In organic chemistry, a lactol is a functional group which is the cyclic equivalent of a hemiacetal () or a hemiketal ().
The compound is formed by the intramolecular, nucleophilic addition of a hydroxyl group () to the carbonyl group () of an aldehyde () or a ketone ().
A lactol is often found as an equilibrium mixture with the corresponding hydroxyaldehyde. The equilibrium can favor either direction depending on ring size and other conformational effects.
The lactol functional group is prevalent in nature as component of aldose sugars.