SolutionsCondensation reactions: Mannich, Knoevenagel and others
Exercise 1
Why is an iminium ion more electrophilic than the aldehyde it comes from, whereas the neutral imine is less electrophilic?
In the neutral imine, nitrogen is less electronegative than oxygen and polarizes the double bond less: the carbon is less electrophilic than in the carbonyl. In the iminium ion nitrogen carries a positive charge, the double bond is strongly polarized and the carbon has strong carbocation character (resonance form C+–NR2), so it is more electrophilic than the neutral carbonyl.