An acyliminium is an acyl iminium ion - an ion of iminium, a form of quaternary ammonium formed by an imine.

R2C=(NR2)+ Note: Nitrogen has a positive charge. It can be generated by treating an aldehyde or ketone with a primary or secondary amine.

The Dakin reaction involves the oxidation of an amine to form an imine using hydrogen peroxide and a base catalyst. In the first step, the amine is oxidized by hydrogen peroxide to form an intermediate called an imine or iminium ion. The base catalyst helps in the formation of the imine through deprotonation of the amine.