Nitrilium

A nitrilium ion is a nitrile that has been protonated, [RCNH]⁺, or alkylated, [RCNR′]⁺.^[1]

The general structures of nitrilium ions

Synthesis

Nitriles are only weakly basic^[2] and are poor nucleophiles, but they will attack very reactive electrophiles such as carbocations.^[3]

Nitrilium salts can be prepared by reacting nitriles with trialkyloxonium salts. The nitrilium ions thus formed can then be reduced to secondary amines with sodium borohydride in diglyme. This is a convenient route to secondary amines of the form RCH₂—NH—R′.^[4]

As intermediates

Nitrilium ions are believed to be intermediates in the hydrolysis of nitriles,^[5] the Beckmann rearrangement, the Friedel-Crafts cyclization of amines to isoquinolines,^[6] the Schmidt reaction with ketones,^[7] and the Ugi, Ritter, Pinner and Passerini reactions.