What is "acetal"

Acetal \Ac"e*tal\, n. [Acetic + alcohol.] (Chem.) A limpid, colorless, inflammable liquid from the slow oxidation of alcohol under the influence of platinum black.

n. (context organic chemistry English) Any diether of a geminal diol, R₂C(OR')₂ (where R' does not = H).

n. any organic compound formed by adding alcohol molecules to aldehyde molecules

An acetal is a functional group with the following connectivity RC(OR'), where both R' groups are organic fragments. The central carbon atom has four bonds to it, and is therefore saturated and has tetrahedral geometry. The two R'O groups may be equivalent to each other or not. The two R groups can be equivalent to each other (a "symmetric acetal") or not (a "mixed acetal"), and one or both can even be hydrogen atoms rather than organic fragments. Acetals are formed from and convertible to carbonyl compounds ( aldehydes or ketones RC=O). The term ketal is sometimes used to identify structures associated with ketones rather than aldehydes and, historically, the term acetal was used specifically for the aldehyde cases.

Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water. The carbocation that is produced is then rapidly attacked by a molecule of alcohol. Loss of the proton from the attached alcohol gives the acetal.

Acetals are stable compared to hemiacetals but their formation is a reversible equilibrium as with esters. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a Dean-Stark apparatus, lest it will hydrolyse the product back to the hemiacetal. The formation of acetals reduces the total number of molecules present and therefore is not favourable with regards to entropy. A way to improve this is to use an orthoester as a source of alcohol. Aldehydes and ketones undergo a process called acetal exchange with orthoesters to give acetals. Water produced along with the acetal product is used up in hydrolysing the orthoester and producing more alcohol to be used in the reaction.

Acetals are used as protecting groups for carbonyl groups in organic synthesis because they are stable with respect to hydrolysis by bases and with respect to many oxidizing and reducing agents. They can either protect the carbonyl in a molecule (by temporarily reacting it with an alcohol) or a diol (by temporarily reacting it with a carbonyl). That is, either the carbonyl, or the alcohols, or both could be part of the molecule whose reactivity is to be controlled.

Various specific carbonyl compounds have special names for their acetal forms. For example, an acetal formed from formaldehyde is sometimes called a "formal" or the methylenedioxy group. The acetal formed from acetone is sometimes called an acetonide.