Carboxylate anion electrochemical oxidation

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 attempt to use these salts originated from the hope that their dissociation constants would be high even in low dielectric media, and the organic nature of perfluorinated alkyls would always assist the solubility of the salts in nonaqueous solvents. Because of the requirement for electrochemical stability, lithium carboxylates (RF-C02Li, where Rp- = perfluorinated alkyls) are excluded from consideration, because their oxidation still occurs at - 3.5 V vs lithium, which is similar to the cases of their non-fluorinated counterparts. Obviously, the electron-withdrawing groups do not stabilize the carboxylate anions sufficiently to alter their oxidative stability. 

The electrochemical results described above indicate that unlike in the cases of other cobalt-catalyzed oxidation processes where the Co /Co redox couple is invariably involved [19b,38], in the present case where cubane clusters of the general formula Co4(p3-0)4( J,-02-CR)4(L)4 are to be employed as catalysts for the air/02 or TBHP oxidation of alkylaromatics, alcohols, etc., we have a catalytic system wherein the oxidation states of cobalt cycle between +3 and +4. The kinetic inertness of Co(lll) coupled with the inadequately explored reactivity of Co(lV) thus make the catalysts based on C04O4 cubanes quite interesting [36]. We shall now discuss the resulting materials prepared by supporting the cubane-like cobalt(lll)-oxo clusters discussed above in this section by following the chemical route in which the carboxylate anion derived from CMS-CH2CH2CO2H binds the in situ or preformed cobalt(III)-oxo tetramers at elevated temperatures. 

When the nickel complexes [NiX2(PPh3)2] (X = Cl, Br) are reduced electrochemically in the presence of C02 and aryl halides, aryl carboxylate anions are formed.585 The reduction leads to nickel(O) complexes which then undergo oxidative addition of the aryl halide followed by insertion of C02. Further reduction then gives the product and regenerates nickel(O) (Scheme 55). 

Elemental analysis and X-ray fluorescence measurements of films electrochemically oxidized in aqueous KCIO4 solutions, at pH values below the pK of the carboxylic acid indicate the absence of perchlorate counterions in the films [25]. This shows that the oxidized polymer backbone is charge compensated by covalently bound carboxylate anions and confirms the self-doping mechanism of poly( -(3-thienyl)octanoic acid). In contrast to films oxidized in aqueous solutions,... 

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). 

Photochemical studies of iron-group metallocene substrates have received much attention, e.g. quenching of excited uranyl ion luminescence, formation of charge-transfer complexes with TCNE, redox reactions of octamethyl-ferrocene and of carboxylate anions of the types FcCOj and Fc(X)COr (X = saturated or unsaturated alkyl chain). Enzyme-catalysed one-electron oxidation of ferrocene derivatives has also been studied. Ferrocene-ferricenium cation redox systems have been incorporated into electrochemical and photo-electrochemical cells, and have found use for catalysis of photo-addition of MeOH to Bu NCO. Inter-valence electron-transfer properties of partially oxidized Fc(C C)nFc ( = 0—2), meso-tetraferrocenylporphyrin, and poly(vinylferrocene) have been assessed. 

Recently, acylsilanes have been utilized as useful intermediates in organic synthesis [57], For example, treatment of acylsilanes with the fluoride ion generates the corresponding acyl anions which react with electrophiles. On the other hand, by using the electrochemical method, acylsilanes serve as acyl cation equivalents because nucleophiles are introduced at the carbonyl carbon. Chemical oxidation of acylsilanes with hydrogen peroxide which affords the corresponding carboxylic acids has been reported [58], However, the anodic oxidation provides a versatile method for the introduction of various nucleophiles... 

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... 

Ferrocene is a well-known guest for CD hosts. Harada and Takashi reported that P-CD and y-CD form 1 1 stoichiometric inclusion complexes with ferrocene, while a-CD forms a 2 1 complex[4]. Electrochemical studies on the interactions of ferrocene carboxylic acid (FCAH) ith p-CD, performed by Evans and coworkers[5], showed that this CD forms inclusion complexes with the FCA anion although no evidence was found for the complexation of its oxidized form. Other reports on the complexation of ferrocene derivatives by p-CD are available, including detailed molecular mechanics investigations on the docking of... 

Figure 4. Such a membrane is permeable to Fe(CN)6 - However, a cationic redox species, Ru(NH3)6, cannot permeate through this cationic supported membrane and this species is electrochemically silent. In contrast at pH 11, the carboxylic acid groups are carboxylate groups and anionic and the amine groups are neutral. The result is reversed activity toward the charged redox species with oxidation/reduction being facile for Ru(NH3)6 and Fe(CN)6 being electrochemically silent. At intermediate pH values, oxidation/reduction occurs for both species. These electrochemical experiments carried out in 3M NaCl show that responsive surfaces can affect the reactivity of their underlying support toward soluble reagents.

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