Anodic Decarboxylation of Carboxylic Acids Kolbe Electrolysis

1 Anodic Decarboxylation of Carboxylic Acids (Kolbe Electrolysis) 

Anodic decarboxylation is a powerful method that generates radicals in a simple procedure for synthetic use. The radicals can be used in homocoupling, heterocoupling (Section 2.6.3.2) and addition reactions (Section 2.6.3.4). The large amount of literature in relation to anodic decarboxylation is covered in a number of reviews [2a, 15, 16] and chapters in books [2b, 17, 18], 

High current densities ( 250 mA cm ) and high carboxylate concentrations favor radical coupling, which is due to a high radical concentration at the anode surface. There is no need for potential control (discharge potentials for carboxylates are in the range of 2.1-2.8 V vs nhe [19]) as long as the current density exceeds 10 mA cm .  

A weakly acidic electrolyte is preferable, which is achieved by neutralizing the electrolyte to an extent of 2 to 10% by an alkali metal hydroxide or alkoxide. This allows the use of an undivided cell, because hydrogen discharge, which continuously regenerates carboxylate that is consumed at the anode, is the exclusive cathode reaction. The endpoint of the electrolysis is indicated by the change of the electrolyte to an alkaline pH. 

Additives can strongly influence the coupling. Foreign anions should be excluded, because they disturb the necessary formation of a carboxylate film at the anode. Their unfavorable effect increases with the charge of the anion. Foreign cations that can form oxide layers or promote radical oxidation (Fe , Co +, Cu +, Mn +) lower the coupling yield. Alkali and alkylammonium ions have no negative effect. Methanol is the solvent of choice, but acetonitrile can also be used if some water is added. 

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