Chemistry Structure of Glucose and Fructose
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Topics Covered :

● Structure of Glucose
● Cyclic Structure of Glucose
● Structure of Fructose

Structure of Glucose :

`=>` Glucose is an aldohexose and is also known as dextrose.

`=>` It is the monomer of many of the larger carbohydrates, namely starch, cellulose.

`=>` It is probably the most abundant organic compound on earth.

`=>` It was assigned the structure given in fig.1 on the basis of the following evidences :

1. Its molecular formula was found to be `color{red}(C_6H_(12)O_6).`

2. On prolonged heating with `color{red}(HI)`, it forms `color{red}(n)`-hexane, suggesting that all the six carbon atoms are linked in a straight chain. See fig.2.

3. Glucose reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen cyanide to give cyanohydrin. These reactions confirm the presence of a carbonyl group (`color{red}(> C = O)`) in glucose. See fig.3.

4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction with a mild oxidising agent like bromine water. This indicates that the carbonyl group is present as an aldehydic group. See fig.4.

5. Acetylation of glucose with acetic anhydride gives glucose pentaacetate which confirms the presence of five `color{red}(–OH)` groups. Since it exists as a stable compound, five `color{red}(–OH)` groups should be attached to different carbon atoms. See fig.5.

6. On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic `color{red}((–OH))` group in glucose. See fig.6.

`=>` The exact spatial arrangement of different `color{red}(—OH)` groups was given by Fischer after studying many other properties.

`=>` Its configuration is correctly represented as `color{red}(I)`. So. gluconic acid is represented as `color{red}(II)` and saccharic acid as `color{red}(III)`. See fig.7.

`=>` Glucose is correctly named as `color{red}(D(+))`-glucose. `color{red}(‘D’)` before the name of glucose represents the configuration whereas `color{red}(‘(+)’)` represents dextrorotatory nature of the molecule.

`=>` It may be remembered that `color{red}(‘D’)` and `color{red}(‘L’)` have no relation with the optical activity of the compound. The meaning of `color{red}(D–)` and `color{red}(L–)` notations is given as follows :

● The letters `color{red}(D)` or `color{red}(L)` before the name of any compound indicate the relative configuration of a particular stereoisomer.

● This refers to their relation with a particular isomer of glyceraldehyde.

● Glyceraldehyde contains one asymmetric carbon atom and exists in two enantiomeric forms as shown in fig.8.

● All those compounds which can be chemically correlated to `color{red}((+))` isomer of glyceraldehyde are said to have `color{red}(D-)` configuration whereas those which can be correlated to `color{red}((–))` isomer of glyceraldehyde are said to have `color{red}(L-)`configuration.

● For assigning the configuration of monosaccharides, it is the lowest asymmetric carbon atom (as shown below) which is compared.

● As in `color{red}((+))` glucose, `color{red}(-OH)` on the lowest asymmetric carbon is on the right side which is comparable to `color{red}((+))` glyceraldehyde, so it is assigned `color{red}(D)`-configuration. For this comparison, the structure is written in a way that most oxidised carbon is at the top. See fig.9.

Cyclic Structure of Glucose :

The structure `color{red}((I))` of glucose explained most of its properties but the following reactions and facts could not be explained by this structure.

1. Despite having the aldehyde group, glucose does not give `2,4`-DNP test, Schiff’s test and it does not form the hydrogensulphite addition product with `color{red}(NaHSO_3).`

2. The pentaacetate of glucose does not react with hydroxylamine indicating the absence of free `color{red}(—CHO)` group.

3. Glucose is found to exist in two different crystalline forms which are named as `color{red}(α)` and `color{red}(β)`. The `color{red}(α)`-form of glucose (m.p. `419 K`) is obtained by crystallisation from concentrated solution of glucose at `303 K` while the `color{red}(β)`-form (m.p. `423 K`) is obtained by crystallisation from hot and saturated aqueous solution at `371 K`.

`=>` This behaviour could not be explained by the open chain structure `color{red}((I))` for glucose.

`=>` It was proposed that one of the `color{red}(—OH)` groups may add to the `color{red}(—CHO)` group and form a cyclic hemiacetal structure.

`=>` It was found that glucose forms a six-membered ring in which `color{red}(—OH)` at `color{red}(C-5)` is involved in ring formation. This explains the absence of `color{red}(—CHO)` group and also existence of glucose in two forms as shown below. These two cyclic forms exist in equilibrium with open chain structure. See fig.1.

`=>` The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at `color{red}(C-1)`, called `color{green}("anomeric")` carbon (the aldehyde carbon before cyclisation). Such isomers, i.e., `color{red}(α)`-form and `color{red}(β)`-form, are called anomers.

`=>` The six membered cyclic structure of glucose is called pyranose structure (`color{red}(α–)` or `color{red}(β–)`), in analogy with pyran.

`=>` Pyran is a cyclic organic compound with one oxygen atom and five carbon atoms in the ring.

`=>` The cyclic structure of glucose is more correctly represented by Haworth structure as given in fig.2.

Fructose :

`=>` Fructose is an important ketohexose.

`=>` It is obtained along with glucose by the hydrolysis of disaccharide, sucrose.

Structure of Fructose :

`=>` Fructose also has the molecular formula `color{red}(C_6H_(12)O_6)` and on the basis of its reactions it was found to contain a ketonic functional group at carbon number `2` and six carbons in straight chain as in the case of glucose.

`=>` It belongs to `color{red}(D)`-series and is a laevorotatory compound.

`=>` It is appropriately written as `color{red}(D-(–))`-fructose. Its open chain structure is as shown in fig.1.

`=>` It also exists in two cyclic forms which are obtained by the addition of `color{red}(—OH)` at `color{red}(C_5)` to the (`color{red}(>C=O)` ) group.

`=>` The ring, thus formed is a five membered ring and is named as furanose with analogy to the compound furan.

`=>` Furan is a five membered cyclic compound with one oxygen and four carbon atoms. See fig.2.

`=>` The cyclic structures of two anomers of fructose are represented by Haworth structures as given. See fig.3.