Nucleophilic attack on the ketone on the ketone or aldehyde by the lone- pair electrons of an amine leads to a dipolar tetrahedral intermediate.
A proton is then transferred from nitrogen to oxygen, yielding a neutral carbinolamine.
Acid catalyst protonates the hydroxyl oxygen.
The nitrogen long-pair electrons expel water, giving an iminium ion.
Loss of H+ from nitrogen then gives the neutral imine product.