Kolbe synthesis of nitriles

Suitable starting materials for the Kolbe electrolytic synthesis are aliphatic carboxylic acids that are not branched in a-position. With aryl carboxylic acids the reaction is not successful. Many functional groups are tolerated. The generation of the desired radical species is favored by a high concentration of the carboxylate salt as well as a high current density. Product distribution is further dependend on the anodic material, platinum is often used, as well as the solvent, the temperature and the pH of the solution.  

A common method for the preparation of alkyl cyanide 2 is the treatment of corresponding alkyl halides 1 with cyanide. The corresponding reaction with aromatic substrates is called the Rosenmund-von-Braun reaction. 

The Kolbe synthesis of nitriles is an important method for the elongation of an alkyl chain by one carbon center (see also the Arndt-Eistert synthesis). The nitrile 2 can for example easily be converted to the corresponding carboxylic acid by hydrolysis. 

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Sandmeyer s synthesis of aromatic nitriles is far more elegant than the removal of water from the ammonium salts of carboxylic acids, which latter reaction is also applicable to benzene derivatives. In particular, the former synthesis permits of the preparation of carboxylic acids via the nitriles, and so provides a complete substitute for Kolbe s synthesis (alkyl halide and potassium cyanide), which is inapplicable to aromatic compounds. The simplest example is the conversion of aniline into benzoic add. The converse transformation is Hofmann s degradation (benzamide aniline, see p. 152). 

The reaction of primary aliphatic halides and alkali metal cyanides - the Kolbe Nitrile Synthesis - gives nitriles in good yields. 

German organic chemist. Kolbe made many important contributions to the development of organic chemistry. These included the synthesis of ethanoic acid from completely inorganic materials, the formation and hydrolysis of nitriles, the electrolysis of solutions of fatty acid salts and the synthesis of salicylic acid from phenol and carbon dioxide. In his later years his influence had a negative effect on the development of chemistry because he opposed concepts such as structure. 

Fig. 3.11. Plausible, but incorrect mechanism of the phase-transfer catalyzed dichlorocyclopropanation of alkenes as often encountered in (close ) analogy to the Bu4N Cl catalyzed Kolbe nitrile synthesis in the two-phase system CH /aqueous solution of NaCN.

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