Forms of Carbohydrates, Chemistry tutorial

Introduction:

Carbohydrates comprise of the elements carbon (C), hydrogen (H) and oxygen (O) having a ratio of hydrogen two times that of carbon and oxygen. Carbohydrates comprise sugars, starches, cellulose and most of the other compounds found in living organisms. Carbohydrates are termed as saccharides or, if they are comparatively small, sugars.

Definition of Carbohydrates:

Carbohydrates are the major energy source for the human body. Chemically, carbohydrates are the organic molecules in which carbon, hydrogen and oxygen bond altogether in the ratio: Cx(H2O)y, here x and y are the whole numbers which differ based on the particular carbohydrate. 

Importance of Carbohydrates:

Carbohydrates are a main source of metabolic energy, both for animals and for plants which based on plants for food. Aside from the sugars and starches which meet up this vital nutritional role, carbohydrates as well serve as the structural material (that is, cellulose), a component of the energy transport compound ATP, recognition sites on cell surfaces, and one of three necessary components of DNA and RNA. 

Classification of Carbohydrates:

Some of the classifications of carbohydrates have proven helpful, and are outlined in the given table.

Fig: Classification of Carbohydrates table

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Ketoses:

If a monosaccharide consists of a carbonyl function on one of the inner atoms of the carbon chain it is categorized as a ketose. Dihydroxyacetone might not be a sugar; however it is included as the ketose all analog of glyceraldehyde. The carbonyl group is generally found at C-2, as described by the following illustrations (chiral centers are colored red). As expected, the carbonyl function of a ketose might be reduced via sodium borohydride, generally to a mixture of epimeric products. 

D-Fructose, the sweetest of the general natural sugars, is for illustration reduced to a mixture of D-glucitol (or sorbitol) and D-mannitol, named after the aldohexoses from which they might as well be obtained via analogous reduction. Mannitol is itself a general natural carbohydrate.

However the ketoses are different isomers of the aldose monosaccharides, the chemistry of both classes is combined due to their facile interconversions in the presence of base or acid catalysts. This interconversion and the corresponding epimerization at sites alpha to the carbonyl functions, takes place by way of an enediol tautomeric intermediate.

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Fig: ketose examples 

Due to base-catalyzed isomerization of this type, the Tollen's reagent is not helpful for differentiating aldoses from ketoses or for particular oxidation of aldoses to the corresponding aldonic acids. The oxidation by HOBr is favored for the later conversion.

Anomeric Forms of Glucose:

Fischer's brilliant explanation of the configuration of glucose didn't get rid of all the uncertainty regarding its structure. The two different crystalline forms of glucose were reported in the year 1895. Each of these provides all the characteristic reactions of glucose, and whenever dissolved in water equilibrated to the similar mixture. This equilibration occurs over a period of many minutes, and the change in optical activity that takes place is termed as mutarotation. These facts are concluded in the diagram shown below.

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Fig: Anomeric Forms of Glucose

Whenever glucose was transformed to its pentamethyl ether (that is, reaction having excess CH3I and AgOH), two different isomers were isolated, and neither showed the expected aldehyde reactions. Acid-catalyzed hydrolysis of the pentamethyl ether derivatives, though, provide a tetramethyl derivative that was oxidized through Tollen's reagent and reduced via sodium borohydride, as expected for the aldehyde. 

The search for scientific truth frequently proceeds in phases and the structural description of glucose serves up as a good illustration. It must be clear from the latest proof presented above, that the open chain pentahydroxyhexanal structure drawn above should be altered. In some way a new stereogenic center should be created, and the aldehyde should be deactivated in the pentamethyl derivative. A simple solution to this dilemma is accomplished via transforming the open aldehyde structure for glucose to a cyclic hemiacetal, termed as a glucopyranose, as illustrated in the diagram below. The linear aldehyde is tipped on its side, and rotation regarding the C4-C5 bond brings the C5-hydroxyl function close to the aldehyde carbon. For easiness of viewing, the six-membered hemiacetal structure is represented as a flat hexagon; however it actually supposes a chair conformation. The hemiacetal carbon atom (C-1) becomes a latest stereogenic center, generally termed to as the anomeric carbon, and the α and β-isomers are known as anomers.

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Fig: Mutarotation equilibrium-Anomeric Forms of Glucose

We can now assume how this modification of the glucose structure accounts for the puzzling facts noted above. Primarily, it is known that hemiacetal are in equilibrium by their carbonyl and alcohol components whenever in solution. As a result, fresh solutions of either alpha or beta-glucose crystals in water must establish an equilibrium mixture of both the anomers, plus the open chain form.

Second, a pentamethyl ether derivative of the pyranose structure transforms the hemiacetal function to the acetal. Acetals are stable to base; therefore this product must not react by Tollen's reagent or be reduced through sodium borohydride. Acid hydrolysis of acetals regenerates the carbonyl and alcohol components, and in the case of glucose derivative this will be tetramethyl ether of the pyranose hemiacetal. This compound will, obviously, undergo typical aldehyde reactions.

Cyclic Forms of Sugars:

The preferred structural form of numerous monosaccharides might be that of a cyclic hemiacetal. Five and six-membered rings are favored over other ring sizes due to their low angle and eclipsing strain. The cyclic structures of this type are known as furanose (that is, five-membered) or pyranose (that is, six-membered), reflecting the ring size relationship to the general heterocyclic compounds furan and pyran shown below. 

1827_Furan and Pyran.jpg

Fig: Furan and Pyran

Ribose, a significant aldopentose, generally adopts a furanose structure, as illustrated in the following illustration. By convention for the D-family, the five-membered furanose ring is drawn in an edgewise projection having the ring oxygen positioned away from the viewer. The anomeric carbon atom (that is, colored red here) is positioned on the right. The upper bond to this carbon is stated as beta, the lower bond then is alpha.

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Fig: Cyclic forms of Sugars

The cyclic pyranose forms of different monosaccharides are often drawn in the flat projection termed as a Haworth formula, after the British chemist, Norman Haworth. As by the furanose ring, the anomeric carbon is positioned on the right having the ring oxygen to the back of the edgewise view. In the D-family, the alpha and beta bonds encompass the similar orientation stated for the furanose ring (that is, beta is up and alpha is down). Such Haworth formulas are convenient for displaying stereochemical relationships; however don't represent the true shape of the molecules.

Such molecules are in reality puckered in a fashion we call a chair conformation. Illustrations of four typical pyranose structures are illustrated below, both as Haworth projections and as the more representative chair conformers. The anomeric carbons are colored red.

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Fig: Examples of some Pyranose forms of hexoses

The size of cyclic hemiacetal ring adopted through a particular sugar is not constant, however might differ with substituents and other structural features. Aldolhexoses generally form pyranose rings and their pentose homologs tend to prefer the furanose form, however there are numerous counter illustrations. The formation of acetal derivatives describes how subtle modifications might modify this selectivity. Acetal derivatives have been made by acid-catalyzed reactions with benzaldehyde and acetone. As a rule, benzaldehyde prepares six-membered cyclic acetals, while acetone favors to prepare five-membered acetals. The top equation exhibits the formation and some reactions of the 4, 6-O-benzylidene acetal, a generally utilized protective group. A methyl glycoside derivative of these compound leaves the C-2 and C-3 hydroxyl groups exposed to reactions like the periodic acid cleavage, illustrated as the last step. The formation of an isopropylidene acetal at C-1 and C-2, center structure, leaves the C-3 hydroxyl as the mere unprotected function.

Selective oxidation to a ketone is then possible. At last, direct di-O-isopropylidene derivatization of glucose by reaction by surplus acetone yields in a change to a furanose structure in which the C-3 hydroxyl is yet again unprotected.

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