Q. 68

On the basis of which evidences D-glucose was assigned the following structure?



Answer :

The reactions of glucose suggest that its molecule contains one primary (-CH2OH) and four secondary (-C-HOH) hydroxyl groups. The evidences that support the following structure of glucose are:-


• Molecular formula of glucose is C6H12O6.


• Reduction - Its reduction to sorbitol in the presence of Ni or sodium amalgam as catalyst.



• Reaction with HI



• Acetylation – A pentaacetyl derivative of glucose is obtained when glucose reacts with acetic anhydride in presence of sulphuric acid. As we know that the presence of two or more hydroxyl groups present on same carbon atom makes the molecule unstable, but since glucose is stable, we conclude that the 5-OH are present on different carbon atoms.



• Oxidation - Glucose on oxidation with mild oxidizing agents like Tollen’s reagent or Fehling’s solution give gluconic acid



On the basis of all these above evidences, the structure of glucose is supported.


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