Acetals

An acetal is an organic functional group in which a carbon atom is bonded to two alkoxy groups (–OR) and to two other substituents. In simple terms, acetals are the fully substituted products of aldehydes or ketones reacting with alcohols, giving structures of the form R2C(OR')2 for acetals derived from aldehydes, or RCR'(OR')2 for those derived from ketones. The carbonyl carbon is replaced by a carbon bearing two –OR groups. Acetals are typically formed from aldehydes or ketones and two equivalents of an alcohol under acid catalysis, with removal of water driving the reaction. The mechanism proceeds through a hemiacetal intermediate before the second alcohol adds. Acetals are generally stable to basic conditions but hydrolyze in the presence of aqueous acid to regenerate the carbonyl compound and alcohols.

Because of their stability under many reaction conditions and their predictable acid-catalyzed hydrolysis, acetals are widely used as protecting groups for carbonyls in organic synthesis. Dimethoxymethane, also called the dimethyl acetal of formaldehyde, is a classic example formed from formaldehyde and methanol. Cyclic acetals form when a diol, such as ethylene glycol, reacts with a carbonyl to give 1,3-dioxolanes or 1,3-dioxanes; these are common protecting groups for aldehydes or ketones (for example, isopropylidene acetals derived from acetone).

In materials science, acetals appear in polymers such as polyoxymethylene, and related acetal linkages are encountered in various industrial reagents and applications. Overall, acetals are a versatile class of compounds both as functional groups and as protective tools in synthesis.