Kolbe reaction 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. 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

One example of the application of in situ electrochemical epr concerns the study of the Kolbe reaction. As was discussed in section 1.3, the Kolbe reaction involves some extremely complex processes and considerable effort has been expended in the search for the identities of the radical intermediates. Evidence for such intermediates remains sparse but one system that has provided such evidence is the electro-oxidation of triphenyl acetic acid (TPA) at a platinum electrode in acetonitrile (Waller and Compton, 1989). The case history of epr in the study of this system is a very good example of the application of the technique to provide details of a reaction mechanism. In... 

Problem 19.18 Outline a mechanism for the (a) Kolbe reaction, (h) Reimer-Tiemann reaction. (a) Phenoxide carbanion adds at the electrophilic carbon of CO,. 

The participation of surface radical species is, then, strongly suspected in the Kolbe reaction, but there are other radical reactions, such as the reductive dimerisation of C02, that are thought to be homogeneous, particularly in non-aqueous solvents. The basic mechanism in this latter case is thought to be ... 

Some additional insight as to possible mechanisms of the photocorrosion process can be gained from a more detailed consideration of the effects of pH on the band levels in SrTiC>3 and on the redox potentials of oxygen formation and the photo-Kolbe reaction. These data, along with the band levels for Ti02, are shown in Figure 5. It is important to remember that the photocorrosion process occurs in com-... 

A notable exception is the discharged ion theory of the Kolbe reaction, dating back to 1891 (Brown and Walker, 1891). This remarkably farsighted suggestion is entirely in accordance with present-day theory of the Kolbe reaction, but was for a long time abandoned in favour of indirect mechanisms (e.g. the hydrogen peroxide theory). 

The effect of concentration gradients in electrode reactions is really not a problem of mechanism but rather a troublesome source of possible systematic ambiguity in the interpretation of the product distributions observed, one of the tasks that lies close to the heart of the organic chemist. To see how this comes about, it is instructive to make the mental experiment that we generate acetoxy radicals by the Kolbe reaction of acetate ion in acetic acid [eqn (52)] at an electrode of 1 cm2 surface area, passing a current of 1 A during... 

The mechanism of the Kolbe reaction involves electrochemical decarboxylation-dimerization via radicals (Scheme 2.34). 

It is possible that some acetate radicals are formed by the direct discharge of the ions as, it will be seen shortly, is the case in non-aqueous solutions but an additional mechanism must be introduced, such as the one proposed above, to account for the influence of electrode material, catalysts for hydrogen peroxide decomposition, etc. It is significant that the anodes at which there is no Kolbe reaction consist of substances that are either themselves catalysts, or which become oxidized to compounds that are catalysts, for hydrogen peroxide decomposition. By diverting the hydroxyl radicals or the peroxide into an alternative path, viz., oxygen evolution, the efficiency of ethane formation is diminished. Under these conditions, as well as when access of acetate ions to the anode is prevented by the presence of foreign anions, the reactions mentioned above presumably do not occur, but instead peracetic acid is probably formed, thus,... 

Brown and Walker first proposed the generally accepted mechanism of the Kolbe reaction, which involves the initial discharge of carboxylates at the anode followed by decarboxylation and subsequent combination of the resulting radicals, leading to the Kolbe dimer [3]. The radical formed may also undergo disproportionation to afford olefins and alkanes as the result of hydrogen abstraction [Eq. (6)]. Actually, olefins and alkanes are found as by-products. 

As already mentioned before, mainly irreversible reactions with organic compounds have been investigated at semiconductor particles. When organic molecules, for example alcohols, are oxidized by hole transfer, O2 usually acts as an electron acceptor or in the case of platinized particles, protons or H2O are reduced. A whole sequence of reaction steps can occur, which are frequently difficult to analyze because cross-reactions may also be possible at particles and a new product could be formed. Concerning the primary electron and hole transfer, certainly there should be no difference between particles and compact electrodes. Since sites at which reduction and oxidation occur are adjacent at a particle, the final product may be different. An interesting example is the photo-Kolbe reaction, studied for Ti02 electrodes and for Pt-loaded particles. Ethane at extended electrodes and methane at Pt/Ti02 particles have been found as reaction products upon photo-oxidation of acetic acid [56, 57]. The mechanism was explained by Kraeutler et al. as follows. 

In many descriptions of electrochemical preparations of organic substances, only the overall current and voltage applied across the cell have been specified. It must be emphasized that this information is generally inadequate for a proper electrochemical specification of the experimental conditions and a characterization of the reaction mechanism. Under constant current conditions, as consumption of the reactant occurs, the potential normally becomes increased (greater polarization) until eventually some new electrode process becomes predominant (see Section 5.1). This may either be decomposition of the solvent or supporting electrolyte or, in some cases, a further reaction with the substrate involved in the electroorganic preparation. In the latter case, it is clear that the preparation will yield more than one principal product. A classical case, first investigated by Haber, is the electroreduction of nitrobenzene referred to above and also the Kolbe reaction. ... 

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 ... 

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