Carboxylate anions electrolysis

The trapping of allyl radicals with other open-shell species can be studied in all reactions in which a sufficiently high concentration of radicals is created and in which the concentration of nonradical trapping agents is low. This prerequisite has been met in Kolbe electrolysis reactions, in which radicals are generated by one-electron oxidation of carboxylate anions. One of the simplest systems, the reaction of methyl radicals with... 

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. 

Following the study of the simple coupling of radicals derived from the salt of a single carboxylic acid, it was found that the electrolysis of a mixture of carboxylate anions or of the salts of half esters of dicarboxylic acids increased the synthetic value of the method. This arises from the possibility of the formation of symmetrical and unsymmetrical coupled products of the derived radicals. These anodic syntheses are illustrated in the synthesis of hexacosane (Expt 5.11), sebacic acid (decanedioic acid), octadecanedioic acid and myristic acid (tetra-decanoic acid), in Expt 5.131. 

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 ... 

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.

Carboxylic acids can be electrochemically decarboxylated to give, ultimately, a hydrocarbon composed of two R groups. This reaction, called the Kolbe electrolysis after Hermann Kolbe (1818—1884), involves an electrochemical oxidation of the carboxylate anion to give the carboxyl radical. Loss of carbon dioxide gives an alkyl radical that can dimerize to give the hydrocarbon (Rg. 17.57). 

The electrochemical reaction of carboxylate anions leads to hydrocarbons (Kolbe electrolysis). Other reactions involving decarboxylations are briefly mentioned... 

Kolbe electrolysis (Section 17.7) The electrochemical conversion of carboxylate anions into hydrocarbons. The carboxyl radical produced loses CO2 to give an alkyl radical that dimerizes. 

Meanwhile, on reaching the anode, carboxylate anions are neutralized by hydrogen ions from the electrolysis of water. As resist solids are removed from the bath at the cathode, there is a gradual build up of ionizer in the bath. Therefore, to maintain bath chemistry, free acid must be removed by ultrafiltration, or drag-out, or the use of semipermeable membranes (anolyte boxes) [3]. 

Kolbe electrolysis The electrolysis of sodium salts of carboxylic acids to prepare alkanes. The alkane is produced at the anode after discharge of the carboxylate anion and decomposition of the radical RCOO- RCOO + e-RCOO —> R + CO2 R + RCOO —> R — R + CO2... 

In the aliphatic series the carboxylic acids furnish the principal material of electrolysis. This is due to the reactive-ness of their anions, which readily split off carbonic acid, thus affording manifold syntheses. In the aromatic series, however, the nitro-compounds are the more interesting, on account- of their easy rcducibility and the importance of their reduction products. The facts which give to electrochemical reduction pre-eminence over oxidation have already been explained in... 

Electrolysis of carboxylic acids in the presence of nitrate [109,110], difluoroamide [111], azide [112], and bromide ions [113] has been examined in which radical species generated from both carboxylic acids and additive anions are formally coupled to give substituted products ... 

Thiophene is not reducible by direct electroreduction, but by indirect reduction (Chapter 29) in DMF using biphenyl radical anion as electron transfer agent it is possible to reduce thiophene to 2,5-dihydrothiophene and tetrahydrothiophene in high yield [192] thiophenes substituted at C-2 with carboxyl, however, may be reduced by direct electrolysis to the 2,5-dihydro derivatives [193]. 

Ionic additives to the electrolyte can influence the Kolbe electrolysis in a negative way. Anions other than the carboxylate should be excluded, because they hinder the formation of a carboxylate layer at the anode, that seems to be a prerequisite for the decarboxylation. In the electrolysis of phenyl acetate the coupling to dibenzyl is totally suppressed when sodium perchlorate is present in concentrations of 5 x 10" mol 1" benzyl methyl ether, the nonKolbe product, is formed instead. This shift from the radical... 

The anionic ED coating is assumed to be induced by the reverse reaction of the electric dissociation of surface carboxylic groups on the resin particles of a paint, which is motivated by the accumulation of h ions upon the electrolysis of water (30) ... 

Whilst medium-chain-length carboxylic acids (six or more carbon atoms) can be directly oxidized in anhydrous fluorosulfuric acid containing potassium fluorosulfate, those of lower molecular weight are not oxidized before the oxidation of the fluorosulfate anion. For their conversion, solutions of peroxydisulfuryl difluoride, which is known to react also with short-chain alkanes " , are prepared by constant-current electrolysis of potassium fluorosulfate and added to the carboxylic acids in fluorosulfonic acid, y- and -Lactones are obtained in yields between 7 and 50% (equation 19) . 

During the electrolysis of a carboxylic acid 3, an electron is removed (at the anode) from the car-boxylate anion 1, leading to the corresponding carboxyl radical 5. This unstable species rapidly loses CO2, generating the C-centered radical 6. Coupling between two radicals affords the dimer 2 (Scheme 2) [13]. 

Terminal acetylenes can be carbonylated (1 atm. CO-CuC -PdCL catalyst-ROH) to give a,/S-acetylenic esters in 60—70% yields this reaction would seem to be a very expedient alternative to the conventional carboxylation procedure using acetylenic anions.The acetylenic ester anion (182) can be derived from methyl propiolate using Bu"Li at -120 and condenses cleanly with carbonyls. Ethyl alk-2-ynoates (183) can be converted into the alk-3-ynoates (184) by electrolysis in HMPA or DMF containing excess alkyl halide (R X) yields are 60—80% when R = alkyl but the reaction fails when R =... 

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