The open chain structure of glucose can’t explain the following propertiesGlucose does not react with Schiff’s reagent.
Glucose does not react with sodium hydrogen sulphite.
Penta acetate derivative of glucose does not react with hydroxylamine.
Glucose in open chain structure does not show mutarotation In order to explain these properties, which are not consistent with the open chain structure, a ring structure was proposed for glucose.
Cyclic structure of glucose:
Ring structure of Glucose explain the properties which are not explained by open chain structure because Ring structure has no free aldehydic group, glucose does not respond to certain characteristic tests of aldehydes, like Schiff’s test and addition reaction with sodium- -bisulphite. The cyclic structure is attributed to the formation of hemiacetal the cyclic structure thus formed is a six-membered ring.
This cyclic structure has one chiral centre more than the open chain structure. Therefore, two possible isomeric forms are possible
Fischer projection formula:
These two isomers differ in the configuration of the hydroxyl group at C1.Stereoisomers that differ in the configuration at C1 are known as anomers.
C1 is called the anomeric carbon.
The six-membered ring structures of glucose can be called pyranose structures, in analogy with pyran. Hence, the anomers are called alpha D plus gluco pyranose and beta D plus gluco pyranose.
Cyclic structure of Fructose:
Fructose is a ketone and consists of six carbon atoms in a straight chain with the keto functional group at position 2 of the carbon chain.
As the open chain structure fails to explain certain facts like its existence in two isomeric forms and the formation of hydrogen sulphite addition product, the ring structure was established.
The cyclic structure is a five-membered ring. Therefore, there are two possible isomeric forms. The two cyclic forms differ in the configuration of the hydroxyl group at C 2. These isomers are called anomers.
The five-membered ring structure of fructose is called furanose. The cyclic structures of the two anomers are named alpha D minus fructofuranose and beta D minus fructofuranose.