Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Addition Reactions Involving Carbonyls and Nitriles
9.8 Addition of Amines to Carbonyl Compounds

In this section, we will examine the addition of ammonia and other types of amines to carbonyl compounds. Reaction (9-61) shows the addition of ammonia to 2-butanone.

(9-61)

Reaction (9-62) gives the reaction of 2-butanone with methanamine, which is a primary amine.

(9-62)

Imines that have an alkyl group bonded to the nitrogen of the carbon—nitrogen double bond are also known as a Schiff base, named after Hugo Schiff, an Italian chemist, who discovered these types of reactions. The mechanism for the formation of imines from the addition of ammonia and amines to carbonyl compounds involves a combination of two reactions introduced in Chapter 6, addition—elimination reactions. In the first step of the reaction mechanism for Reaction (9-62), the carbonyl group is protonated since the medium is acidic, followed by the addition of methylamine as shown in Reaction (9-63).

(9-63)

Once the addition of methanamine to 2-butanone occurs, there is a loss of a proton. In the next step in the sequence of reactions, the protonation of the OH group occurs converting it to a good leaving group, H₂O, followed by the loss of water and the formation of a resonance-stabilized carbocation, which is stabilized from the lone-pair of electrons from the adjacent nitrogen, as shown in the reaction steps in Reaction (9-64).

(9-64)

In the last step of the mechanism, a proton is lost to form the imine, as shown in Reaction (9-65).

(9-65)

Other addition reactions involving different types of amines are shown in Reactions.

(9-66)

(9-67)

(9-68)

Problem 9.13

Give a mechanism for the addition—elimination (of water) reactions for the reaction shown in Reaction (9-66).