hemiketal

Meaning: A hemiketal forms when an alcohol oxygen attacks the electrophilic carbonyl carbon of a ketone (nucleophilic addition). The carbonyl carbon becomes tetrahedral, bonded to an OH, an OR, and two carbon substituents. The reaction is reversible and catalyzed by acid or base. For simple ketones and separate alcohols, the equilibrium often favors the ketone. When the alcohol is in the same molecule and a five- or six-membered ring can form, the cyclic hemiketal is favored. With acid and a second alcohol, the OH is replaced by OR and water is lost, giving a ketal.

Equation: R₂C=O + R'OH ⇌ R₂C(OH)(OR'), where R is a carbon group on the ketone carbon and R' is the alkyl group of the alcohol. These are structural labels, so no SI units apply.

On the MCAT: Carbohydrate passages are the usual setting. In fructose, the C2 ketone reacts with the C5 hydroxyl to form a furanose ring (or with the C6 hydroxyl to form a pyranose ring). The hemiketal carbon is the anomeric carbon, so α and β anomers interconvert through the open-chain form (mutarotation). Questions ask you to spot the hemiketal in a ring drawing, or to predict that adding an alcohol with acid gives a ketal, as in a glycosidic bond.

Common confusion: A hemiacetal comes from an aldehyde, so its carbon bears an H. A hemiketal comes from a ketone, so its carbon bears two carbon groups. A ring ether is not a hemiketal unless the same carbon also carries an OH. A ketal has two OR groups and no OH.