Kolbe electrosynthesis mechanism

In non-aqueous solutions the Kolbe electrosynthesis takes place with high eflSciency at platinized platinum and gold, as well as at smooth platinum, anodes increase of temperature and the presence of catalysts for hydrogen peroxide decomposition, both of which have a harmful effect in aqueous solution, have relatively little influence. The mechanism of the reaction is apparently quite different in non-aqueous solutions and aqueous solutions in the former no hydroxyl ions are present, and so neither hydroxyl radicals nor hydrogen peroxide can be formed. It is probable, therefore, that direct discharge of acetate ions occurs at a potential which is almost independent of the nature of the electrode material in a given solvent. The resulting radicals probably combine in pairs, as in aqueous solution, to form acetyl peroxide, which subsequently decomposes as already described. ... 

It can be seen that the Involvement of free radicals in the Kolbe electrosynthesis is actually not the question. Rather, it is whether these radicals are free to react homogeneously in solution —although formed by a concerted electron transfer/decarboxylation process at the electrode surface—or are adsorbed In an excellent review of the anodic reactions of carboxylates, Utley " has presented a reaction scheme (shown in part below) which is adequate for most purposes. In a concluding section of this review, the question of free radical intermediates vis a vis adsorbed radicals is discussed in considerable detail. However, it would seem that the only conclusion one can draw is that the mechanism of this electro-organic process is still by no means settled. However, the following steps are likely to be involved ... 

The principal theories of the mechanism of the Kolbe electrosynthesis, as discussed earlier, have been criticized. A satisfactory summary of these has been presented by Dickinson and Wynne-Jones. The main points of disagreement among these theories are, (a) whether the hydroxyl ion or the carboxylate ion is discharged at the anode, (b) the explanation of the effect of the electrode material on the reaction, and (c) the reason for the occurrence of the Kolbe synthesis in preference to oxygen evolution which could occur at lower anode potentials. Recent experimental investigations, particularly those of Dickinson and Wynne-Jones and of Conway and Dzieciuch permit a detailed analysis of the mechanism of the Kolbe reaction, in both aqueous and nonaqueous media. 

Eberson L (1967) Mechanism of the kolbe electrosynthesis. Electrochim Acta 12 1473-1478... 

Some aspects of the mechanism of the Kolbe reaction are still very much a matter for debate as in some respects is the mechanism of electroreduction of aldehydes and ketones key questions appear to be the nature of the intermediates and if they are adsorbed or not. It has been well established " that in aqueous electrolytes the Kolbe reaction does not occur below a certain critical anodic potential (--1.9 V vs. normal hydrogen electrode). The results of a thorough study of this electrosynthesis in both aqueous and anhydrous systems were interpreted in terms of adsorbed carboxyl radicals. With increasing anodic potential, these intermediate species gradually cover the electrode surface until discharged to a second adsorbed radical at anode potentials above the critical potential. Subsequent dimerization and desorption occurs to yield the product hydrocarbon ... 

The dependence of Kolbe products on anode potential and electrode material has been the subject of considerable study. Only a few examples from recent studies will be discussed here since the interest is in mechanism determination and not electrosynthesis. Dickinson and Wynne-Jones analyzed the composition of gases evolved from acetate solutions under various conditions and their results are presented in Table 5. They also observed that with a current density of 30mA/cm in citrate and phthalate solutions, oxygen was evolved at current efficiencies of not less than 90% on the metals they examined (Pt, Ir, Pd, Au, and Ni). The results of Conway and Dzieciuch with aqueous potassium formate are seen in Table 6. These results agree with those of Dickinson and Wynne-Jones, showing that the decarboxylation reaction is almost completely inhibited by the oxygen evolution process. The anodic reaction at Pd and Au in aqueous potassium trifluoro-acetate was... 

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