Kolbe electrolysis

Mechanism and side-reactions

The reaction mechanism involves a two-stage radical process: electrochemical oxidation first gives an 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.