lactol

thumb|right|120px|The lactol functional group, highlighted in blue, is present in many sugars such as ribose shown here.

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 ().