What observations suggest a cyclic structure for the glucose molecule?

i) Aldehydes and ketones are known to form hemiacetals and acetals with alcohols. Acetals are more stable to basic solutions and nucleophilic attack. ii) Glucose itself exists in two forms- α and β form. Both, in solution, undergoes equilibration called ‘Mutarotation’ and have the same optical rotation of 52.7. Like an aldehyde and alcohol mixture, aldehyde and hydroxyl group of the glucose can undergo an intramolecular hemiacetal and acetal formation. 

Cyclic hemiacetal formation between the aldehyde and with the hydroxyl group of 5th carbon atom yields a very stable six-membered cyclic system. The cyclic structure is similar to the six-membered ring exhibited by a heterocyclic compound known as pyran. So this cyclic structure is referred to as glucopyranose, Bending, followed by a rotation of the fourth carbon and fifth-carbon bond, of glucose chain brings the C5-hydroxyl group and the aldehyde groups nearer to form hemiacetal containing a six-membered ring. The six-membered hemiacetal structure of glucose can be drawn as a flat hexagon like structure. (HAWORTH FORMULA). But, the glucose molecule cannot be planar because of the hybridization. It can exist in three forms as chair, boat or skew form with relative stability in the same order.

The hemiacetal carbon atom (C-1) is a new stereogenic centre and is referred to as anomeric carbon. Depending upon the orientation of the OH groups at this anomeric carbon, the hemiacetal can be of two forms (anomers), as α- glucose and β- glucose. Structure with hydroxyl group above the plane is designated as β-form and if below the plane as α-form. Both cyclic hemiacetals (anomers) and the open-chain form are in equilibrium in solution. 

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