Question

Excess ammonia must be used when a primary amine is synthesized by reductive amination. What product will be obtained if the reaction is carried out with excess carbonyl compound?

Short answer

The product of the reaction will be a tertiary amine.

Step-by-step solution

Reductive amination

The reaction of a carbonyl compound with ammonia to form an imine which on reduction gives a primary amine, is called reductive amination.

Mechanism of reductive amination

The reductive amination reaction generally involves two steps: imine formation and its reduction to form amine.

During the imine formation, the electrophilic carbon atom of the carbonyl compound reacts with the nucleophilic nitrogen atom of ammonia to form the imine through three tetrahedral intermediates.

Use of excess carbonyl in reductive amination

The reductive amination of a carbonyl using ammonia gives a primary amine. If an excess of carbonyl is present in the solution, the formed primary amine can undergo another reductive amination to form secondary amine.

The secondary amine formed also reacts with the carbonyl in solution to form enamine, which is reduced to a tertiary amine. Since tertiary amines cannot form imines, the reaction is stooped at the tertiary amine stage.