section_path: Monosaccharides/Ring-straight chain isomerism
section_name: Ring-straight chain isomerism
paragraph_text: Ring-straight chain isomerism
  
  The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, forming a heterocyclic ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called furanose and pyranose forms, respectively, and exist in equilibrium with the straight-chain form.
  
  During the conversion from straight-chain form to the cyclic form, the carbon atom containing the carbonyl oxygen, called the anomeric carbon, becomes a stereogenic center with two possible configurations: The oxygen atom may take a position either above or below the plane of the ring. The resulting possible pair of stereoisomers is called anomers. In the α anomer, the -OH substituent on the anomeric carbon rests on the opposite side (trans) of the ring from the CH OH side branch. The alternative form, in which the CH OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called the β anomer.