Acetale

Acetale is a term used in some languages to refer to what is more commonly called an acetal in English. An acetal is a carbon atom bonded to two alkoxy groups (OR) and two other substituents, typically written as R2C(OR')2. Acetals are formed from aldehydes or ketones that react with alcohols or diols under acidic conditions.

Formation and equilibrium

Acetals are typically prepared by acid-catalyzed reaction of a carbonyl compound with one or two equivalents of an alcohol or diol. The reaction is reversible and produces water as a byproduct. Removal of water or use of an excess of alcohol shifts the equilibrium toward acetal formation. Ketones and aldehydes each form acetals, though the rates and stabilities differ depending on substituents and reaction conditions.

Stability and deprotection

Acetals are generally stable under neutral and basic conditions but are sensitive to acid. Acidic hydrolysis cleaves the acetal back to the original carbonyl compound and the corresponding alcohols. This property makes acetals useful as protecting groups for carbonyl functionality in multi-step syntheses.

Types and examples

Cyclic acetals, such as 1,3-dioxolanes and 1,3-dioxanes, arise when diols react with aldehydes or ketones to form ring structures. Dimethyl acetal of formaldehyde (methoxymethane) is a simple acetal example derived from formaldehyde and methanol. Cyclic acetals derived from acetaldehyde or acetone are common protecting groups in organic synthesis.

Applications

The primary use of acetals is as protecting groups for aldehydes and ketones during reactions that would otherwise modify or consume the carbonyl. They are later removed by acidic hydrolysis to regenerate the carbonyl functionality.