Kolbe Nitrile Synthesis
The reaction of primary aliphatic halides and alkali metal cyanides - the Kolbe Nitrile Synthesis - gives nitriles in good yields.
Mechanism of the Kolbe Nitrile Synthesis
The Kolbe Nitrile Synthesis is a typical SN2 reaction, which runs best in polar aprotic solvents (DMSO, acetone):
Primary alkylating agents work best, while secondary bromides and chlorides react in moderate yields to give the desired nitriles. Tertiary halides mainly undergo side reactions, one of which is the E2 elimination:
Cyanide is an ambident nucleophile, and can also react on nitrogen to yield isonitriles.
According the HSAB principles, the carbon center is more basic and more nucleophilic. When protic solvents are used, the resulting greater solvation of this carbon center is thought to favor the competing reaction at the weaker nitrogen center. A similar rationale explains why the more covalent cyanide salts such as silver cyanides and cuprous cyanides also give isonitriles as main product.
As the isonitriles are rapidly hydrolyzed to amines and formic acid, an extraction step with hydrochloric acid is normally sufficient in practice to remove these impurities from a desired nitrile product.