Kolbe electrolysis

"Kolbe reaction" redirects here. For the carboxylation of phenols, see Kolbe–Schmitt reaction.

The Kolbe electrolysis or Kolbe reaction is an organic reaction named after Hermann Kolbe.^[1] 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.^[2] 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:^[3]

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:^[4]

R₁COO⁻ + R₂COO⁻ → R₁−R₁ and/or R₁−R₂ and/or R₂−R₂

The reaction process can be enhanced and the Hofer–Moest reaction alternative supressed, by performing the reaction under weakly acidic conditions in protic solvents, and using a high curent density and a platinum anodic electrode.^[4]

In 2022, it was discovered that the Kolbe electrolysis is enhanced if an alternating square wave current is used instead of a direct current.^[5][6]

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.^[7] 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.^[4]

Applications

Kolbe electrolysis has a few industrial applications.^[8] The reaction typically yields <50%.^[4]

In one example, sebacic acid has been produced commercially by Kolbe electrolysis of adipic acid.^[9]

Kolbe electrolysis has been examined for converting biomass into biodiesel^[10][11] and for grafting of carbon electrodes.^[12][13]

See also

• Electrosynthesis
• Wurtz reaction