Q. 67

Write the reactions of D-glucose which can’t be explained by its open-chain structure. How can cyclic structure of glucose explain these reactions?

Answer :

The open chain structure if glucose could explain most of its properties but failed to explain the following:-


• In spite of the presence of an aldehyde group, glucose does not restore the pink colour of Schiff’s reagent, does not give 2,4-DNP test and also it does not form any addition product with sodium hydrogen sulphite and ammonia.


• Glucose pentaacetate does not react with hydroxyl amine thus it indicates the absence of free –CHO group.


• An aqueous solution of glucose shows mutorotation, which means that the specific rotation decreases from +110° to +52.5° for α-glucose and increases from +19.7° to +52.5° for β-glucose.


• Glucose exists in two crystalline forms alpha(α) and beta(β). The α-form (m.p. = 419 K) crystallises from a concentrated solution of glucose at 303 K and the β-form (m.p = 423 K) crystallises from a hot and saturated aqueous solution at 371 K.


The above facts of glucose can be explained in terms of cyclic structure of glucose which is formed through intramolecular hemiacetal formation which leads to cyclization.



Glucose


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