The Kolbe electrolysis or Kolbe reaction is an organic reaction named after Hermann Kolbe. The Kolbe reaction is formally a decarboxylative dimerisation of two carboxylic acids (or carboxylate ions). The overall reaction is:
Mechanism and side-reactions The reaction mechanism involves a two-stage radical process: electrochemical oxidation first gives a alkylcarboxyl radical, which decarboxylates almost immediately to give an alkyl radical intermediate. The alkyl radicals which combine to form a covalent bond. As an example, electrolysis of acetic acid yields ethane and carbon dioxide:
CH3COOH → CH3COO− → CH3COO· → CH3· + CO2 2CH3· → CH3CH3 Another example is the synthesis of 2,7-dimethyl-2,7-dinitrooctane from 4-methyl-4-nitrovaleric acid:
Other compounds can trap the radicals formed by decarboxylation, and the Kolbe reaction has also been occasionally used in cross-coupling reactions. If a mixture of two different carboxylates are used, the radical cross-coupling reaction generally gives all combinations of them:
R1COO− + R2COO− → R1−R1 and/or R1−R2 and/or R2−R2 The reaction process can be enhanced and the Hofer–Moest reaction alternative suppressed, by performing the reaction under weakly acidic conditions in protic solvents, and using a high current density and a platinum anodic electrode. In 2022, it was discovered that the Kolbe electrolysis is enhanced if an alternating square wave current is used instead of a direct current.
Hofer–Moest reaction In the Hofer–Moest reaction, the alkyl radical undergo further oxidation to form a carbocation, rather than coupling with another alkyl radical, which then reacts with an available nucleophile. The Hofer–Moest reaction, rather than Kolbe radical-coupling, always occurs if the carboxylic acid bears a carbocation-stabilizing side-substituent at the α position, but only sometimes otherwise.
Applications Kolbe electrolysis has a few industrial applications. The reaction typically yields <50%. In one example, sebacic acid has been produced commercially by Kolbe electrolysis of adipic acid. Kolbe electrolysis has been examined for converting biomass into biodiesel and for grafting of carbon electrodes.
See also Electrosynthesis Wurtz reaction Hunsdiecker reaction
References
Further reading Kolbe, Hermann (1848). "Zersetzung der Valeriansäure durch den elektrischen Strom" [Decomposition of valeric acid by an electric current]. Annalen der Chemie und Pharmacie. 64 (3): 339–341. doi:10.1002/jlac.18480640346. Kolbe, Hermann (1849). "Untersuchungen über die Elektrolyse organischer Verbindungen" [Investigations of the electrolysis of organic compounds]. Annalen der Chemie und Pharmacie. 69 (3): 257–294. doi:10.1002/jlac.18490690302.
External links "Kolbe Electrolysis". Organic Chemistry Portal. Retrieved 22 October 2007.