The Kolbe nitrile synthesis is a method for the preparation of alkyl nitriles by reaction of the corresponding alkylhalide with a metal cyanide [1]. A side product for this reaction is the formation of an isonitrile because the cyanide ion is an ambident nucleophile and according to Kornblum's rule is capable of reacting with either carbon or nitrogen. The reaction is named after Hermann Kolbe.
The ratio in which both isomers form depends on the solvent and the reaction mechanism. With the application of alkali cyanides such as sodium cyanide and polar solvents the reaction type is an SN2 reaction whereby the alkylhalide is attacked by the more nucleophilic carbon atom of the cyanide ion. This type of reaction together with dimethyl sulfoxide as a solvent is a convenient method for the synthesis of nitriles.[2]
When silver cyanide is reacted with diethyl ether as a solvent the reaction runs according to an SN1 reaction mechanism: the product is mainly isonitrile due to the attack of the more nucleophilic nitrogen atom.
In particular primary alkyl halogenides and benzyl halogenides give good chemical yields when reacted with alkali cyanides to form nitriles. Secondary alkylhalides, on the other hand, only react to form undesirable tertiary halides.
Once formed the nitriles can be converted to the corresponding carboxylic acids by hydrolysis and then to more functional groups. In this way the Kolbe Nitrile synthesis is an important method in homologization.