Question

How do you explain the absence of the aldehyde group in the pentaacetate of D-glucose?

Answer 1

Hint: One can make pentaacetate of D-glucose to react with hydroxylamine, tollen reagent, and Fehling solution to detect the presence of the aldehyde group, which will display only these reactions if it contains the aldehyde group.

Complete step by step answer
D-glucose pentaacetate belongs to the glucose family and has the chemical formula $ {C_{16}}{H_{22}}{O_{11}} $ and its molecular weight is 390. We can detect the presence or absence of the aldehyde group in D-glucose pentaacetate by allowing it to react with certain reagents or the chemical compounds with which it demonstrates the reactions if only the aldehyde group is present in D-glucose pentaacetate, otherwise no reaction to those reagents would be seen, suggesting the absence of the aldehyde group.
To detect the presence of the aldehyde group in D-glucose pentaacetate, the following reactions can be applied;
Hydroxylamine reaction: when D-glucose is reacted with hydroxylamine, due to the aldehydic group present in it, oxime formation occurs. However, D-glucose pentaacetate does not react with hydroxylamine because the aldehyde group is absent.
Reaction with the reagent test of Tollens (ammoniacal silver nitrate solution): It is the test provided only by certain compounds containing the aldehyde group since it oxidizes the aldehyde group present in that very compound, but D-glucose pentaacetate does not give this reaction suggesting the aldehyde group's absence.
Fehling solution reaction: Like the tollen reagent, since it oxidizes the aldehyde group but D-glucose pentaacetate due to the absence of the aldehyde group in it, it is only the test supplied by certain compounds containing the aldehyde group; it does not give this reaction.
The absence of the aldehyde group in D-glucose pentaacetate is therefore indicated by all of the above reactions.

Note
The ammonium solution of the silver nitrate solution is Tollen's reagent. On the other side, Fehling reagents consist of types of aqueous copper sulphate (blue) Fehling solution A and alkaline sodium potassium tartrate Fehling solution B (colourless).