Question

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

Answer 1

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

Complete step by step solution:
-Pentaacetate of D-glucose belongs to the glucose family and has the chemical formula as${{C}_{16}}{{H}_{22}}{{O}_{11}}$ and its molecular weight is 390.34.
-We can detect the presence or absence of the aldehyde group in the pentaacetate of D-glucose by making it to react with those reagents or the chemical compounds with which it shows the reactions if only the aldehyde group is present in pentaacetate of D-glucose, otherwise, it would not show any reaction with those reagents, indicating the absence of the aldehyde group.
The following reactions can be applied to detect the presence of the aldehyde group in pentaacetate of D-glucose;
1. Reaction with the hydroxylamine: when D-glucose is reacted with the hydroxylamine, it results in the formation of oxime due to the aldehydic group present in it. But pentaacetate of D-glucose doesn’t give any reaction with the hydroxylamine as it lacks the aldehyde group.
2.Reaction with the Tollens’ reagent test( ammoniacal silver nitrate solution): It is the test given by those compounds only which contains the aldehydic group because it oxidizes the aldehyde group present in that very compound but pentaacetate of D-glucose doesn’t give this reaction indicating the absence of the aldehyde group in it.
3. Reaction with the Fehling solution: Like tollen’s reagent, it is the test given by those compounds only which contains the aldehydic group because it oxidizes the aldehyde group but pentaacetate of D-glucose due to the absence of the aldehyde group in it; doesn’t give this reaction.
Thus, all the above reactions indicate the absence of the aldehyde group in the pentaacetate of D-glucose.

Note: Tollen’s reagent is the ammonical solution of the silver nitrate solution. On the other hand, Fehling reagent consists of types of solutions Fehling solution A of aqueous copper sulphate (blue) and Fehling solution B of alkaline sodium potassium tartrate (colourless).