Kolbe electrolysis mechanism

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

Non-Kolbe electrolysis of carboxylic acids in acetonitrile/water leads to acetamides as main products [294] (Table 10). The mechanism has been investigated by using " C-labeled carboxylic acids. The results are rationalized by assuming a reaction layer rich of carboxylate resulting in the formation of a diacylamide that is hydrolyzed... 

The discussion of this reaction can be limited to a short summary (see Table 11 for some representative examples) since there are exhaustive and critical reviews on both the mechanism 14>21>324) and the synthetic applications 14>32S> 3 2 6 of the Kolbe electrolysis available. 

Carboxylate anions are oxidized at the anode of an electrochemical cell to produce radicals and ultimately hydrocarbons in a reaction known as the Kolbe electrolysis. Suggest a mechanism for the Kolbe electrolysis shown in the following equation ... 

The oxidation of acetate by peroxodisulphate is much slower than that of formate. Glasstone and Hickling showed that the products, which include carbon dioxide, methane, ethane, and ethylene, are similar to those produced by the anodic oxidation of acetate ions (Kolbe electrolysis), and they inferred that the same organic radicals are formed as intermediates. Similar results are reported by Eberson et al. for the oxidations of ethyl terf.-butyl-malonate, tert.-butyl-cyanoacetate, and ferl.-butyl-malonamate ions. The oxidations of these ions and of acetate by peroxodisulphate are first order with respect to peroxodisulphate and zero order with respect to the substrate. Mechanisms involving hydroxyl radicals are excluded because the replacement of peroxodisulphate by Fenton s reagent leads to different products, so Eberson et al. infer that the initial attack on the substrate is by sulphate radical-ions. Sengar and Pandey report that the rate of the silver ion-catalysed oxidation of acetate is independent of the peroxodisulphate concentration. 

Major contributions to the mechanism of the Kolbe electrolysis have been made by Conway et al., who mainly used electroanalytical techniques, and by Eberson and Utley, who drew their conclusions from the dependence of product structures on reaction conditions. 

In the past, widely differing mechanisms have been proposed for the Kolbe electrolysis. The free-radical theory, which assumed acyloxy radicals as intermediates, was suggested by Crum-Brown and Wal-ker. According to Glasstone and Hickling in aqueous solution hydroxyl radicals and hydrogen peroxide were formed, which converted the carboxylic acids into dimers and alcohols or esters, the so-called Hofer-Moest products. Schall and Fichter proposed that the carboxylates are oxidatively coupled to diacyl peroxides, that subsequently decomposed. These mechanistic proposals have been reviewed. - The presently accepted mechanistic schemes have been reviewed, - and the general scheme summarized in Scheme 2 is assumed. 

Decarboxylation reactions may be induced (depending on the acid) in a variety of ways thermally, bacteriologically, photochemically, or even by electrolysis as in the anodic reaction of the Kolbe synthesis. Thermally induced decarboxylation of many carboxylic acids in solution proceeds by a bimolecular mechanism involving addition of a nucleophilic solvent molecule to an electrophilic carbon atom on the root molecule - preferably at a carbon adjacent to (a) or one removed from (P) the carboxyl carbon (Fraenkel et al. 1954 Clark 1958, 1969). An electrophile-nucleophile pair is formed in the transition state, which subsequently undergoes heterolytic fission (i.e., decomposition of a molecule into two ions of opposite charge) to yield CO2, a proton, and a carbanion the latter two species are reactive intermediates, which then combine rapidly (Brown 1951). The solvent molecule departs unaffected and in this sense the solvent may be considered... 

However, not all the phenomena can be explained in terms of the free-radical theory. Thus, there is now no doubt that the formation of hydrocarbons during electrolysis of carboxylic acids (the Kolbe synthesis) takes place by a radical mechanism [119]. In spite of this no case has been mentioned where metal allgrls have been formed during electrolysis of carboTQrlic acids. 

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