hemiacetaler

Hemiacetal, sometimes referred to as hemiacetaler in some languages, is a functional group in organic chemistry. It features a carbon atom bearing both a hydroxyl group (-OH) and an alkoxy group (-OR') attached to the same carbon that originated from a carbonyl compound. The typical formation is by addition of an alcohol to an aldehyde, giving R-CH(OH)-OR'. If the starting carbonyl compound is a ketone, the analogous product is a hemiketal (R2C(OH)-OR').

Formation and reactivity: A hemiacetal forms when an alcohol adds to an aldehyde (or a ketone to give a hemiketal). This process is often acid- or base-catalyzed and is commonly encountered in protecting-group strategies and acetal formation. In many solvents, especially water, hemiacetals are in equilibrium with the corresponding carbonyl compound and the alcohol; they can be short-lived unless stabilized.

Stability and significance: Hemiacetals are generally reactive intermediates. They tend to convert into acetals or revert to the original carbonyl compound under hydrolytic conditions. A notable exception occurs in carbohydrate chemistry, where intramolecular cyclization of an aldehyde with a nearby hydroxyl group yields cyclic hemiacetals (and the related hemiketals). The cyclic forms are central to the structure and chemistry of sugars and determine anomeric configuration (alpha and beta) in many monosaccharides.

Applications: Hemiacetals and hemiketals are important in organic synthesis as intermediates in acetal/ketal formation, in protecting carbonyl groups, and in the study of carbohydrate structure and reactivity.