Electrolysis carboxylation, cathodic

The nature of the cathode material is not critical in the Kolbe reaction. The reduction of protons from the carboxylic acid is the main process, so that the electrolysis can normally be conducted in an undivided cell. For substrates with double or triple bonds, however, a platinum cathode should be avoided, as cathodic hydrogenation can occur there. A steel cathode should be used, instead. 

Ion-exchange membranes for chlor-alkali electrolysis generally contain a sulphonic layer which faces the anode and a carboxylic layer which faces the cathode, joined by lamination. The Na+ transport number is higher in the carboxylic layer than in the sulphonic layer, and a region of low Na+ concentration therefore tends to form at the interface between the two layers during electrolysis, as shown in Fig. 17.5. 

Carboxylic acids cannot normally be reduced directly electrochemically1 they can however be reduced to aldehydes by electrolysis in an undivided cell containing tributylphos-phine and methanesulfonic acid72. The conversion involves an interesting combination of anodic and cathodic reactions (Scheme 9). 

Electro-organic chemistry is the study of the oxidation and reduction of organic molecules and ions, dissolved in a suitable solvent, at an anode and cathode respectively in an electrolysis cell, and the subsequent reactions of the species so formed. The first experiment of this type was reported in 1849 by Kolbe, who described the electrolysis of an aqueous solution of a carboxylate salt and the isolation of a hydrocarbon. The initial step involves an anodic oxidation of the carboxylate anion to a radical which then dimerises to the alkane. 

Also in the cathodic reduction of carboxylic acids, electrolysis is in competition with catalytic methods. However, catalytic hydrogenations in this area do not always proceed so smoothly that electrochemical processes are without any prospects from the outset. 

The glow electrolysis technique (electrolysis with an anode immersed in the solution and the cathode above the surface) at 600-800 V dc and 300-500 mA converts a solution of starch into ethylene, methane, hydrogen, and both carbon mono- and dioxides.323 Electrochemical methods for converting polysaccharides and other biomass-derived materials have been reviewed briefly by Baizer.324 These methods are mainly oxidations along a potential gradient, which decreases the activation energy of the reactants. Starch in 5 M NaOH solution is oxidized on platinum electrodes to carboxylic acids with an activation energy of about 10 kcal/mol. In acidic media oxidation takes place at C-l followed by decarboxylation and oxidation at the C-2 and C-6 atoms.325... 

Aromatic carboxylic esters may be reduced to produce the corresponding primary alcohol by electrolysis at a mercury, lead or cadmium cathode. - For example, methyl benzoate is readily reduced to benzyl alcohol (91%) at a mercury cathode in MeOH containing Me4NCl. Ring substituents in the ben-... 

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. 

The reduction of lactones of polyhydroxy carboxylic acids to the corresponding aldoses with sodium amalgam can be successfully achieved by electroreduction at a mercury cathode, provided that the catholyte contains salts of amalgam-forming metals. The electroreduction of the lactones of n-ribonic and n-arabinonic acids to n-ribose and n-arabinose, respectively, is performed at a mercury cathode, with sodium, potassium, or zinc sulfate (or their mixtures) as the catholyte, and platinum gauze as the anode, immersed in aqueous, 15% sulfuric acid. The electrolyzer compartments are separated by a diaphragm, and the electrolysis is performed with intensive stirring of the catholyte, which is maintained at a temperature of +5 to +12° and at a constant pH (adjusted by additions of dilute sulfuric acid). Yields of monosaccharide are increased by addition of boric acid to the reaction mixture. 

Before discussing impurity effects, let us examine the pH profile across the membrane during the course of electrolysis. Using experimental data from a laboratory cell, Ogata and coworkers [18,103] and Obanawa and coworkers [104] found the pH in a sulfonate-carboxylate bilayer to be in the range 9-12 over the bulk of the membrane with the exception of narrow regions near the membrane/solution interfaces. On the anode side, the pH decreased steeply to about 3, while on the cathode side, it increased to 14. Thus, the pH of 9-12 in the membrane can indeed force the precipitation of metal hydroxides [105] when the metal ion concentration exceeds the dictates of the solubility product (Fig. 4.8.34). It should be noted that Hg and Fe are electrodeposited on the cathode and oxides of Mn, Pb, and Fe are formed on the anode as a result of oxidation of the relevant ionic species by the active chlorine in the anolyte. 

The dimerization reaction, illustrated by the formation of the pinacol in the electroreduction of acetophenone, is an obvious example of a coupling reaction. Such reactions may occur between radicals or radical ions generated in either an anodic or a cathodic process. For example, an electrochemical process in which radicals are generated anodically is the classical Kolbe reaction, the electrolysis of carboxylates ... 

A weakly acidic medium favors the Kolbe product. Therefore, the carboxylic acid is partially neutralized to the extent of 2-5 %. The concentration of the carboxylate anion remains constant during the whole electrolysis process since the base is regenerated at the cathode at the same rate as the carboxylate is consumed at the anode. While water has been used before, methanol or aqueous methanol is now the solvent of choice. Obviously, the selection of the best solvent rests mostly on experimental investigations. Temperature is usually not a critical... 

Scheme 9.100. A representation of the Kolbe electrolysis of the sodium salt of propanoic acid. The voltage for the electrolysis must be greater than that required for the reduction of water to hydroxide anion (OH") and hydrogen (H2) which occurs at the cathode. It is generally accepted that one electron is lost by the carboxylate anion at the anode to generate a carboxyl radical. The arrows shown on the carboxyl radical (with half-heads) are drawn to account for the apparent movement of one electron (in contrast to the usual cartoons showing two electron arrows) at a time to produce carbon dioxide (CO2) and the ethyl radical (CH3CH2 ).The latter dimerizes to butane or loses a hydrogen to a second radical, producing ethane and ethene.

Oxidation at an anode follows the same principles as oxidation by chemical electron transfer, with one exception. In the case of anodic oxidation, the radical cations are produced in a very small volume near the cathode. Even if their lifetimes are short, they are therefore liable to undergo bimolecular reactions. This effect is seen in the Kolbe synthesis. Electrolysis of a salt of a carboxylic acid, RCOOH, usually leads to good yields of the hydrocarbon R2 by the following process ... 

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