MBIO 2360 Lecture Notes - Lecture 13: Furanose, Ribose, Ketose
1 Dec 2016
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Document Summary
In solution, many carbohydrates exist in cyclic form. This is because of a tendency or carbonyl groups to react reversibly with alcohols: draw. Since each of the carbohydrates we have discussed has both a carbonyl and one or more alcoholic hydroxyls, an intramolecular hemiacetal or hemiketal is possible, giving a cyclic structure. The main restriction is steric, small rings (4 or fever atoms in the ring) are unstable, so the stable rings will be those containing 5 or 6 atoms (6-membered are more stable than 5 membered) The reactions create one additional chiral centre: the anomeric carbon. This 6-membered ring is preferred over the less stable 5-membered ring: the structure is called a pyranose because of the ring pyran, draw. Fischer structures (non-cyclic) are correct for c3 or c4 sugars in solution, otherwise ring structures are more stable. In water - all three forms exist in equilibrium.
