Problem 5 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

Fehling's solution and Benedict's solution cannot distinguish between glucose and fructose.

Step 1: Understanding the Problem

We need to determine which reagent, among (a) Tollen's reagent, (b) Fehling's solution, and (c) Benedict's solution, cannot be used to distinguish between glucose and fructose.

Step 2: Understanding the Reagents and Their Reactions

Tollen's reagent is used to test for aldehydes, which will oxidize and produce a silver mirror. Fehling's solution and Benedict's solution both test for reducing sugars, and both glucose (an aldose) and fructose (a ketose) will react with them under heated conditions.

Step 3: Analyzing Reaction with Glucose and Fructose

Fructose can interconvert to its aldose form under basic conditions, allowing it to also react positively with Fehling's and Benedict's solutions. Thus, both glucose and fructose will give positive reactions in these solutions.

Step 4: Conclusion

Since Tollen's reagent will only react with the aldehyde group in glucose and not initially with ketones like fructose, it can distinguish glucose from fructose, while Fehling's and Benedict's solutions cannot.

Key Concepts

Tollen's Reagent

Tollen's reagent is a powerful tool in organic chemistry for identifying certain functional groups. Specifically, it is used to differentiate between aldehydes and ketones. This reagent contains silver nitrate (\( \text{AgNO}_3 \)) in ammonia, which under certain conditions will oxidize aldehydes to carboxylic acids. More excitingly, this reaction results in the famous 'silver mirror' effect, where metallic silver deposits inside the test tube.
Tollen's reagent, therefore, is especially useful because it does not react with ketones, making it an excellent choice for distinguishing between glucose, an aldose sugar (which has an aldehyde group), and fructose, a ketose (which initially does not). The difference in their ability to react with Tollen's reagent is what allows us to identify them. While glucose will produce the silver mirror, fructose will not because under neutral conditions, ketones like fructose do not form aldehydes to react with Tollen’s.

Fehling's Solution

Fehling's solution is an older yet widely recognized method for identifying reducing sugars. This solution is composed of two separate solutions: Fehling's A, which contains copper(II) sulfate (\( \text{CuSO}_4 \)), and Fehling's B, which contains a mixture of potassium sodium tartrate and a strong base. Upon mixing, the solution becomes an active blue complex. When heated with a reducing sugar, the copper(II) ions in Fehling’s solution undergo reduction to form a brick-red precipitate of copper(I) oxide (\( \text{Cu}_2\text{O} \)).
Both glucose and fructose are reducing sugars, but for slightly different reasons. Glucose, being an aldose, directly reduces the copper(II) ions. Fructose, although a ketose, can rearrange to become an aldose under the basic conditions provided by Fehling's solution, thus also giving a positive result. Consequently, Fehling's solution cannot be used to distinguish between glucose and fructose, as both will react and form the precipitate.

Benedict's Solution

Similar to Fehling's solution, Benedict's solution is used to determine the presence of reducing sugars. It consists of a mixture of copper(II) sulfate, sodium carbonate, and sodium citrate. The citrate ensures the copper ions stay in solution, while the sodium carbonate provides an alkaline environment. During the test, if reducing sugars are present, the copper(II) ions will reduce to form a reddish-brown precipitate of copper(I) oxide when heated.
Like Fehling’s solution, Benedict's solution will react with both glucose and fructose. Glucose will directly engage the copper(II) ions due to its aldehyde group. Fructose modifies itself in the alkaline conditions and behaves similarly, thus reducing the copper ions. Since both sugars can reduce the copper present in Benedict's solution, this reagent also cannot be used to differentiate between glucose and fructose.