Chapter 9: Problem 10
D-Glucose and D-Fructose can be differentiated by
(A) Fehling solution
(B) Tollen's reagent
(C) Benedict solution
(D)
Short Answer
Expert verified
D-Glucose and D-Fructose can be differentiated by the reaction with bromine water ( ), as it can detect the presence of an aldehyde functional group in D-Glucose and does not react with the ketone functional group in D-Fructose. Therefore, the correct answer is (D) .
Step by step solution
01
Analyze the reaction with Fehling solution
Fehling's solution is a deep-blue alkaline solution used to test for reducing sugars. The main component of Fehling's solution is copper(II) ions, which are reduced to copper(I) ions when reacting with reducing sugars. Both D-Glucose and D-Fructose are reducing sugars, so they both react with Fehling's solution, and this reagent cannot differentiate between them.
02
Analyze the reaction with Tollen's reagent
Tollen's reagent is a colorless, aqueous solution containing silver ions. When a reducing sugar is added to Tollen's reagent, silver ions are reduced to metallic silver, which appears as a "silver mirror" on the walls of the test tube. Both D-Glucose and D-Fructose are reducing sugars, so they both react with Tollen's reagent, and this reagent cannot differentiate between them either.
03
Analyze the reaction with Benedict solution
Benedict's solution is another complex reagent containing copper(II) ions and is used to test for reducing sugars. It functions similarly to Fehling's solution as it can detect the presence of reducing sugars, which in this case are both D-Glucose and D-Fructose. Therefore, Benedict solution cannot differentiate between the two sugars.
04
Analyze the reaction with
Bromine water, , is a reagent used to detect the presence of aldehyde functional groups. When an aldehyde (such as D-Glucose) reacts with bromine water, the solution gets decolorized due to the reaction between the aldehyde group and the bromine. As D-Fructose has a ketone functional group, it does not react with bromine water and the solution remains colored. Thus, bromine water can differentiate between D-Glucose and D-Fructose.
Since only the bromine water reagent can distinguish between D-Glucose and D-Fructose, the correct answer is:
05
Conclusion
(D)
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Fehling Solution and Reducing Sugars
Fehling solution is a commonly used chemical test to determine whether a carbohydrate is a reducing sugar. A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a free aldehyde or ketone group. When Fehling's solution is mixed with a reducing sugar and heated, a redox reaction occurs where the blue copper(II) ions are reduced to red copper(I) oxide, which precipitates out of the solution.
This color change, from blue to a red precipitate, is a positive indication of the presence of a reducing sugar. Both D-Glucose and D-Fructose are considered reducing sugars as they have free aldehyde and ketone groups, respectively. However, because Fehling's solution reacts with both in a similar manner, turning the solution from blue to red, it does not serve as a differentiating test between these two monosaccharides.
This color change, from blue to a red precipitate, is a positive indication of the presence of a reducing sugar. Both D-Glucose and D-Fructose are considered reducing sugars as they have free aldehyde and ketone groups, respectively. However, because Fehling's solution reacts with both in a similar manner, turning the solution from blue to red, it does not serve as a differentiating test between these two monosaccharides.
Tollen's Reagent Reaction
Tollen's reagent, also known as the silver mirror test, is used to identify the presence of aldehydes. It consists of silver nitrate in ammonia, which forms a complex with a free aldehyde group under alkaline conditions, reducing the silver ions to metallic silver. The result is the appearance of a striking silver mirror on the inside of the test tube.
Because Tollen's reagent reacts with aldehydes, it is an effective test for monosaccharides with aldehyde groups, such as D-Glucose. However, since D-Fructose is a ketose sugar with a ketone group, it would typically not react. Yet, under basic conditions, D-Fructose can undergo keto-enol tautomerization to form an enediol structure, which presents a reactive form for the Tollen's test. Thus, both D-Glucose and D-Fructose can give a positive Tollen’s test by displaying a silver mirror, rendering it inadequate for differentiating between them.
Because Tollen's reagent reacts with aldehydes, it is an effective test for monosaccharides with aldehyde groups, such as D-Glucose. However, since D-Fructose is a ketose sugar with a ketone group, it would typically not react. Yet, under basic conditions, D-Fructose can undergo keto-enol tautomerization to form an enediol structure, which presents a reactive form for the Tollen's test. Thus, both D-Glucose and D-Fructose can give a positive Tollen’s test by displaying a silver mirror, rendering it inadequate for differentiating between them.
Bromine Water Test for Aldehydes
Bromine water serves as a test for aldehydes by manifesting a color change in the presence of these functional groups. The test exploits the fact that aldehydes can be readily oxidized to carboxylic acids, while ketones, lacking the necessary hydrogen atom, cannot. When an aldehyde compound reacts with bromine in water, the solution loses its characteristic orange-brown color as the aldehyde is oxidized and the bromine is reduced.
For instance, D-Glucose contains an aldehyde functional group and will thus react with bromine water, leading to decolorization of the bromine solution. D-Fructose, on the other hand, has a ketone functional group and does not react with bromine water, so the solution retains its color. This specific reaction is what allows bromine water to differentiate between D-Glucose and D-Fructose, as it will only indicate a positive result with aldehyde-containing sugars.
For instance, D-Glucose contains an aldehyde functional group and will thus react with bromine water, leading to decolorization of the bromine solution. D-Fructose, on the other hand, has a ketone functional group and does not react with bromine water, so the solution retains its color. This specific reaction is what allows bromine water to differentiate between D-Glucose and D-Fructose, as it will only indicate a positive result with aldehyde-containing sugars.