hemiacetal

A hemiacetal is an organic functional group formed by the addition of an alcohol to a carbonyl compound, either an aldehyde or a ketone. In the aldehyde case, the product has the form R-CH(OH)-OR′, where the carbon bearing the hydroxyl and alkoxy groups is attached to a hydrogen and an R group. In the ketone case, the product is R1-C(OH)(OR′′)-R2, where the carbonyl carbon is bonded to two carbon substituents as well as the new hydroxyl and alkoxy groups. The central carbon in a hemiacetal is typically a stereogenic center when R groups are different.

Hemiacetals are often intermediates in the acid-catalyzed conversion of carbonyls to acetals. Addition of an alcohol to a carbonyl under acid catalysis yields the hemiacetal, which can then be further reacted with another molecule of alcohol to give an acetal. The two-step process explains why acetals are typically prepared in the presence of an alcohol and an acid: the hemiacetal stage is an intermediate that is stabilized momentarily before forming the acetal.

Cyclic hemiacetals are particularly important in carbohydrate chemistry. Intramolecular attack of a hydroxyl group on the carbonyl carbon can generate six-membered (pyranose) or five-membered (furanose) rings, yielding cyclic hemiacetals. This intramolecular reaction underlies mutarotation in aldoses and ketoses, and helps explain the existence of multiple anomeric forms in solution.

Hemiacetals generally exist in equilibrium with their parent carbonyl compound and alcohol, often favoring the carbonyl form in the absence of internal stabilization. They are distinguished from acetals by having one OH and one OR group on the same carbon rather than two OR groups.