Lactol

File:Lactol group in ribose.png

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 (−CH(OH)O−) or a hemiketal (>C(OH)O−). The compound is formed by the intramolecular, nucleophilic addition of a hydroxyl group (−OH) to the carbonyl group (C=O) of an aldehyde (−CH=O) or a ketone (>C=O).

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.

File:Lactol equilibrium.png

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

Chemical reactivity

Lactols can participate in a variety of chemical reactions including:

• Oxidation to form lactones
• Reaction with alcohols to form acetals
  • The reaction of sugars with alcohols or other nucleophiles leads to the formation of glycosides
• Reduction (deoxygenation) to form cyclic ethers