Figure 6.8. Acetal and Ketal Formation
Once a hemiacetal or hemiketal is formed, the hydroxyl group is protonated and released as a
molecule of water; alcohol then attacks, forming an acetal or ketal.KEY CONCEPT
In the formation of hemiacetals and hemiketals, alcohol is the nucleophile and the carbonyl
carbon is the electrophile. In the formation of acetals and ketals, alcohol is the nucleophile,
and the carbocation carbon (formerly the carbonyl carbon) is the electrophile.IMINES AND ENAMINES
Nitrogen and nitrogen-based functional groups act as good nucleophiles due to the lone pair of
electrons on nitrogen, and react readily with the electrophilic carbonyls of aldehydes and ketones.
In the simplest case, ammonia adds to the carbon atom and water is lost, producing an imine, a
compound with a nitrogen atom double-bonded to a carbon atom. This reaction is shown in Figure
6.9. Because a small molecule is lost during the formation of a bond between two molecules, this is
an example of a condensation reaction. Because nitrogen replaces the carbonyl oxygen, this is also
an example of a nucleophilic substitution. Some common ammonia derivatives that react with
aldehydes and ketones are hydroxylamine (H 2 N–OH), hydrazine (H 2 N–NH 2 ), and semicarbazide
(H 2 N–NH–C(O)NH 2 ); these form oximes, hydrazones, and semicarbazones, respectively.