Kolbe electrolysis

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.{{cite journal :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: :[[File:Kolbe electrolysis.svg|Kolbe electrolysis, synthesis of 2,7-Dimethyl-2,7-dinitrooctane]]

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. :[[File:Hofer-Moest-reaction.svg]]

Applications

Kolbe electrolysis has a few industrial applications. The reaction typically yields

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.

References

References

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7. (1979). "Development of Kolbe Electrosynthesis of Sebacic Acid". CEER, Chemical Economy & Engineering Review.
8. (2020-01-18). "Kolbe Electrolysis of Biomass-Derived Fatty Acids Over Pt Nanocrystals in an Electrochemical Cell". ChemCatChem.
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