Note on: Cyclic structure of glucose. - Vidyadip

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Note on: Cyclic structure of glucose.

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Answer

→Limitations of open chain structure of glucose shows that free -CHO group is absent in glucose.
→It was proposed that one of the -OH groups may add to the -CHO group and form a cyclic hemiacetal structure.
→It was found that glucose forms a six membered ring in which-OH at C-5 is involved in ring formation.
→This explains the absence of -CHO group and also existence of glucose in two forms as shown below:

→These two cyclic forms exist in equilibrium with open chain structure.
→The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at $C_1$ called anomeric carbon. (the aldehyde carbon before cyclisation)
→Such isomer i.e. a-form and ẞ-form are called anomers.
→The six membered cyclic structure of glucose is called pyranose structure (a- or ẞ-) in analogy with pyran.
→Pyran is a cyclic organic compound with one oxygen atom and five carbon atoms in the ring.
→The cyclic structure of glucose is more correctly represented by haworth structure as given below.

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