Oxidation of Monosaccharides - Online Tutor, Practice Problems & Exam Prep

The oxidation of monosaccharides involves converting aldehyde sugars, known as aldoses, into aldonic acids through reactions like Benedict's test, which produces a brick red precipitate of copper(I) oxide. Aldoses and ketoses can act as reducing sugars; ketoses can rearrange to form aldoses in basic solutions. For example, D-fructose (a ketose) can convert to D-glucose (an aldose). This highlights that both aldoses and ketoses are reducing sugars, essential for understanding carbohydrate chemistry.

Oxidation of Monosaccharides Concept 1

In this video, we're going to take a look at the oxidation of monosaccharides. Now recall that in Benedict's test, an aldehyde undergoes oxidation to a carboxylic acid and forms a brick-red precipitate. Here we're going to say that our aldose monosaccharides, so aldehyde sugars, produce what we call an aldonic acid. And aldonic acids are sugar acids, and again this happens upon oxidation. And the key takeaway from here is that we're changing the ending of our sugar name from 'ose' to 'onic acid'.

If we take a look here at this reaction we have D-ribose, which is a typical aldose sugar, so we have an aldehyde here. We're using copper(II) ion to oxidize this aldose sugar. We oxidize it into a carboxylic acid group, and now we're still be D, a D-sugar. It's still going to be ribe, and remember we're changing the 'ose' to 'onic acid', so it becomes ribonic acid. The brick-red precipitate happens in the form of copper(I) oxide here. So this is just a byproduct which would be our solid brick precipitate.

Now here, in addition to this, we have what's called a reducing sugar. This is a carbohydrate that produces a sugar acid upon oxidation, like D-ribose. And we're going to say here, Benedict's test can be used to detect a reducing sugar in solution. So, just remember in Benedict's test we're basically changing an aldose sugar into a carboxylic acid group, this transitions it away from an aldose sugar to an aldonic acid or sugar acid.

Oxidation of Monosaccharides Example 1

Here in this example question, it says, draw and name the product of the reaction when d-glucose is treated with a basic copper(II) solution. So we're undergoing Benedict's test. Now remember, under Benedict's test, we're going to transform our aldose sugar into a sugar acid. This happens by changing the aldehyde group into a carboxylic acid group through oxidation. So here, that aldehyde group is now going to be a carboxylic acid. Everything else would stay the same. It's just that group that gets changed into a carboxylic acid. So all these H's and hydroxyl groups are going to stay in the same position. Remember when it comes to our sugar acid, we transform the 'ose' ending to 'onic acid'. So this would be d-gulonic acid. This would be its structure as a carboxylic acid or sugar acid, and d-gulonic acid would be its name.

Ketoses as Reducing Sugars Concept 2

Now, besides all those sugars, we can say here that keto sugars can also be reducing sugars. We're going to say in basic solutions, ketones undergo rearrangement to form aldoses. So here we have D-fructose. D-fructose represents a ketose. This ketose itself can undergo a rearrangement where it was a ketone here, but now it is an aldehyde group. The number of carbons stay the same; it's just that we've changed from going from a ketone to an aldehyde. So here now it is an aldose sugar. We've transitioned from being D-fructose to D-glucose. Because of this possibility of rearrangements with keto sugars, we can say that all Aldose and Ketose monosaccharides are reducing sugars in basic solutions. So keep this in mind, reducing sugars are not only limited to aldose sugars; keto sugars can be included as well.

Ketoses as Reducing Sugars Example 2

Which of the following statements is incorrect about oxidation of monosaccharides? All monosaccharides are reducing sugars. Yes. That's true. That's because all those sugars automatically can be oxidized in basic solutions into sugar acids. Ketoses can rearrange to become aldose sugars, which again can be oxidized into sugar acids. Ketoses undergo rearrangement to produce aldoses in basic solution. That is true. Some keto or ketose monosaccharides are reducing sugars. This is incorrect. It's not some, it's all. Because those keto sugars can undergo rearrangement to become aldose sugars, and all those aldose sugars can be oxidized. All reducing sugars produce a brick red precipitate in Benedict's test. Yes, they do. That brick red precipitate is copper(I) oxide. So, out of my four statements, the only one that's incorrect is option C.

Write common names for the reduction and oxidation products of D-allose.

• A. D-allosol and D-allonic acid
• B. D-allitol and D-allinic acid
• C. D-allitol and D-allonic acid
• D. D-allositol and D-allitonic acid

Complete the following reduction/oxidation reaction of D-idose.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is the oxidation of monosaccharides?

The oxidation of monosaccharides involves converting aldehyde sugars (aldoses) into carboxylic acids (aldonic acids). This process can be observed in reactions like Benedict's test, where an aldehyde group in the sugar is oxidized to a carboxylic acid group, resulting in a brick red precipitate of copper(I) oxide. For example, D-ribose (an aldose) is oxidized to ribonic acid. This reaction is crucial for identifying reducing sugars, which include both aldoses and ketoses, as ketoses can rearrange to form aldoses in basic solutions.

How does Benedict's test work in detecting reducing sugars?

Benedict's test detects reducing sugars by oxidizing the aldehyde group of aldoses to form carboxylic acids, producing a brick red precipitate of copper(I) oxide. In this test, the sugar solution is mixed with Benedict's reagent and heated. If a reducing sugar is present, the aldehyde group is oxidized, and the copper(II) ions in the reagent are reduced to copper(I) oxide, forming a red precipitate. This indicates the presence of reducing sugars, which include both aldoses and ketoses (the latter can rearrange to form aldoses in basic solutions).

What are aldonic acids and how are they formed?

Aldonic acids are sugar acids formed by the oxidation of aldose sugars. In this process, the aldehyde group of an aldose is oxidized to a carboxylic acid group. For example, D-ribose (an aldose) is oxidized to ribonic acid. This transformation is typically observed in reactions like Benedict's test, where the aldehyde group is converted to a carboxylic acid, resulting in a brick red precipitate of copper(I) oxide. The name of the sugar changes from 'ose' to 'onic acid' to reflect this oxidation.

Can ketose sugars act as reducing sugars?

Yes, ketose sugars can act as reducing sugars. In basic solutions, ketoses can undergo rearrangement to form aldoses, which can then be oxidized. For example, D-fructose (a ketose) can rearrange to form D-glucose (an aldose). This rearrangement allows ketoses to participate in reactions like Benedict's test, where they can be detected as reducing sugars. Therefore, both aldoses and ketoses are considered reducing sugars in basic solutions.

What is the significance of reducing sugars in carbohydrate chemistry?

Reducing sugars are significant in carbohydrate chemistry because they can participate in oxidation-reduction reactions, such as Benedict's test. These sugars, which include both aldoses and ketoses, have free aldehyde or ketone groups that can be oxidized. This property is essential for identifying and characterizing carbohydrates. For instance, the ability of ketoses to rearrange into aldoses in basic solutions expands the range of detectable reducing sugars. Understanding reducing sugars is crucial for studying metabolic pathways and diagnosing certain medical conditions.