12.24: Aldehydes and Ketones with Amines: Imine and Enamine Formation Overview
Primary amines react with carbonyl compounds—aldehydes and ketones—to generate imines. Imines consist of a C=N double bond and are named Schiff bases after its discoverer—the German chemist Hugo Schiff. On the other hand, secondary amines react with carbonyl compounds to give enamines. In enamines, the presence of a C=C double bond adjacent to the nitrogen atom leads to the delocalization of the lone pair.
Both imine formation and enamine formation are reversible and acid-catalyzed. Maintenance of proper pH of the reaction medium is necessary to control the formation rate. Both reaction mechanisms follow similar steps and can be divided into carbinolamine formation and water elimination. The only difference lies in the final step—imine formation involves proton abstraction from N–H of the iminium ion, whereas enamine formation involves proton abstraction from the α C–H of the iminium ion.
Since the reactions are reversible, both imines and enamines can be converted to carbonyl compounds by hydrolysis. The reaction of carbonyl compounds with hydroxylamines, hydrazines, and semicarbazides results in imine derivatives such as oximes, hydrazones, and semicarbazones, respectively.