivatives of formic acid, like formyl chloride (−COCl), undergo spontaneous decarbonylation at room temperature (or below). Reactions involving oxalyl chloride (COCl) 2 (e.g., hydrolysis, reaction with carboxylic acids, Swern oxidation, etc.) often liberate both carbon dioxide and carbon monoxide via a fragmentation process.
The decarboxylation of this compound by heat is essential for the psychoactive effect of smoked cannabis, and depends on conversion of the enol to a keto group when the alpha carbon is protonated. Upon heating, Δ9-tetrahydrocannabinolic acid decarboxylates to give the psychoactive compound Δ9- Tetrahydrocannabinol . [ 13 ]
The Barton decarboxylation is a radical reaction in which a carboxylic acid is converted to a thiohydroxamate ester (commonly referred to as a Barton ester). The product is then heated in the presence of a radical initiator and a suitable hydrogen donor to afford the decarboxylated product. [1] [2] This is
As this compound still contains the carboxylic acid it is then free to re enter the catalytic cycle where it undergoes coupling at the carbon 2 position, along with the expulsion of carbon dioxide to form a biaryl heteroatom. As this pathway competes with the decarboxylation step, two products are formed making this reaction less selective.
As an example, electrolysis of acetic acid yields ethane and carbon dioxide: CH 3 COOH → CH 3 COO − → CH 3 COO· → CH 3 · + CO 2 2CH 3 · → CH 3 CH 3. Another example is the synthesis of 2,7-dimethyl-2,7-dinitrooctane from 4-methyl-4-nitrovaleric acid: [3] The Kolbe reaction has also been occasionally used in cross-coupling reactions.
Water and carbon dioxide are byproducts: [1] 2 RCO 2 H → R 2 CO + CO 2 + H 2 O. 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. [1]
It is an example of both a decarboxylation and a halogenation reaction as the product has one fewer carbon atoms than the starting material (lost as carbon dioxide) and a halogen atom is introduced its place. [2] [3] A catalytic approach has been developed. [4]
In the process, in addition to water, an equivalent of ethanol and carbon dioxide are lost in decarboxylation. Ethyl glyoxylate 2 and glutaconate (diethyl-2-methylpent-2-enedioate) 1 react to isoprenetricarboxylic acid 3 (isoprene (2-methylbuta-1,3-diene) skeleton) with sodium ethoxide.