Why are carbohydrates called hydrated carbons

Subdivision of carbohydrates and their projections

There are several criteria for classifying carbohydrates:


1. According to the size of n according to the general formula:

$ C _ {\ color {Blue} n} (H_2O) _ {\ color {Blue} n} $


$ C _ {\ color {Blue} n} H_ {2 {\ color {Blue} n}} O _ {\ color {Blue} n} $:


Depending on the number of carbon atoms (n) the Greek prefixes are assigned, which are already known from the nomenclature of alkanes, among other things. However, are n= 1 and n= 2 only formally to be regarded as carbohydrates, n= 3 is denoted by Tri- instead of Prop- and n= 4 instead of but- with tetr-:



















The two parent substances glyceraldehyde and dihydroxyacetone would have the prefix Tri-!

left: glyceraldehyde, right: dihydroxyacetone, both: n = 3 = trioses

The fact that both parent substances have the same prefix makes it clear that a further differentiation is necessary, but this is extremely simple: it occurs after the Derived from one of the two parent substances.

Carbohydrates can also link with each other, as we will see later, a single carbohydrate molecule is therefore also known as a monosaccharide.

2. According to the relationship to glyceraldehyde or dihydroxyacetone:

As already mentioned, carbohydrates are either polyhydroxy aldehydes or ketones, i.e. they have either primary or secondary carbonyl carbon atoms.



D-glucose and D-fructose: aldohexose and ketohexose

Combined with the syllable for n the names are clearer: D-glucose is an aldohexose and D-fructose is a ketohexose.


3. Chirality and Optical Activity

But what does the "D" before glucose mean? Let us remember the chapter on stereochemistry D comes from dextro, this in turn from Latin dexter: right and means the configuration at the farthest chiral carbon atom from the most oxidized group of the molecule in the Fischer projection. In the case of D, it is on the right, which is what gave D-glucose the name “dextrose”.


D- and L-glucose, the last chiral and thus decisive carbon atom is marked in green, the direction of the carbonyl group was also chosen to the right or left, but this does not matter because the carbonyl-C is sp²-hybridized, thus planar and completely achiral.

L.Evo-left carbohydrates also exist, but play a rather subordinate role - almost a mirrored ratio if you compare this fact with the amino acids, which occur almost exclusively in L-form in nature.

Mirrored is a good keyword: The D&L shapes behave like an image and a mirror image is Enantiomers.

If a molecule is chiral, it is able to rotate the polarization plane of the light, it is optically active. In doing so, clockwise should NOT be equated with D and counterclockwise not with L, instead most authors use “(+)” for clockwise (clockwise and “(-)” for counterclockwise connections).

As a specific rotation $ \ left [{\ alpha} \ right] $ of carbohydratesolutions the rotation value $ \ alpha $ in degrees (°), which 1 g of carbohydrate in 1 ml of water in a tube with a length of 10 cm causes. Since this property also depends on the wavelength of the light used and the temperature of the solution, it is necessary to specify the wavelength and temperature for which the rotation value was determined. This is expressed as follows:



4. Ring-chain tautomerism

As we already learned in the introduction, carbohydrate molecules are able to form a ring. This property is called ring-chain tautomerism (intramolecular rearrangement). Some of them, such as fructose, can form both pentagonal (“furanose”) and hexagonal (“pyranose”) rings. Since each ring can come about in two ways, which differ in the position of an OH group, there are even five forms in which fructose can occur! This alone in the D-form, the L-form has another five - L-fructose plays practically no role for living beings.

The five forms of D-fructose


The mechanism of the ring closure might sound familiar: namely nothing more than a cyclic hemiacetal is formed!

The ring closure occurs through hemiacetal formation, both in the furanose and in the pyranose forms.

The ring closure, also known as Mutarotation is called, therefore runs through a nucleophilic attack by a hydroxyl group:



Depending on whether the attacking hydroxyl group is attached to the 5th or 6th carbon atom, either a pyranose or a furanose is formed.

One can see further the coming about of the two anomeric forms $ \ alpha $ & $ \ beta $, so-called. More Compliant, to illustrate with a classic belt: You can put the end of the belt “normally” or turned 180 ° into the belt loop, ie “the wrong way round”.

The "belt analogy": left: "normal", right: "the wrong way round"

If you do it the wrong way round (which probably happens to very few people), the back of the end partially points away from the body instead of towards it. When it comes to carbohydrates, however, there is no right or wrong way around, although with some one form is energetically more favorable than the other. It was arbitrarily determined that alpha = below and beta = above. Here are two tips:

a) For people who can better remember words and sounds:

"Alphunten" or "Beethoven"


b) For those who remember things better in pictures:





There are two common representations for pyranoses, the so-called Haworth projection and the armchair projection. The former seems clearer, the latter reflects the spatial relationship more realistically, so that the choice should be left to personal taste:

Projection forms

For furanoses, only the Haworth flat projection is common; the names furanose and pyranose were borrowed from the two compounds, the structures of which are similar to the respective carbohydrate ring:

Furan and pyran: basic structural elements of furanoses and pyranoses


5. Glycosidic bond: mono-, di-, oligo- and polysaccharidee

The hemiacetal OH groups of the Monosaccharides are quite reactive, they can, for example, form a bond with the alcoholic OH groups of other carbohydrates or with their hemiacetal OH groups, which are known as glycosidic bond referred to as.

One molecule of water is released for each bond.

For example, can two Glucose molecules react with each other in this way and form a disaccharide. Depending on whether both hemiacetal OH groups or a hemiacetal OH group and one of the alcoholic OH groups form the link, a distinction is made between:

a) Maltose: The two monomers are α-1 → 4-bridged:




b) Cellobiose: The two monomers are β-1 → 4-bridged:



c) Gentiobiosis: It has a β-1 → 6 bridge




If $ \ alpha $ -D-glucose is linked with $ \ alpha $ -D-fructose, the result is sucrose - ordinary table sugar, whether from sugar beet or sugar cane:



Milk sugar, lactose, is also a disaccharide, in this case made from D-galactose and D-glucose:


Connect between 2 and 10 Monosaccharides with each other, so form Oligosaccharides (Greek: oligos - little), but this can vary depending on the author - strictly speaking, all disaccharides are also oligosaccharides.

Raffinose, as found in beet molasses, is a trisaccharide and therefore also an oligosaccharide

If more units (> 10) come together, they form Polysaccharides, as the Glycogen the liver:

Glycogen or "animal starch" often has side chains, similar to an elastomer

the Amylose (generally as Strength referred to, from Latin Amylum)

Amylose is "the starch" and consists of many monomers, it is the storage carbohydrate of plants - just like the glycogen for animals - some varieties, such as potato starch, also have side groups.


or the Cellulose:

Cellulose, the structural substance of the plant kingdom, n is often between 500 and 5000