Question

Which of the following reagents cannot distinguish between glucose and fructose?
A. Tollen’s reagent
B. Fehling’s solution
C. Benedict’s solution
D. All of these

Answer 1

Hint: Benedict's solution (often called Benedict's qualitative solution or Benedict's reagent) is a chemical reagent and complex mixture of sodium carbonate, sodium citrate and copper(II) sulfate pentahydrate. It is often used in place of Fehling's solution to detect the presence of reducing sugars.

Complete step by step answer:
Glucose is a simple sugar with the molecular formula $$C_6{H}_{12}{O}_6$$ . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight, where it is used to make cellulose in cell walls, which is the most abundant carbohydrate. Glucose is an aldohexose monosaccharide and has the aldehydic group as the most primary group in it.
Fructose, or fruit sugar, is a simple ketonic monosaccharide found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the blood during digestion.
Benedict's test detects the presence of aldehydes, alpha-hydroxy-ketones, and hemiacetals, including those that occur in certain ketoses. Thus, although the ketose fructose is not strictly a reducing sugar, it is an alpha-hydroxy-ketone and gives a positive test because it is converted to the aldoses glucose and mannose by the base in the reagent. Oxidation of the reducing sugar by the cupric ($$C{u}^{2+}$$ ) complex of the reagent produces a cuprous ($$C{u}^{+}$$ ), which precipitates as insoluble red copper(I) oxide ($$C{u}_{2}O$$ ).

Note:
The presence of the reducing substances in a solution also gives a positive Benedict test result. The tests that use Benedict’s reagent are called the Benedict's tests. A positive test with Benedict's reagent is shown by a color change from clear blue to brick-red with a precipitate.