Oxidation of carboxylate anions

Radicals, (34), that subsequently dimerise, are also obtained through the anodic oxidation of carboxylate anions, RCO20, in the Kolbe electrolytic synthesis of hydrocarbons ... 

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... 

If these anions are oxidized at carbon anodes instead of Pt anodes, the main product is not the Kolbe dimer but an ester of the original carboxylic acid. This reaction (Hofer-Moest) is explained by the inherent instability of C radicals at highly anodic potentials, which are necessary for the anodic oxidation of carboxylate anions. At these potentials, the C radicals are oxidized to carbonium ions that react with carboxylate anions forming esters ... 

Considerable effort has been expended by electrochemists in the quest for radical intermediates in the Kolbe reaction, i.e. the anodic oxidation of carboxylate anions in solution. This reaction produces a variety of products dependent upon several factors such as solvent, pH, electrode material, etc. 

Electrodes coated with poly(4-vinyl-4, 4 -dibromotriphenylamine) mediated the oxidation of carboxylate anions... 

A further effect which has been known for many years is that of anions, which are specifically adsorbed at high anodic potentials on platinum, on the products of the oxidation of carboxylate ions. For example, carbonium ion-derived products can be obtained in the presence of such specific adsorption and this demands a complete change in reaction route (Fioshin and Avrutskaya, 1967 Glasstone and Hickling, 1934). 

The decarboxylation of carboxylate anions is carried out chemically by a variety of one-electron oxidants such as lead tetraacetate, uranyl nitrate, peroxides, quinones, pyridinium cations, etc.199 Importantly, the carboxylate anion (as... 

For anion-radicals, air (i.e., oxygen, carbon dioxide, and water [moisture]), on the whole, is an active component of the medium and so it should be removed before conducting reactions. Understandably, air inhibits anion-radical reactions The anion-radicals primarily formed are consumed at the expense of oxidation, carboxylation, and protonation. Certainly, oxidation can take place only if the acceptor organic molecule possesses a lower affinity for an electron than oxygen does or if one-electron oxidation of the anion-radical by oxygen proceeds more rapidly than the anion-radical decomposition into a radical and an anion (RX R + X ). 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

Nonetheless, a number of electroorganic synthesis reactions are known whose outcome i.e., whose yield and selectivity, is decisively determined by the nature of the electrode so that heterogeneous acceleration of at least one of several competitive reactions of the electrogenerated reactive intermediates might be anticipated. A famous case is the Kolbe reaction, which is essentially the anodic dimerization of alkyl radicals that are generated at platinum anodes by anodic oxidation of the anions of carboxylic acids ... 

Thermodynamically, the latter reaction is to be expected, as C radicals exhibit an oxidation potential that is at least 1 V more negative than the oxidation potential of carboxylate anions (213, 214). 

Runs 29-34 show the effects of carboxylate anions as adhesion promoters. Simple dipping in sodium acetate solution is as effective as seven hours immersion—hence the reaction here is rapid (Runs 29,30). Acetates of other cations are effective, as is glycine. Thus, carboxylate anions as well as the acids promote the SiOR-metal oxide reaction. 

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. 

The anodic oxidation of the carboxylate anion 1 of a carboxylate salt to yield an alkane 3 is known as the Kolbe electrolytic synthesis By decarboxylation alkyl radicals 2 are formed, which subsequently can dimerize to an alkane. The initial step is the transfer of an electron from the carboxylate anion 1 to the anode. The carboxyl radical species 4 thus formed decomposes by loss of carbon dioxide. The resulting alkyl radical 2 dimerizes to give the alkane 3 " ... 

The final step is the nucleophilic displacement of the oxyphosphonium group by the carboxylate anion via a SN2-mechanism, yielding ester 3 with inverted configuration at the stereogenic center, and triphenylphosphine oxide. A hydrolysis of the ester 3 will leave the new configuration unchanged, and yield the inverted alcohol 4 ... 

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