Reducing Sugars Tests - Video Tutorials & Practice Problems
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Reducing Sugar Tests
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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 failings test for reducing sugars. And so failings test is named after the scientist who helped develop it and so failings 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. And so really this color change reaction is just again testing for the presence of reducing sugars. And so we'll see that this failing's test uses a blue cupric ion solution as the oxidizing agent and so in the presence of a non reducing sugar, the blue cupric ion is not going to be reduced. And so 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. And so 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. So, 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 and we can tell that it's reducing because when we look at the Numeric carbon right here, notice that it's present forming a Hemi Acetal. And so, of course, Hemiacetals we know are going to be relatively unstable And so, they're unstable where they can actually, react to, generate its linear form again. So 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 And so notice that in the presence of the cupric ion oxidizing agent, the aldehyde group is being oxidized here. So you can see that it's turning into a carboxylic acid. And notice that 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. And so the idea here is that when there are no reducing sugars, the cupric ion solution is going to remain blue and so 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, of course, this cupric ion oxide, precipitate is going to form. So we will get a red color which is our positive control in the of reducing sugars and so we will get a red cupric oxide precipitate in the presence of reducing sugars. And so really the main summary of the failings test is that if the solution does turn this reddish brownish brick color over here, 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. And so 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 failings test so I'll see you guys in our next video
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Reducing Sugar Tests
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Now your professors and your textbooks may also mention another test for reducing sugars called the Benedict's test. And so the Benedict's test is really just a very very very similar test to the failings test in terms of the overall reaction and the overall test result and so the Benedict's test is a color change test and so 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 failings 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 failings 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 failings test as pretty much being equivalent to the Benedict's test and so again, they're going to be very, very, very similar and so that concludes our lesson on the Benedict's test and 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.
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Reducing Sugar Tests
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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 Phelan's and Benedict's tests used. However, Talon's test is still going to be testing for the presence of reducing sugars just like failings and Benedict's test test for the presence of reducing sugars. And so if we take a look down below at our image, notice that Failing's and Benedict's test does use that blue cupric ion complex. And so, in the presence of a reducing sugar like D Mannose, which has a free aldehyde group, we already know from our previous awesome videos that Phelan's and Benedict's test is going to oxidize the, reducing sugar here so that it is forming this carbox silic acid, this d monoic acid here 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. And so there is a color change and this red brick, color change indicates the presence of a reducing sugar. Now, with the reducing sugar. Now with the tolerance 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 Pfalen's and Benedicte's test uses. And you can see that there are a variety of different solutions that can be used for Talon's test, but all of them include the silver. And so, in the presence of a reducing sugar like D Mannose, the Talens test, is also going to oxidize, the reducing sugar. So, you can see that it's oxidized to this carboxylic acid here and it's also going to create a color change. However, the color change is not gonna be a red brick precipitate. Instead, the color change is just going to be a silver mirror with the tones test since it's using silver and so you can see over here, we have a positive test and you can see the 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 sugars, the color would change to this silver mirror and so, the Talens test is still a color change test testing for the presence of reducing sugars. And so this here concludes our introduction to the Tolerance 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.
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Problem
Problem
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.
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Problem
Problem
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.
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Problem
Problem
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.