Isocyanides undergo addition reactions, but cyanides do not. Why?
Isocyanide can undergo addition reaction due to the fact that it is capable to attach both the electrophile and nucleophile in the same carbon atom while it is not achieved by cyanide. The negative charge on carbon atom accepts the electrophile to form an intermediate species in which the positive charge on nitrogen atom gets neutralized by the addition of the nucleophile to the carbon atoms and the nucleophile gets attached in the same carbon atom. Cyanide is incapable of undergoing such mechanism because of less nucleophilicity of nitrogen atom of cyanide and thus don't undergo addition reaction. The mechanism is as follows: