Reducing Sugars Tests: Videos & Practice Problems

The tests for reducing sugars include the Fehling's, Benedict's, and Tollens tests, each indicating the presence of reducing sugars through distinct color changes. Fehling's and Benedict's tests utilize cupric ions, changing from blue to red precipitate, while Tollens test employs silver ions, resulting in a silver mirror effect. These tests rely on the oxidation of aldehyde groups in reducing sugars, demonstrating their reactivity. Understanding these tests is crucial for identifying reducing sugars in various biological contexts.

Reducing Sugar Tests

In this video, we're going to begin our discussion on reducing sugars tests. So it turns out that there are actually several different experimental tests that can detect the presence of reducing sugars, and one of these tests is the Fehling's test for reducing sugars. So, Fehling's test is named after the scientist who helped develop it, and so Fehling's test is really just an experimental color change reaction, and as we'll see, the color is going to change from a blue color to a reddish color in the presence of a reducing sugar. Really this color change reaction is about testing for the presence of reducing sugars.

We'll see that this Fehling's test uses a blue cupric ion solution as the oxidizing agent, and in the presence of a non-reducing sugar, the blue cupric ion is not going to be reduced. That means that the blue cupric ion is going to remain its blue color in the presence of non-reducing sugars. However, when the blue cupric ion is in the presence of reducing sugars, then the blue cupric ion will be reduced and it will ultimately generate this red cupric oxide precipitate.

What you'll notice is that the blue solution in the presence of a reducing sugar will change from a blue color to a red color. So when the color changes to a red color, that indicates the presence of a reducing sugar. Let's take a look down below at our image, and notice up here at the top left, we have a cyclic sugar that is actually a reducing sugar. We can tell that it's reducing because when we look at the anomer carbon here, notice that it's present forming a hemiacetal. Hemiacetals are known to be relatively unstable and can react to generate its linear form again.

Here we have the linear form of the same cyclic sugar, and notice that it has a free aldehyde group that's capable of being oxidized. In the presence of the cupric ion oxidizing agent, the aldehyde group is oxidized, turning into a carboxylic acid. In the process, the ion molecule here is being reduced in its charge, and ultimately, the reduced cupric ion can react to form a cupric oxide precipitate that is going to be a red color.

The idea here is that when there are no reducing sugars, the cupric ion solution is going to remain blue. This is going to be our negative control with no reducing sugars present, and no reducing sugars means that no precipitate is going to form. However, in the presence of a reducing sugar, this cupric ion oxide precipitate is going to form. So, we will get a red color which is our positive control in the presence of reducing sugars, and we will get a red cupric oxide precipitate.

The main summary of the Fehling's test is that if the solution does turn this reddish-brownish brick color, then that indicates the presence of a reducing sugar and if it does not turn this reddish-brownish color over here and it remains blue, then that means that there are no reducing sugars present, and any sugars that are present would be non-reducing. Really it's just a simple color change reaction, and in our next video, we'll be able to talk about another test for reducing sugars called the Benedict's test, which is very similar to the Fehling's test. So, I'll see you guys in our next video.

Reducing Sugar Tests

Now your professors and your textbooks may also mention another test for reducing sugars called the Benedict's test. So, the Benedict's test is really just a very very very similar test to the Fehling's test in terms of the overall reaction and the overall test result. The Benedict's test is a color change test, and the change in color will indicate the presence of reducing sugars. Now, Benedict's solution, which is used in Benedict's test, is going to use slightly different reagents than the Fehling's test but again, overall, the reaction is very similar and the test results are very similar. However, one thing to note about Benedict's solution is that it actually does have a longer shelf life than the Fehling's test and so in labs, you might find that the Benedict's test is more readily available because of its longer shelf life. But other than this longer shelf life, you can pretty much think of the Fehling's test as being equivalent to the Benedict's test. So again, they're going to be very, very, very similar and that concludes our lesson on the Benedict's test. In our next lesson video, we'll be able to talk about the last test for reducing sugars that we'll cover which is the Tollens' test for reducing sugars. So I'll see you guys in that video.

Reducing Sugar Tests

Alright, so the last test for reducing sugars that we're going to talk about is the Tollens test. Now, the Tollens test will actually use silver or Ag as the oxidizing agent instead of using cupric ion like the Fehling's and Benedict's tests used. However, the Tollens test is still going to be testing for the presence of reducing sugars just like Fehling's and Benedict's tests. And so, if we take a look down below at our image, notice that both Fehling's and Benedict's tests do use that blue cupric ion complex. In the presence of a reducing sugar like D-Mannose, which has a free aldehyde group, we already know from our previous videos that Fehling's and Benedict's tests will oxidize the reducing sugar so that it is forming this carboxylic acid, this D-monoic acid and in the presence of a reducing sugar like this one, the color is going to change from the blue cupric ion complex to the red brick cupric oxide precipitate. This red brick color change indicates the presence of a reducing sugar.

Now with the Tollens test that we are introducing now, what you'll notice is that it uses silver as the oxidizing agent instead of using the blue cupric ion like Fehling's and Benedict's tests. And you can see that there are a variety of different solutions that can be used for the Tollens test, but all of them include the silver. In the presence of a reducing sugar like D-Mannose, the Tollens test is also going to oxidize the reducing sugar so that it's oxidized to this carboxylic acid and it's also going to create a color change. However, the color change is not going to be a red brick precipitate. Instead, the color change is just going to be a silver mirror with the Tollens test since it's using silver. Over here, we have a positive test and you can see how it's really creating just a silver mirror kind of color change and over here we just have the clear solution. So, this is what it would end up looking like very clear if there were no reducing sugars. But if there is a reducing sugar, the color would change to this silver mirror. The Tollens test is still a color change test testing for the presence of reducing sugars.

This here concludes our introduction to the Tollens test and as we move forward we'll be able to apply the concepts that we've learned. So, I'll see you guys in our next video.

Which of the following would you predict to be the LEAST reactive in a Fehling's test for reducing sugars?

• A. D-Glucose.
• B. L-Glucose.
• C. D-Mannose.
• D. D-Ribose.
• E. D-Fructose.

Answer: (E)

Which of these statements is false?

• A. The Fehling's test allows us to detect the presence of reducing sugars.
• B. The Benedict's test allows us to detect the presence of sugars with a free aldehyde or ketone group.
• C. All simple, linear monosaccharides are reducing sugars.
• D. All disaccharides have exposed carbonyl groups and are also reducing sugars.
• E. Sucrose and other non-reducing sugars will not react with Tollens' solution.

Answer: (D)

In the table below, indicate what the result of a Fehling's, Benedict's, and Tollen's test would be for each of the two sugars and what conclusion can be made about the properties of the sugars.

So this practice problem says, in the table below shown right here, indicate what the result of a Fehling's, Benedict's, and Tollen's test would be for each of the 2 sugars shown here and here and what conclusion can be made about the properties of these 2 sugars. And so, of course, we know from our previous lesson videos that the Fehling's, Benedict's, and Tollen's test are all color change experiments that are testing for the presence of reducing sugars. And so, in the presence of a reducing sugar, we expect to see a color change. And so what we want to do is take a look at these 2 sugar molecules and determine if they are reducing sugars. And of course, when it comes to cyclic monosaccharides like these 2 sugars here, we need to determine if the anomeric carbon is part of either a hemiacetal, hemiketal, acetal, or ketal. And so when we take a look at the anomeric carbon of this sugar right here, notice it's right here and recall that the anomeric carbon is the only carbon atom that's bonded to 2 oxygen atoms. And so notice that this anomeric carbon is part of a hemiacetal. So this is a hemiacetal and so we know that it's a hemiacetal because notice that it only has 1 OR group. It has a free hydroxyl group and, of course, going down, it still has its hydrogen. And so, of course, hemiacetals are going to be susceptible to breaking apart into its linear form and generating an aldehyde group. And the aldehyde group is capable of being oxidized. And once it's oxidized, of course, it's going to create a color change. And so with the Fehling's and Benedict's test, we would expect there to be a color change. So we would expect the color to change. And of course, we would expect to end up getting a red color, upon the result. And so we would actually expect a very similar result for the Benedict's test and so we could put that in here as well. And then for the Tollen's test, we would also expect a color change, but this time instead of getting a red color, we would expect to get a silver mirror. And the final conclusion here is that this is a reducing sugar because of the color changes and because of the presence of the hemiacetal. Now, when we take a look at this sugar over here and we try to look for its anomeric carbon, notice that it actually does not have an anomeric carbon because recall that the anomeric carbon is the only carbon atom bonded to 2 oxygen atoms. But if you look at all of these carbon atoms, notice that none of them are bonded to 2 oxygen atoms directly. And so what this means is that there is no hemiacetal or hemiketal. And that means that this ring right here is not capable of opening up and generating a free aldehyde group. And of course, that means that we would expect to get no color change. And so if there's no color change, then of course, it's going to remain a blue color. So, we can put in a blue color here and we would expect to get a very similar result for the Benedict's test. And then, of course, for the Tollen's test, there would also be no color change, but instead of being a blue color, it would just be clear and would not be silver. And so, of course, this is an indication that this sugar right here is not a reducing sugar. Instead, this is going to be a non-reducing sugar. And so relatively easy practice problem. And so the main idea again is to be able to look at sugars and identify if they have hemiacetals or hemiketals and determine if they are reducing or non-reducing. And so that concludes this practice problem and I'll see you guys in our next video.

Here’s what students ask on this topic:

What is the Fehling's test for reducing sugars?

The Fehling's test is a chemical test used to detect the presence of reducing sugars. It involves a color change reaction where a blue cupric ion solution acts as the oxidizing agent. In the presence of reducing sugars, the blue cupric ion is reduced to form a red cupric oxide precipitate. This color change from blue to red indicates the presence of reducing sugars. The test is based on the ability of the free aldehyde group in reducing sugars to be oxidized, which in turn reduces the cupric ion. If no reducing sugars are present, the solution remains blue.

How does the Benedict's test for reducing sugars work?

The Benedict's test is similar to the Fehling's test and is used to detect reducing sugars. It involves a color change reaction where Benedict's solution, which contains cupric ions, is used. When reducing sugars are present, the cupric ions are reduced, leading to a color change from blue to a red precipitate. This indicates the presence of reducing sugars. Benedict's solution has a longer shelf life compared to Fehling's solution, making it more commonly used in laboratories. The test relies on the oxidation of the aldehyde group in reducing sugars, which reduces the cupric ions.

What is the Tollens test for reducing sugars?

The Tollens test is another method to detect reducing sugars, but it uses silver ions (Ag⁺) as the oxidizing agent instead of cupric ions. In the presence of reducing sugars, the silver ions are reduced to metallic silver, forming a silver mirror on the inner surface of the test tube. This silver mirror effect indicates the presence of reducing sugars. The test is based on the oxidation of the aldehyde group in reducing sugars, which reduces the silver ions. If no reducing sugars are present, the solution remains clear.

What are the differences between Fehling's, Benedict's, and Tollens tests for reducing sugars?

Fehling's and Benedict's tests both use cupric ions (Cu²⁺) as the oxidizing agent and result in a color change from blue to red precipitate in the presence of reducing sugars. The main difference is that Benedict's solution has a longer shelf life. The Tollens test, on the other hand, uses silver ions (Ag⁺) and results in a silver mirror effect when reducing sugars are present. All three tests rely on the oxidation of the aldehyde group in reducing sugars, but they use different oxidizing agents and produce different visual results.

Why is it important to test for reducing sugars in biological contexts?

Testing for reducing sugars is crucial in biological contexts because these sugars play significant roles in metabolism and energy production. Reducing sugars, such as glucose, are involved in cellular respiration and provide energy for various cellular processes. Identifying the presence of reducing sugars can help diagnose metabolic disorders, monitor blood sugar levels in diabetic patients, and understand carbohydrate metabolism in different organisms. Additionally, these tests can be used in food industry to determine sugar content in products, ensuring quality and safety standards.