Is C6H1206 a compound

carbohydrates

Carbohydrates are important nutrients. In addition to fats, they are our most important energy suppliers. They are also often called sugar. On this page we clarify which different types of carbohydrates exist and how they are structured!

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The carbohydrates are organic compounds made up of carbon, hydrogen and oxygen. Depending on their molecular structure, they can be divided into monosaccharides (single sugars consisting of one sugar molecule), disaccharides (double sugars consisting of two sugar molecules) and polysaccharides (multiple sugars consisting of at least ten sugar molecules).

Carbohydrates have special properties, which are described in the following chapters.

Table of contents on this page:

Basics of carbohydrates: optical activity

Carbohydrates are so-called optically active substances that rotate linearly polarized light through a certain angle. What this means exactly is described below:

Light waves from a light source usually run in different directions of oscillation. If light is filtered through a polarizer, all light waves are filtered out, except those that are perpendicular. The remaining light waves run in the same (vertical) direction of oscillation. They are linearly polarized light. If this is now passed through a cuvette in which an optically active substance is dissolved in water, the direction of oscillation of the linearly polarized light is rotated by a certain angle. This angle, also known as the angle of rotation, can be measured using a second polarizer.

Lactic acid is also an optically active substance. However, there are two different types of lactic acid: right-handed and left-handed lactic acid.
Turning to the left means that the direction of oscillation is counterclockwise. These molecules are marked with a (-). The levorotatory lactic acid is also called (-) - lactic acid.
Clockwise means that the direction of oscillation is clockwise. These molecules are marked with a (+). The dextrorotatory lactic acid is called (+) - lactic acid. The right- and left-rotating molecules are enantiomers (image and mirror image), which is why they also have different physical properties.

If there is a mixture of the same amount of left- and right-rotating substance, the plane of oscillation is not rotated, since the two directions of oscillation are balanced. Such a mixture is called a racemate.

Molecular chirality

If four different substituents are attached to an atom, it is a symmetrical atom. This symmetrical atom is called the center of chirality. These are marked with an asterisk on the asymmetric atom.

To determine whether it is a chiral center, it is not enough to look at the four atoms directly bound to an atom. In the following molecule, for example, two further carbon atoms are bonded to the middle carbon atom. It is still a center of chirality, as the atoms attached to these carbon atoms are different from each other. A -CH_3 group is bonded at the bottom, a -COOH group at the top. Four different groups are bound to the middle carbon atom and it is a center of chirality.

Molecules with at least one center of chirality are, as a rule, chiral and thus optically active.

Fisherman projection

Since the spatial arrangement of the substituents plays an important role in carbohydrates, it would be important to be able to use a spatial representation of the molecules. These are very confusing for large molecules. The Fischer projection was developed to show the carbohydrates more clearly.

In order to represent a molecule in the Fischer projection, the following rules must be observed:

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  1. The longest carbon chain points from top to bottom. The terminal carbon atom with the highest oxidation number is on top.
  2. The molecule is rotated so that the substituents on the asymmetric carbon atoms point backwards.
  3. The bindings that point to the back are now represented by vertical lines. All lines that are horizontal point forward.
Example: lactic acid molecule
  1. The longest carbon chain should point from top to bottom, with the terminal carbon atom with the highest oxidation number at the top.

    At the beginning we first determine the oxidation numbers of the two carbon atoms at the end of the chain:

    The carbon atom of the acid group has the higher oxidation number and is therefore on top. This results in the following basic structure for the fisher projection:

  2. In the next step the substituents have to be drawn in, in this case a hydroxyl group and a hydrogen atom.
    To do this, the molecule is rotated so that the single bonds between the carbon atoms point to the rear and the substituents to the front (we look from the left at the shape shown above):
  3. The substituent that now points to the right is also shown on the right in the Fischer projection. The substituent that points to the left is drawn accordingly on the left. All bindings that point to the front are drawn horizontally. All bindings that point to the back are drawn vertically.
    This then results in the following fisherman projection:

Stereoisomerism

The spatial arrangement of the substituents plays a major role in chiral molecules. If the connection of the atoms in two molecules at a center of chirality is different, then we are dealing with isomers.

In the Fischer projection, the spatial arrangement of the substituents at a center of chirality is symbolized by the positions on the right or left. If the two substituents are now swapped from right to left and vice versa, this spatial arrangement is changed. If the spatial arrangement is changed at all centers of chirality, then we are dealing with enantiomers (image and mirror image). The two molecules behave like an image and a mirror image. If the spatial arrangement is only changed at a part of the chiral centers, it is a question of diastereomers. If there are n centers of chirality, there are a maximum of 2 ^ n isomers.

Example: tartaric acid
If the spatial arrangement of the molecule is changed at all centers of chirality, i.e. if the substituents are swapped from right to left at all of them, the result is a mirror image, i.e. an enantiomer.

If the spatial arrangement is changed only at one of the two centers of chirality, diastereomers are formed.

Diastereomer that is formed when the spatial arrangement at the upper chiral center is changed.
Diastereomer that is formed when the spatial arrangement at the lower chiral center is changed.

In the case of tartaric acid, the diastereomers are a special form. Since both diastereomers are identical, it is a meso compound. Despite the two centers of chirality, there are not 2 ^ 2 = 4 isomers, but only 3. These meso compounds are not optically active despite their centers of chirality.


D and L nomenclature

With carbohydrates, as with tartaric acid, there are always two isomers that behave like an image and a mirror image (enantiomers). These enantiomers are distinguished by a prefixed D- or L- in their names. The name is derived from the position of the lower hydroxyl group in the Fischer projection - if the lower hydroxyl group is on the right, it is the D-shape, if it is on the left, it is the L-shape. As an example, consider the two enantiomers of tartaric acid:

Since this nomenclature only indicates the position of the lower hydroxyl group, diastereomers must be given by different names (only enantiomers can have the same name with the addition of D or L).

ATTENTION: The D- / L-nomenclature has nothing to do with the angle of rotation!

glucose

Glucose is a monosaccharide that occurs naturally only as D-glucose (also called grape sugar). It is the most common monosaccharide that belongs to carbohydrates.
The molecular formula of glucose is C_6 H_12 O_6.

The position of the hydroxyl groups is very important at the centers of chirality. There is an easy-to-remember donkey bridge for this purpose: ta tü ta ta. In the case of D-glucose, the hydroxyl groups are on the right-hand side at ta and on the left-hand side at tü. With L-glucose it is exactly the opposite.

Since glucose is an aldehyde, it can be detected with the help of the silver mirror test or the Fehling test.

As a rule, glucose is not in the open-chain form shown above, but in a ring form (pyranose structure), which is formed by a nucleophilic addition between the hydroxyl group on the C5 atom and the carbonyl group on the C1 atom. The resulting six-membered ring is very stable and is called glucopyranose.

The ring closure creates a new center of chirality at the C1 atom.

Haworth projection

If carbohydrates are present in the closed ring form, the Fischer projection is no longer a suitable form of representation, since the new bonds here are much longer than the others, although in reality they are of the same length. Therefore, a different form of representation is required for the ring shape of the carbohydrates: the Haworth projection. In order to rewrite a carbohydrate molecule from the Fischer projection to the Haworth projection, the following rules must be observed:

  1. The molecule in the Fischer projection is rotated 90 ° clockwise so that the longest carbon chain is no longer vertical, but horizontal. Substituents that are on the left in the Fischer projection are now at the top. Substituents that are on the right in the Fischer projection are now at the bottom.
  2. The chain is "bent" in such a way that an even ring is created, whereby the positions of the substituents (top and bottom) are not changed. The oxygen atom, which becomes part of the ring, will always be placed on the top right.
  3. The ring is closed in a nucleophilic addition.
Example: D-glucose
  1. The molecule in the Fischer projection is rotated 90 ° clockwise.

    Substituents that are on the left in the Fischer projection are now on top, substituents that are on the right in the Fischer projection are now on the bottom.

  2. The chain is bent in such a way that an even ring is formed, whereby the positions of the substituents (top and bottom) are not changed). The oxygen atom that becomes part of the ring is always written on the top right.

  3. The ring is closed in a nucleophilic addition.

    Here is the Fischer projection of the ring-shaped glucose compared to the Haworth projection:

Mutarotation

The ring closure creates a new center of chirality at the C1 atom. It is therefore important here whether the hydroxyl group is on top or bottom. Both shapes can be formed by the ring closure.

If the hydroxyl group is at the bottom of the C1 atom, it is α-D-glucose. If it is at the top, it is β-D-glucose. These two forms are so-called anomers (isomers that differ in the spatial arrangement of the hydroxyl group on the C1 atom in the ring-shaped sugar). This is why the new chiral center is also called the anomeric carbon atom.

The forms “open-chain”, “α-form” and “β-form” can be present in aqueous solution. By closing the open-chain form into one of the two rings and separating the ring again, they are repeatedly converted into one another. These reactions are equilibrium reactions in which an equilibrium is established. The process by which the two anomers are converted into one another is called mutarotation.

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Fructose

Fructose is another important monosaccharide. The molecular formula of fructose and glucose (C_6 H_12 O_6) are identical, the two constitutional isomers have little in common. Fructose is not an aldehyde like glucose, but a ketone.

Like glucose, fructose is usually ring-shaped. There are two options for ring closure with fructose:

  1. Reaction between the carbonyl group and the hydroxyl group on the C5 atom. A five-membered ring is created. This form of fructose is called fructofuranose.
  2. Reaction between the carbonyl group and the hydroxyl group on the C6 atom. A six-membered ring is created. This form of fructose is called fructopyranose.

Keto-enol tautomerism

Although fructose is not an aldehyde but a ketone, the silver mirror test and the Fehling test are positive. But how can that be?
An aldehyde group must be present in a molecule for positive silver mirror and Fehling samples. This means that the keto group of fructose has to be converted into an aldehyde group. This conversion is called keto-enol tautomerism and it proceeds as follows:

In the first step a proton is split off from the C1 atom, the single bond folds in and becomes a double bond, and therefore the double bond between the carbon atom and the oxygen atom has to fold out to the oxygen atom. The positively charged proton is attracted and absorbed by the negative charge created. A proton is then split off from the upper hydroxyl group, and a negative charge arises on the oxygen atom of the hydroxyl group. The proton is attacked by the electron pair of the double bond, whereby the lone pair of electrons on the upper oxygen atom can fold inwards and become a double bond. In this way, an aldehyde was created from a ketone.

Disaccharides

Disaccharides are carbohydrates that are composed of two monosaccharides. The two monosaccharides are linked by a glycosidic bond. When such a glycosidic bond is formed, two hydroxyl groups react with one another in a condensation reaction and form an oxygen bridge.
We consider three different important disaccharides to illustrate the principle of the glycosidic bond:

Maltose: Maltose is a disaccharide with an α-1,4-glycosidic bond between two α-glucose molecules. So we consider two α-glucose molecules. An α-1,4-glycosidic compound should be created, i.e. the hydroxyl group on the C1 atom of one α-glucose molecule reacts with the hydroxyl group on the C4 atom of another α-glucose molecule. A water molecule emerges in the process.

Cellobiose is a disaccharide with a β-1,4-glycosidic bond between β-glucose and α-glucose. So we consider an α- and a β-glucose molecule. A β-1,4-glycosidic compound should be created, i.e. the hydroxyl group on the C1 atom of one β-glucose molecule reacts with the hydroxyl group on the C4 atom of another α-glucose molecule. A water molecule emerges in the process.

So that the two hydroxyl groups can react with one another, both must be oriented either upwards or downwards. In this case, one up and one down. Since the spatial arrangement cannot simply be changed, we have to turn the entire molecule around (mirror it at the drawn mirror plane).

As soon as one of the two molecules has been rotated, the two hydroxyl groups are both up and they can react with one another.

Sucrose: Sucrose is a disaccharide with an α-1,2-glycosidic bond between α-glucose and β-fructose. So we consider an α-glucose and a β-fructose molecule. An α-1,2-glycosidic compound should be created, i.e. the hydroxyl group on the C1 atom of an α-glucose molecule reacts with the hydroxyl group on the C2 atom of a β-fructose molecule. A water molecule emerges in the process.
So that the two hydroxyl groups can react with one another, they must be in the same direction again. So we have to rotate the molecule so that the C2 atom of fructose is next to the C1 atom of glucose and that both hydroxyl groups are oriented downwards.

Now the two hydroxyl groups are next to each other and can react with one another.

Disaccharides, in which the hydroxide group on the anomeric carbon atom of one of the two monosaccharides is not involved in the formation of the glycosidic formation, run in a positive silver mirror test or Fehling test. A monosaccharide whose hydroxyl group is retained on the anomeric carbon atom can change from the ring form to the open-chain form by mutarotation. In the open-chain form, the aldehyde group is present, which shows a positive silver mirror or Fehling test. Maltose and cellobiose are among these disaccharides.

Disaccharides, in which the glycosidic bond is formed by the reaction between the hydroxyl groups on the two anomeric carbon atoms, cannot be converted into the open-chain form, which is why the molecule never has a free aldehyde group. Therefore, the silver mirror test and the Fehling test with such disaccharides are negative. Sucrose is one of these disaccharides.

Polysaccharides

Polysaccharides are carbohydrates that are composed of at least ten monosaccharides via glycosidic bonds.
An important polysaccharide is starch.It is an important vegetable storage substance and consists of two components: amylose and amylopectin.

Amylose makes up about 20% of the starch. It is a polysaccharide consisting of glucose molecules with α-1,4-glycosidic bonds.

The macromolecule, however, is not built linearly, but in a spiral shape.

Amylopectin makes up about 80% of the starch. It is a polysaccharide which, in addition to the α-1,4-glycosidic bonds between the glucose molecules, also forms α-1,6-glycosidic bonds, which means that the macromolecule is more branched.

Another important polysaccharide is cellulose, a vegetable building material. It is a macromolecule made up of β-glucose molecules with β-1,4-glycosidic bonds. The cellulose macromolecule has a linear structure, which is why it has the typical fiber-like structure.

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