Condensation reactions: Mannich, Knoevenagel and others
Imines and iminium ions are the nitrogen analogues of carbonyl compounds and undergo nucleophilic additions similar to aldol reactions. Iminium ions, in particular, are very reactive electrophiles.
An iminium ion forms easily from a secondary amine and an aldehyde in acidic medium:
The order of reactivity towards nucleophiles is:
C=NR < C=O < C=N+R2 < C=O+H
Neutral imines are less electrophilic than carbonyls, because nitrogen is less electronegative than oxygen; once protonated or alkylated at nitrogen, however, they are more reactive than neutral carbonyls. That is why the reactions in this section are carried out in slightly acidic medium or with amines as catalysts.
