Question

Which of the following reagents cannot distinguish between glucose and fructose?
a) Tollens reagent
b) Fehling’s solution
c) Benedict’s solution
d) All of the above

Answer 1

Aldehydes react with Tollen's reagent and Fehling's solution however ketones do not react with these reagents. And we know that in glucose, aldehyde, and in fructose, ketone is present as a functional group.

Complete step by step answer:
Let us know first about glucose and fructose.
Glucose is considered as a simple sugar with the molecular formula, ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{12}}}{{\text{O}}_{\text{6}}}$. Glucose is the most frequently found monosaccharide (simplest carbohydrates). The glucose molecule has an open and unbranched backbone of six carbon atoms, C-1 through C-6. C-1 is the carbon of aldehyde group H(C=O)−, and the other five carbons bear one hydroxyl group −OH. All the remaining bonds of the backbone carbons are satisfied by hydrogen atoms (−H). Therefore, glucose is considered as an aldohexose. This is because it is a hexose as well as an aldose. The aldehyde group makes glucose is a reducing sugar, giving a positive reaction with the Fehling test and Tollen’s test.
- Fructose is a simple ketonic monosaccharide that is found in many plants and is hence called fruit sugar. As in fructose, the keto group is present, which results in the formation of the intramolecular hemiacetal. In this arrangement, ${{\text{C}}_{\text{5}}}\text{-OH}$ combines with the ketonic group present in the second position (C=2). This results in the formation of chiral carbon and two arrangements of $\text{C}{{\text{H}}_{\text{2}}}\text{OH}$ and OH group. Fructose is both hexose and ketose or ketohexose.
- However, Benedict's solution cannot be used to distinguish between glucose and fructose because it is used to differentiate between reducing and non-reducing sugars, and Fructose and glucose are both reducing sugars.

So, the correct answer is “Option C”.

Note: Tollen’s reagent and Fehling’s solution are used to differentiate the carbohydrates having ketone and aldehyde as a functional group. And Benedict’s solution is used to differentiate reducing and non-reducing sugar.