hemiketaal

Hemiketaal, or hemi-ketal, is a functional group that forms when a ketone reacts with an alcohol. The resulting carbon is bonded to both a hydroxyl group and an alkoxy group, giving a tetrahedral carbon center with four substituents: two carbon substituents from the original ketone, one OH, and one OR group. The general form is R-CR'(OH)-OR'', where the central carbon bears the OH and OR'' groups.

Formation and properties are typically reversible. Under acid or base catalysis, a ketone plus an alcohol can add to give a hemiketal; the reaction may equilibrate with the starting ketone and alcohol. Hemiketals can further react with a second equivalent of an alcohol under acidic conditions to form an acetal or ketal, a common strategy for protecting carbonyl groups in organic synthesis. Because the formation involves a carbonyl-derived intermediate, hemiketals are often in equilibrium with their starting materials in solution and are generally less stable than acetals, depending on the substituents and solvent.

In carbohydrate chemistry, hemiketals are central to the formation of cyclic forms of ketoses. For example, fructose can cyclize to give cyclic hemiketals such as the furanose and pyranose forms. This intramolecular hemiacetal/hemiketal formation explains the stability of cyclic sugars in solution and underpins many protective-group strategies and analytical approaches in carbohydrate measurements.

Applications of hemiketals mainly arise during carbonyl protection and the synthesis of acetals/ketals. They are typically transient intermediates, with stability influenced by ring size, substituents, and reaction conditions.