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Cyclic Structure of Glucose

Carbohydrates
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Cyclic Structure of Glucose

The cyclic structure of glucose has been proposed to explain those facts that could not be explained by the open-chain structure of glucose. In fact, the open-chain structure of glucose explained most of its properties, but the following facts could not be explained by this structure-

(1) Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with NaHSO3.

(2) The pentaacetate of glucose does not react with hydroxylamine indicating the absence of the free -CHO group.

(3) Glucose is found to exist in two different crystalline forms namely α and ꞵ-forms. The α-form of glucose (m.p. = 419K) is obtained by crystallization from a concentrated solution of glucose at 303K, while the ꞵ-form (m.p. = 423K) is obtained by crystallization from hot and saturated aqueous solution at 371K. This behavior could not be explained by the open-chain structure for glucose.

In fact, the -OH group of the fifth carbon atom of glucose molecule adds to the -CHO group and forms a cyclic hemiacetal structure. Thus, glucose forms a six-membered ring in which -OH at C-5 is involved in a ring formation. This explains the absence of -CHO group and also the existence of glucose in two cyclic forms which exist in equilibrium with open-chain structure.

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Important Reference Books

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                            (2) Modern’s abc of Objective Chemistry

(3) Biology-         (1) Biology by Trueman 

                            (2) Modern’s abc of Objective Biology

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