Kolbe electrolysis electrochemical oxidation

In Kolbe electrolysis, electrochemical oxidation occurs in aqueous sodium hydroxide solution, leading to the formation of a hydrocarbon. 

Electrochemical methods are very useful in structural studies but are barely applicable for preparative aims. The cause is the limited stability of cation radicals. It is difficult to do low-temperature preparative electrolysis, and the main problem is to dispose of the large amount of heat generated during the electrode work. That is, not much current can be passed through an ordinary-sized electrode without generating too much heat. When potential and temperature control are necessary, only small quantities of a material can be obtained in a reasonable period of time. When potential and temperature control are not necessary, as in Kolbe electrolysis, anodic oxidation is indeed useful as a preparative method. 

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 ... 

Kolbe electrolysis also allows some comparisons with analogous homogeneous reactions with regard to dimerization, substitution, or addition reactions of the generated radicals. Photolytic or thermal decarboxylation of diacylperoxides is a source of alkyl radicals similar to those afforded by the Kolbe electrolysis. The anodic oxidation of propionate has been compared with the thermal decomposition of dipropionyl peroxide [28]. Examination of the yields shows that reaction between radicals is favored in the electrochemical process, whereas in peroxide decomposition hydrogen atom abstraction from the solvent or the substrate occurs to a higher extent. This illustrates the effect of the higher radical concentration at the electrode. 

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). 

Corey and coworkerssuggested that when the reactant in the Kolb6 reaction is such that the R group will form a relatively stable carbocation, products other than Kolbe dimers (RR) or monomers (RH) may be formed by an additional electrochemical step. To test this idea they oxidized compounds which can generate stable carbocations. For instance, the electrolysis of either exo- or endo-5-norbornene-2-carboxylate (16) in methanol gave 3-methoxynortricyclene (17) in 50 % yield. 

The anodic oxidation of carboxylic acids has also been carried out using a solid-supported base in methanol. Non-Kolbe-type electrolysis takes place to give the corresponding methoxylated product (Equation 12.7). The acid-base reaction between a carboxylic acid and a solid-supported base seems to reduce the cell voltage and makes the electrochemical reaction possible. Kolbe-type carbon-carbon coupling using aliphatic and benzylic carboxylic acids has also been accomplished using this method [26]. Based on a similar concept, anodic fluorination by an alkali metal... 

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