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Home » Chemistry Homework Help » Biochemistry » Glucose-Cyclic Structure
Glucose-Cyclic Structure
The open chain structure of glucose, could explain most of its properties, but failed to explain following properties:

1. Inspite of the presence of an aldehydic group, glucose does not restore the pink colour of Schiff’s reagent does not give 2, 4-DNP test and does not form addition products with sodium hydrogen-sulphite and ammonia.

2. Glucose pentaacetate does not react with hydroxyl amine thus indicating the absence of free –CHO group.

3. When glucose was crystallized from a concentrated ethanolic solution at 30˚C, it gives –form having melting point 146˚C and specific rotation + 111˚. On the other hand, crystallization from hot pyridine gives -form having melting point 150˚C and specific rotation + 19.2˚. when either of the –or -form of glucose is dissolved in water and allowed to stand, the specific rotation changes slowly and attains as equilibrium value of + 52.7˚.

The spontaneous change of specific rotation of an optically active compound toward an equilibrium value is called mutarotation.

4. When dry hydrogen chloride gas is passed through a solution of glucose in methyl alcohol, a reaction takes place and a mixture of methyl -D-glucoside and methyl –D-glucoside is formed. These glucosides do not reduce Fehling’s solution and also do not react with HCN and hydroxylamine. This indicates that glucosides do not have free –CHO group.

During the formation of glucosides only one molecule of CH3OH combines with a mixture of glucose.

The above facts about glucose can be explained in terms of cyclic structure of glucose. The cyclic structure of glucose is formed through intramolecular hemiacetal formation which leads to cyclisation. In glucose –OH group of C-5 reacts with carbonyl carbon to form a ring structure consisting of five carbon atoms and one oxygen atom. Such a ring structure of five carbon atoms and one oxygen atom is called pyranose ring. It gets its name from heterocyclic compound pyran. In the similar way, the ring structure consisting of four carbon atoms and oxygen atom is called furanose in analogy with furan.

As a result of cyclisation the carbonyl carbon, C-1 also becomes chiral carbon atom and has two possible arrangements of H and OH groups around it. These two arrangements of glucose which differ only in the orientation of hydroxyl groups at C-1 are called anomers and are represented as –D-glucose and –D-glucose. The carbon C-1 is called anomeric carbon atom.

These two forms of glucose (–D-glucose and –D-glucose) are stereoisomers but are mirror image of each other. Hence, these are not enantiomers but are diastereomers. The six membered cyclic structure of glucose was established by R.D.Haworth.

Mutarotation can be accounted for by considering the gradual conversion of either from of glucose in aqueous solution into other till an equilibrium mixture aqueous into other till an equilibrium mixture is formed. This process of conversion of one anomeric form of glucose into the other chain structure. The equilibrium mixture also contains a small amount of open chain form. For example,
       


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