Question

Assertion: Fructose is a reducing sugar
Reason: Fructose has a ketonic group.
A. Both assertion and reason are correct and reason is the correct explanation for assertion
B. Both assertion and reason are correct but reason is not the correct explanation for assertion
C. Assertion is correct but reason is incorrect
D. Assertion is incorrect but reason is correct

Answer 1

Hint:Fructose is a simple ketonic monosaccharide. A reducing sugar is any sugar containing aldehyde or ketone group, which acts as reducing agent that may reduce the other molecule and itself gets oxidized to acid group

Complete answer:
The reducing sugar is the one that reduces another molecule and oxidizes itself. A sugar is classified as reducing sugar only if it has an open-chain form with free aldehyde group. All the monosaccharide is reducing sugars, along with some disaccharide and some polysaccharide. The monosaccharide can be divided into two groups – the aldose group that contain aldehyde group, and second is ketose, that contain ketone group. The aldehyde group can be oxidized via redox reaction, but the sugars with ketone group in their open chain form are capable of isomerizing via a series of tautomeric shifts to produce aldehyde group. Therefore ketone containing monosaccharide like fructose are considered reducing sugar but it is the isomer containing an aldehyde group which is actually reducing since ketones cannot be oxidized. This type of isomerization is catalyzed by a base present in the solution already. Like tollens reagent, an oxidizing agent is basic in nature therefore, the ketonic group gets isomerized to the aldehyde group and then can be oxidized to the acid group.
So we can say that reducing sugar are those which can reduce reagents like tollen’s reagent or Benedict solution. So fructose is reducing sugar. Fructose has a ketonic group but it is a reducing sugar not because it has a ketonic group.

So the correct option will be B.

Note:
Disaccharide consists of two monosaccharide units and both units. It can be reduced or non reducing depending. In non reducing sugar the two monosaccharide units are linked at their respective anomeric carbons, therefore the carbonyl group will not be free. In reducing disaccharide, the reducing sugar pair has the free hemiacetal group that can perform as the reducing aldehyde group.