Kolbe electrolysis
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. 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: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:
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.