Organic Chemistry

Section 18.6 Reactions of Aldehydes and Ketones with Nitrogen Nucleophiles 747

18.6 Reactions of Aldehydes and Ketones

with Nitrogen Nucleophiles

Aldehydes and ketones react with a primaryamine to form an imine. An imine
is a compound with a carbon–nitrogen double bond. The imine obtained from the
reaction of a carbonyl compound and a primary amine is often called a Schiff base.

The orbital model of a group (Figure 18.1) is similar to the orbital model of a
group (Figure 17.1 on p. 676). The imine nitrogen is hybridized. One
of its orbitals forms a bond with the imine carbon, one forms a bond with a sub-
stituent, and the third contains a lone pair. The porbital of nitrogen and the porbital of
carbon overlap to form a bond.p

sp^2 s s

C“O sp^2

C“N

C

C

O ++R

H

R

NH 2 H 2 O

an aldehyde or

a ketone

a primary amine an imine

a Schiff base

C

C

H

N

(RNH 2 )

C N

C C

C

>Figure 18.1

Bonding in an imine

Aldehydes and ketones react with a secondary amineto form an enamine (pro-
nounced “ENE-amine”). An enamineis an -unsaturated tertiary amine—a tertiary
amine with a double bond in the -position relative to the nitrogen atom. Notice that
the double bond is in the part of the molecule that comes from the aldehyde or ketone.
The name “enamine”comes from “ene”“amine,”with the “e”omitted in order to
avoid two successive vowels.

When you first look at the products of imine and enamine formation, they appear to
be quite different. However, when you look at the mechanisms for the reactions, you
will see that the mechanisms are exactly the same except for the site from which a pro-
ton is lost in the last step of the reaction.

Addition of Primary Amines
Aldehydes and ketones react with primary amines to form imines. The reaction requires
a catalytic (small) amount of acid—we will see that the pH of the reaction mixture must
be carefully controlled.

C

O ++

R

H

R R

R

NH H 2 O

an aldehyde or

a ketone

a secondary amine an enamine

C

C

C N

+

a,b

a,b