Lactol

In organic chemistry, a lactol 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.