An acetal is a functional group characterized by the presence of two ether groups bonded to a single carbon atom.

Acetals are synthesized through the reaction of an aldehyde or ketone with an alcohol, facilitated by an acid catalyst. This process begins with the nucleophilic addition of the alcohol to the carbonyl group of the aldehyde or ketone, resulting in the formation of a hemiacetal (from aldehydes) or a hemiketal (from ketones) intermediate. The acid catalyst subsequently protonates the oxygen atom in the hemiacetal or hemiketal, leading to the formation of an acetal or ketal, respectively.

In organic chemistry, acetals serve as crucial protecting groups for aldehydes and ketones. By converting an aldehyde or ketone into an acetal, the reactive carbonyl group is effectively masked. This alteration allows for the modification of other functional groups within a molecule without interfering with the carbonyl functionality. The acetal can then be removed easily through hydrolysis under acidic conditions, which regenerates the original aldehyde or ketone.

Additionally, acetals are present in biological systems and play significant roles in various metabolic pathways. For instance, acetaldehyde, a widely used industrial chemical and a key metabolic intermediate, can react with two molecules of ethanol to produce the acetal ethylidene diacetate. This compound is noted for its fruity aroma, contributing to the distinctive flavors of certain fruits and alcoholic beverages.