Fructose, also known as levulose, is a simple monosaccharide carbohydrate with the molecular formula of C6H12O6. It contains six carbons, making it a hexose, and is classified as a ketose sugar due to the presence of a ketone functional group when the molecule is in its acyclic linear form.
Naturally occurring fructose is present exclusively as the levorotatory stereoisomer. In solution, fructose adopts a cyclic structure with a dynamic equilibrium between pyranose and furanose structural isomers. The pyranose form of fructose contains a six-membered heterocyclic ring, whereas the furanose form of fructose contains a five-membered ring. Both cyclic forms of fructose contain hemiketal functional groups.