Nitrilium

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Nitrilium ions are reactive forms of nitriles. A nitrile becomes a nitrilium ion when it is protonated (giving [RCNH]+) or alkylated (giving [RCNR′]+). Although nitriles are weak bases and not very good nucleophiles, they can attack highly reactive electrophiles such as carbocations.

Nitrilium salts can be made by reacting nitriles with trialkyloxonium salts. The resulting nitrilium ions can be reduced to secondary amines with sodium borohydride in diglyme, giving compounds of the form RCH2–NH–R′.

Nitrilium ions are thought to be key intermediate species in several important reactions: hydrolysis of nitriles, the Beckmann rearrangement, Friedel–Crafts cyclization of amines to isoquinolines, the Schmidt reaction with ketones, and the Ugi, Ritter, Pinner, and Passerini reactions.