Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

Reactions of Primary Amines

imine

Summary

• Primary amines, R-NH₂ or ArNH₂, undergo nucleophilic addition with aldehydes or ketones to give carbinolamines which then dehydrate to give substituted imines.
• Imines are the N analoges of O systems in aldehydes and ketones
• The reactions are usually carried out in an acidic buffer to activate the C=O and facilitate dehydration but without inhibiting the nucleophile.
• Systems of the general type Z-NH₂ (see table) undergo this type of reaction and can be used as derivatives to aid in the identification of aldehydes and ketones.

Related Reactions

• Formation of Acetals
• Dehydration of Alcohols

NUCLEOPHILIC ADDITION OF A PRIMARY AMINE GIVING AN IMINE
Step 1: An acid/base reaction. Protonation of the carbonyl activates it and makes it more susceptible to attack by a neutral nucleophilic like the N of a primary amine.
Step 2: Attack of the N nucleophile at the electrophilic C of the C=O group with the electrons from the π bond going to the +ve O.
Step 3: An acid/base reaction. Removal of the proton neutralises the +ve charge on the N and forms the carbinolamine intermediate.
Step 4: To form the imine we need to dehydrate. However, before -OH leaves it needs to be protonated, so a simple acid/base reaction.
Step 5: Use the electrons of the N to help push out the leaving group, a neutral water molecule, forming a C=N in the form of an iminium ion.
Step 6: An acid/base reaction. Deprotonation of the iminium N reveals the imine product and regenerates the acid catalyst.

© Dr. Ian Hunt, Department of Chemistry