Why is there a mutarotation of fructose

List of experiments on carbohydrates

Whenever you talk about carbohydrates, you think of ring structures. Some even use it to define the term `` carbohydrates '': only those polyhydroxyl carbonyl compounds should be called carbohydrates that can also form stable rings - i.e. only the hexoses and pentoses.

However, carbohydrates are not always in the ring shape. Rather, they are in constant equilibrium with their open-chain form. There are also different ring shapes, depending on whether five or six atoms are involved in the formation of the ring structure: Furanoses and Pyranoses. The terms come from cyclic ethers that have corresponding structures.

 

Pyran and furan


Formation of intramolecular hemiacetals
Carbohydrates have both OH and C = O groups. A typical reaction between carbonyl compounds and alcohols is hemiacetal formation. Since both functional groups are present in one molecule, this reaction can also take place intramolecularly. This is how a ring is formed.

In the reaction to the cyclic hemiacetal, one must first remember that the individual links of the open-chain form can be rotated. This means that when the ring is formed, the individual links are first around the C.5-Turn the atom until its hydroxyl group comes into a position in which it is as close as possible to the terminal carbonyl group. Both react with each other in an addition reaction and a six-membered hemiacetal is formed (Pyranose). A five-membered ring (Furanosis) On the other hand, if you do not use the C5-Atom, but those attached to the C4-Atom bonded hydroxyl group reacts with the carbonyl group.


Which ring shape is preferred?
Whether a furanose or a pyranose is formed depends on the respective carbohydrate. In the case of glucose, the pyranose form predominates. The same applies to fructose when it is in solution or in crystals. Because a ring of 6 atoms with its chair shape, familiar from cyclohexane, guarantees the most stress-free and therefore lowest-energy shape. However, the proportion of the furanose form predominates in the bound fructose - e.g. B. in sucrose.
It is therefore not possible to establish a simple rule as to whether one or the other form is generally preferred.


Anomer formation - a consequence of ring closure and mutarotation
When looking at the product that results from the ring closure from the linear carboxyl compound, it becomes apparent that two further isomers are possible due to the spatial arrangement of the newly formed hydroxyl group at C atom 1. This is called Anomers. They are designated by the Greek letters α and β: α-D-glucose or β-D-glucose.

Formation of anomers of D-glucose

The entire reaction group represents a chemical equilibrium system. As a result, a state of equilibrium is also established between the two anomeric forms.

If a pure a- or b-anomer is put into solution, the equilibrium between the anomers is established spontaneously, albeit quite slowly. The reaction can be accelerated by adding a weak base such as soda or ammonia. Because this promotes the ring opening.

One speaks here of Mutarotation (lat. mutare, change). The chemical reaction can be followed by changing the optical activity of a solution. The a-anomer has a specific rotation [a]D. = + 112.2 , the b-anomer on the other hand only + 18.7 . These are also the initial values ​​of the respective solutions. After reaching the state of equilibrium one measures + 52.7 . Incidentally, this is also the rotation value that can be found in tabular form for a glucose solution.

In the case of glucose, the b-form is preferred with 64%. On another website we explain what it depends on which form is formed in the first place. Here we also bring the reaction kinetic equilibrium calculations. We also describe how to produce the pure anomers.

Incidentally, anomers are examples of the special case of stereochemistry, diastereomerism.

For the sake of completeness, it should be said that there are other configurational isomers of the anomeric forms shown above. The chair shape of the ring can actually fold over. In the picture above, the carbon atom 1 is facing up, the carbon atom 4 is facing down. However, this is only really important for understanding some enzymatic reactions.


Further texts on the subject of `` carbohydrates ''