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The lactol functional group, highlighted in blue, is present in many sugars such as ribose shown here.

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. [1]

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.

Lactol equilibrium.png

The lactol functional group is prevalent in nature as component of aldose sugars.

Chemical reactivity[edit]

Lactols can participate in a variety of chemical reactions including:[2]


  1. ^ IUPAC Gold Book lactols
  2. ^ Lundt, Inge (2001). "Oxidation, reduction and deoxygenation of carbohydrates". Glycoscience (1): 501–531.