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Glucose is a six-carbon aldohexose. The straight chain hexose structures can become cyclic. When aldehydes and ketones undergo reactions with alcohols, hemiacetals or hemiketals are formed. In aldohexoses, the cyclic structure is formed when the hydroxyl group in the fifth carbon reacts (nucleophilic addition) with the carbonyl carbon of the aldehyde group. The product formed is a hemiacetal.

A few facts worth remembering

  1. The six-membered (hemiacetal) ring formed is called the pyranose ring.
  2. The oxygen in the ring is from the hydroxyl group involved in the ring closure.
  3. The carbon which is denoted as “carbon one” in the cyclic structure is the carbonyl carbon of the straight chain (the aldehyde group).
  4. Notice that the cyclic structure can go either way in the sense that the resulting cyclic structure can either be an α- or a β-anomer.
  5. The carbon number `one’ shown in the cyclic structure is called the anomeric carbon.
  6. In aqueous solutions, the α- and β-anomers can interconvert, and the process is termed mutarotation.


Figure: The formation of cyclic structures of glucose


Fructose is a ketohexose sugar. The cyclic structure formed is called a hemiketal, and the five membered ring that is formed is called furanose ring. The straight chain structure and the cyclic structures of fructose are shown in the below figure.


Epimers are any two isomeric sugars which differ in their configurations only at one of the chiral carbon atoms. Glucose and mannose are epimers. Glucose and galactose are also epimers.

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