Kolbe electrolysis reaction conditions

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

The yield and selectivity of Kolbe electrolysis is determined by the reaction conditions and the structure of the carboxylate. The latter subject is treated in chaps 3, 4. Experimental factors that influence the outcome of the Kolbe electrolysis are the current density, the temperature, the pH, additives, the solvent, and the electrode material. 

Kolbe electrolysis conditions. The 6,)/-unsaturated carboxylic acids (VII) give the isomeric 1,5-dienes (IX-XI) via the allylic radicals (VIII) [Eq. (11)] [54]. Through this reaction, the configuration of the double bonds is almost totally (> 90%) retained. 

Major contributions to the mechanism of the Kolbe electrolysis have been made by Conway et al., who mainly used electroanalytical techniques, and by Eberson and Utley, who drew their conclusions from the dependence of product structures on reaction conditions. 

In the past three decades, more sophisticated electrolysis conditions suited for the Kolbe dimerization, for hydrogen abstraction, for substitution, for rearrangement, and for other radical- or cation-induced reactions have accumulated for a number of carboxylic acids and have been used for many synthetic purposes. Most recently, in the field of the... 

TV-Bromosuccinimide is recommended as a mild, efficient reagent for the a-bromination of acids this reaction, previously considered to be a free-radical process, has now been shown to be ionic in nature, and also to be subject to acid catalysis. A general synthesis of per-acids is provided by per-hydrolysis of acyldiethylphosphates under acid catalysis (Scheme 9) the intermediate esters are prepared by acylation of silver diethyl phosphate (24). This method is useful for the preparation of methoxyaromatic and sterically hindered per-acids, e.g. trimethylperacetic acid teing obtained from pivalic acid in 84% yield. A complementary method involving the intermediacy of acid imid olides has been published. The electrolysis of some a-ketoacids has been studied Kolbe processes can be suppressed under certain conditions. ... 

In organic chemistry well known is the electrosynthesis reaction of Kolbe, in which the electrolysis of sodium salts of carboxylic acids, carried out imder appropriate conditions, leads to the formation of free radicals at the anode. These free radicals undergo decarboxylation and linking with the formation of saturated hydrocarbons ... 

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