Monosaccharides - Definition, Glucose, Fructose, Structure of Glucose and Fructose & Lobry de Bruyn-van Ekenstein Rearrangement Reaction

Look at this picture. Have you ever seen this at your home or at shops or in TV ads? Of course yes. Have you ever drunk it? Of course yes. Each one of us has had Glucon D. But what does it do? Why do we drink it, especially in the summer? Why do we start feeling energetic after having it?

As it contains 99.4% glucose which is the most common monosaccharide that fulfils the energy that the body needs, Glucon-D gives instant energy when we get tired on summer days.

Now, let’s see and try to understand what monosaccharides are and how it is related to glucose and how glucose gives us the energy,

Table of contents

• What are Monosaccharides?
• Classification of Monosaccharides
• Glucose
• Structure of Glucose
• Fructose
• Lobry De Bruyn van Ekenstein rearrangement reaction
• Video Link
• Practice Problems
• Frequently Asked Questions - FAQs

What are Monosaccharides?

Monosaccharides are polyhydroxy aldehydes or ketones with an unbranched carbon chain in most of the cases.

Monosaccharides are the most basic type of sugar. Breaking down the monosaccharides glucose and harvesting the energy produced is how most organisms make and store energy. The number of carbon atoms and the functional groups associated with this form of glucose are used to classify it. Aldose is a monosaccharide that contains aldehyde, while ketose is a monosaccharide that contains a ketone group.

Classification of Monosaccharides

Monosaccharides can be classified into the given categories as:

Let’s now understand them one by one.

Classification of Monosaccharides in terms of the number of carbon atoms present in a molecule:

Classification of Monosaccharides in terms of the functional groups present in a molecule:

Monosaccharides can contain aldehyde as well as ketone groups.

E.g., Aldotriose and Aldotetrose monosaccharides have an aldehyde group in them whereas Ketotriose and Ketotetrose have a keto group in them.

These two classifications are frequently combined.

A four-carbon aldose is called an aldotetrose, and a five-carbon ketose is called a ketopentose as shown below.

Glucose is the most common monosaccharide. Let’s talk about glucose now.

D and L designations of monosaccharides:

The simplest optically active monosaccharide is glyceraldehyde.

(+)-Glyceraldehyde is designated D-(+)-glyceraldehyde, and (-)-glyceraldehyde is designated L-(‒)-glyceraldehyde. These two compounds serve as configurational standards for all monosaccharides.

A monosaccharide whose highest-numbered chiral centre (the penultimate carbon) has the same configuration as D-(+)-glyceraldehyde is designated as a D-sugar.

One whose highest numbered chiral centre has the same configuration as L-(‒)-glyceraldehyde is designated as a L-sugar.

Point to remember:

• D sugars have the ‒OH on their last (highest-numbered) chiral carbon on the right.
• L sugars have the ‒OH on their last (highest-numbered) chiral carbon on the left.

Glucose

Glucose is an aldohexose kind of a monosaccharide with six carbon atoms and an aldehyde group. The glucose molecule can exist as an acyclic open-chain or cyclic ring structure. Fruits and other plant components contain free glucose, which is a naturally occurring substance.

The most abundant monosaccharide, a type of carbohydrate, is glucose. Glucose has the chemical formula C₆H₁₂O₆ and is a simple sugar. Some of the properties of glucose is given below:

• It is known as dextrose because it occurs in nature principally as the optically active dextrorotatory isomers.
• It acts as a reducing agent (reduces both Fehling’s and Tollens' reagent).
• Sources of glucose are ripe grapes, honey, cane sugar, starch, and cellulose.

Structure of glucose

Glucose has two types of structures:

• Open chain structure of glucose
• Cyclic structure of glucose

Open chain structure of glucose:

Limitation of open structure of glucose:

1. The penta-acetate of glucose does not react with NH₂OH.

2. Even after having an aldehydic group, glucose does not give Schiff's test and does not form hydrogensulphite addition product with NaHSO₃

Cyclic structure of Glucose:

The cyclic structures of monosaccharides can be better represented by Haworth Projection.

Groups on the right in a Fischer projection are down in the Haworth projection. Groups on the left in a Fischer projection are up in the Haworth projection. This is represented by the structure given below.

This structure can be represented in two different ways as 𝛂-D-Glucopyranose and another as 𝛃-D-Glucopyranose as shown below. They are basically termed as anomers.

An anomeric carbon is a stereocenter and is the carbon centre that is derived from the carbonyl carbon, which has a ketone or aldehyde functional group in its open chain version. Hence the process of conversion of the open-chain structure of the monosaccharides to its respective cyclic structure is called anomerization.

It can be defined as: “In carbohydrate chemistry, diastereomers differing only at the hemiacetal or acetal carbon are known as anomers, and the hemiacetal or acetal carbon atom is ​ known as the anomeric carbon atom.’’

Depending on where the substituent is located at the anomeric carbon, carbohydrates can exist in two different cyclic forms: -anomer and -anomer. Since the two forms are isomers at the anomeric centre, they are referred to as "anomers."

Let us take a quick look at the relative locations of the -CH₂OH group and -OH (or -OR) group at the anomeric centre to determine if the cyclic form of a carbohydrate is the -anomer or -anomer form.

In -anomer, the hydroxyl group present at the anomeric carbon is cis to the exocyclic oxygen present at the anomeric centre.

In -anomer, the hydroxyl group present at the anomeric carbon is trans to the exocyclic oxygen.

After understanding glucose, we have one more common monosaccharide known as fructose. Let’s see what fructose is and how it is different from Glucose.

Fructose

Fructose is a simple sugar having the chemical formula C₆H₁₂O₆. It is a monosaccharide and is an important ketohexose as shown below:

Fruit sugar is another name for it. Augustin-Pierre Dubrunfaut, a French chemist, discovered it in the year 1847. It's a polyhydroxy ketone with six carbons. It belongs to D-series and is

a laevorotatory compound. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose.

Due to the stability of the hemiketal and internal hydrogen bonding, crystalline fructose has a cyclic six-membered structure. D-fructopyranose is the name for this kind of fructose. Honey, vine fruits, most of the root vegetables, berries, and flowers are the most common sources. Sugar beetroot, maize, and sugar cane are some of the commercial sources.

Structure of fructose:

Based on how the -OH group is positioned in the fifth carbon, fructose can either be D-fructose or L-fructose. D-fructose is present if the fifth carbon's -OH group is on the right side. L-fructose is present if it is on the left side. The Fischer projections of D- and L-fructose are given below:

Cyclic structure of fructose:

The cyclic structure of fructose is given as 𝛂-D-Fructose and 𝛃-D-Fructose which can be represented as given below.

The & -anomers fructose refers to the position of the hydroxyl group at the anomeric carbon. If the hydroxyl group (-OH) is present on the same side of the ring as the C-6 then it is the -anomer. If If the hydroxyl group (-OH) is present on the opposite side of the ring as the C-6 then it is the -anomer.

Lobry de Bruyn-Van Ekenstein rearrangement reaction

The Lobry de Bruyn–Van Ekenstein transformation, also known as the Lobry de Bruyn–Alberda van Ekenstein transformation, is a base or acid catalysed transformation of an aldose into the ketose isomer, or vice versa, using a tautomeric enediol as a reaction intermediate in carbohydrate chemistry. Ketoses can be converted into 3-ketoses, and so on. The enediol is also a step in the process of epimerization of an aldose or ketose.

The following scheme describes the interconversion between an aldose and a ketose, where R is any organic residue.

Video Link:

https://www.youtube.com/watch?v=ZcDIetuGKEk

Practice problems:

Question: Which of the following statements about glucose indicates the presence of five –OH groups?

1. Penta-acetyl derivative of glucose
2. Cyanohydrin formation of glucose
3. Reaction with Fehling's solution
4. Reaction with Tollens' reagent

Answer: A

Solution: Acetylation of glucose with acetic anhydride gives glucose pentaacetate which confirms the presence of five –𝑂𝐻 groups as given in the reaction.

So, option A is the correct answer.

Question: Which of the following is correct regarding glucose?

1. The molecular formula of glucose is C6H8O6.
2. It can be obtained by base-catalyzed hydrolysis of cane sugar.
3. It is a mono-saccharide.
4. It can’t be oxidized

Answer: C

Solution: We know that the molecular formula of Glucose is C₆H₁₂O₆ and it is an easily

accessible monosaccharides. Glucose is a non-oxidizing substance. Furthermore, glucose is classified as a lowering sugar (a sugar that has the ability to act as a reducing agent owing to its free aldehyde group or free ketone group) and can undergo oxidation to form gluconic acid HOCH₂-(CHOH)₄-COOH.

So, option C is the correct answer.

Question: Glucose is known as _______

1. Aldopentose
2. Aldohexose
3. Ketopentose
4. Ketohexose

Answer: B

Solution: The structure of glucose can be represented as HOCH₂-(CHOH)₄-CHO. It is a molecule containing six carbons in which an aldehyde functional group is present. So we can call such a molecule aldohexose

Option B is the correct answer.

Question: How is fructose different from glucose?

Solution: Glucose is a monosaccharide that contains six carbon atoms and an aldehyde group. As a result, it's both a hexose and an aldose. It contains the following structure and four hydroxyl groups.

On the other hand, fructose is a type of sugar known as hexose. It also contains a keto group, making it a ketose. Fruits, sugar cane, sugar beets, corn, and other foods contain fructose. The structure of fructose is as follows.

Frequently asked questions - FAQs

Question: How many carbon atoms are present in glucose?
Answer: We know that the molecular formula of Glucose is C6H12O6, so it has 6 carbon atoms. It is made up of extra oxygen and hydrogen atoms as well as a chain of six carbon atoms that are bound together. Glucose is an aldohexose because it is a monosaccharide with six carbon atoms and an aldehyde group. The glucose molecule can exist as an acyclic open-chain or cyclic ring structure. Fruits and other plant components contain free glucose, which is a naturally occurring substance.

Question: Is glucose reducing sugar?
Answer: Glucose is a reducing sugar because it is an aldose, which means it has an aldehyde group in its open-chain form. In general, carboxylic acids are quickly oxidised from aldehydes.

Question: What exactly are the components of glucose?
Answer: Carbon, hydrogen, and oxygen are the three elements that makeup glucose. The most abundant monosaccharide, a type of carbohydrate, is glucose. Glucose has the chemical formula C6H12O6. A monosaccharide with an aldehyde group is glucose (-CHO). It is composed of 12 hydrogen atoms, 6 oxygen atoms, and 6 carbon atoms. Aldohexoses include glucose.

Typically, glucose exists as a closed pyran ring monohydrate in solid form (dextrose hydrate). On the other hand, it is mostly found as - or -pyranose in aqueous solution, where it is open-chain to a modest extent and interconverts.

Question: What is the source of glucose?
Answer: Glucose or sugar is found in the foods we eat. Glucose can be found in a variety of foods such as fruit, bread, pasta, and cereals. In our stomachs, these nutrients are broken down into sugar, which is then taken into the bloodstream.

Question: What is the chemical name of glucose?
Answer: With six carbon atoms and one aldehyde group, glucose is a simple sugar. The molecular formula for this monosaccharide is C₆H₁₂O₆. Dextrose is another name for it. Because it has six carbon atoms and an aldehyde group, it's termed aldohexose.

Question: What is blood sugar and why is it important?
Answer: When we talk about blood sugar levels, we're referring to glucose, the molecule that powers our cells and has a significant impact on our health. At the cellular level, glucose is the fundamental fuel that drives our bodies. Glucose is obtained from the foods we consume.

Question: Which Isomers are Glucose and Fructose?
Answer: Because they have the same molecular formula, C₆H₁₂O₆, but distinct functional groups in their chemical formula, glucose and fructose are functional isomers of each other. The functional group of glucose is aldehyde, whereas the functional group of fructose is ketone. The nature of the functional group differs. Fructose is a ketone while glucose is an aldehyde.

Question: What are the differences between fructose and glucose?"
Answer: Monosaccharide sugars fructose and glucose are both simple sugars. Both starch and sugar, whether sucrose or high-fructose corn syrup (HCFS), produce large amounts of glucose when digested.

Question: What is the role of fructose in the body?
Answer: Fructose, like glucose, supplies energy to the cells. Fructose is converted to energy by cells using a process known as aerobic breathing, which involves burning fructose in the presence of oxygen to produce ATP, the cellular energy molecule.

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