Carboxylic acids can be decarboxylated by anodic oxidation to radicals (Kolbe-electrolysis) and/or carbocations (non-Kolbe electrolysis). The procedure and necessary equipment is simple, a scale-up easy, the choice of carboxylic acids wide, the selectivity towards radicals or carbocations can be controlled by reaction conditions and the structure of the carboxylic acid, the yields are in general good. The radical pathway can be used for the preparation of e.g. 1,n-diesters, pheromones, or rare fatty acids. Electrolysis in the presence of olefins affords additive dimers and monomers or by intramolecular addition five membered carbocycles and heterocycles. By non-Kolbe electrolysis carboxylic acids can be converted into ethers, acetals, olefins or acetamides. Rearrangements and fragmentations lead to stereospecifically substituted cyclopentanoids and one- or four-carbon ring extensions.

Recent Contributions of Kolbe Electrolysis to Organic Synthesis

Electro-oxidation of thiosulphate ion on gold: Study by means of cyclic voltammetry and auger electron spectroscopy

Studies are reported on the kinetics of anodic formation of Br2 at Pt in relation to chemisorption of the Br– ion, and the role of the adsorbed Br˙ intermediate in the reaction steps. The results are compared with those for the analogous anodic Cl2 evolution reaction. Cyclic voltammetry, recording of potential-relaxation transients and impedance spectroscopy methods are used to elucidate the question of chemisorption of Br– and Br˙ species in competition with surface oxide formation at Pt, which otherwise occurs in Br–-free aqueous solutions, and to evaluate the kinetic features of the reaction.Measurements have been made of the kinetics of Br2 formation at ‘freshly reduced’ Pt electrodes, at which virtually no anodic oxide film exists, and at ‘pre-oxidized’ ones where a well defined oxide film has been grown prior to the anodic Br2 formation experiments.The kinetics are shown to be dominated by the step of Br˙ recombination under all conditions, as similarly found for the Cl2 evolution reaction. However, the role of Br3– ion in the reaction in relatively concentrated Br– solutions has to be taken into account as well as chemisorption of Br– as a partially discharged reactant anion.

Surface electrochemistry and kinetics of anodic bromine formation at platinum

The effect of ultrasound upon the oxidation of thiosulphate on stainless steel and platinum electrodes

The electrochemical reactions of carboxylic and lactone groups on carbon nano-onions (CNOs) in aqueous solutions result in non-Kolbe products: alcohols, ketones, ethers and epoxides. The anodic/cathodic conversion of ox-CNOs was assessed by Boehm titrations and by Raman and DRIFTS-FTIR (diffuse reflectance infrared Fourier transform spectroscopy). The electrochemical properties of oxidized carbon nano-onions were investigated by cyclic voltammetry in aqueous solutions. The ox-CNOs are electrochemically active as a result of the reduction of the oxygen-containing groups.

S. Glasstone

Papers

The Hydrogen Peroxide Theory of Electrolytic Oxidation.