Properties of carbohydrates

This ppt explains the properties of monosaccharides, polysaccharides. the properties like mutarotation, reduction, optical activity, caramerlization, osazone is given in the ppt. Also the determination of ring size of the monosaccharide is explained/

1. Dr.N.C.J.Packia Lekshmi Allied Health Sciences Noorul Islam Centre for Higher Education Properties of Carbohydrates

2. Reducing sugar Aldehydes and keto groups have reducing character and reduce Tollens reagent and Fehling’s (Benedict’s) solution. Carbohydrates containing free aldehyde and keto functional group are thus reducing sugars. Example: Glucose, lactose.

3. Non-Reducing Sugar Sugar which does not have free aldehyde or ketone functional group is the non-reducing sugar. Also , they do not get oxidized. Sucrose is their most common source. They give a negative reaction for Fehling’s as well as Benedict’s test. All polysaccharides are non-reducing sugars.

4. Comparison Between Reducing and Non-Reducing Sugar Reducing sugars are carbohydrates with free aldehyde or ketone group while non-reducing one does not have free aldehyde or ketone instead they are present in bond formation. Reducing sugar is in hemiacetal or hemiketal form whereas non-reducing form is in acetal or ketal form. Reducing sugar exhibits mutarotation , on the other hand, its non-reducing form does not exhibit it. Reducing sugar form osazones while non-reducing does not form osazones . Reducing sugar is mainly monosaccharides while non-reducing is mainly polysaccharides.

5. Properties of Monosaccharides Colour – Monosaccharides are colourless Shape – They are crystalline compounds Solubility – They are readily soluble in water Taste – They have sweet taste Optical activity – They are optically active. They rotate the plane polarized light. When a monosaccharide rotates the plane polarized light in the clockwise direction or to the right (dextrorotatory) the monosaccharide is called ‘d’ form. When the monosaccharide rotate in the anticlockwise direction or to the left (levorotatory) the monosaccharides is called ‘l’ form.

6. Mutarotation – Monosaccharides exhibit mutarotation . The change in specific rotation of an optically active compound is called mutarotation . When monosaccharide is dissolved in water, the optical rotatory power of the solution gradually changes until it reaches a constant value. The mutarotation is due to the existence of two optical isomers of glucose, namely α -D-glucose with a specific rotation of +112° and β -D-glucose with a specific rotation of +19°.

7. Glucoside formation – Glucose reacts with methyl alcohol in the presence of hydrogen chloride gas to give glucosides . Glucoside formation is due to the reaction of alcohol with the glucosidic OH of monosaccharides . In the same way fructose forms fructosides Glucose Methyl Alcohol Methyl Glucoside

8. Esterification – Glucose reacts with 5 molecules of acetic anhydride to form esters. The ability of sugars to form esters indicates the presence of alcohol groups. As glucose yields a penta acetate derivative on acetylation , it obviously contains five OH groups

9. Reaction with concentrated HCl When hexoses are treated with concentrated HCl , they form 5-hydroxymethylfurfural which on further heating yields levulinic acid and formic acid. This reaction is the basis of the colour test, Molisch test for sugars.

10. Formation of Oximes Aldoses and ketoses react with hydroxylamine to form oximes Glucose Hydroxylamine Aldoxime Water

11. Reaction with hydrogen Cyanide When hydrogen cyanide is added to sugars, cyanohydrin is produced Glucose Hydrogen Cyanide Glucose cyanohydrin

12. Kiliani Synthesis This reaction is the basis for the synthesis of monosaccharides . This was proposed by Kiliani in 1886 and this method of monosaccharide synthesis is called Kiliani synthesis By this method the chain length of a carbohydrate is increased

13. Enolization Glucose, fructose and mannose are interconvertible in weak alkaline solutions such as Ca(OH) 2 and Ba (OH) 2 at low temperature. This reaction is called Lobry de Bruyn-Alberda Ekenslein conversion named after its discoverer.

14. Caramelization When monosaccharide is added with concentrated alkali, it is burnt and this process is called caramelization . It produces a series of decomposition products. Yellow and brown pigments develop, salts may form, many double bonds between carbon atoms are formed and carbon to carbon double bonds may rupture.

15. Phosphorylation of hexoses The formation of a phosphoric acid derivative of hexoses is called phosphorylation . On phosphorylation , the hexoses are converted into phosphoric acid esters Phosphorylation is an important step in metabolism such as glycolysis , kreb’s cycle, glycogenesis , glycogenolysis etc…

16. Fermentation Glucose give ethyl alcohol and CO 2 during fermentation by zymase Zymase Glucose Ethyl Alcohol

17. Osazone formation Osazones are a class of carbohydrate derivatives found in organic chemistry formed when reducing sugars are reacted with excess of phenylhydrazine at boiling temperatures. The osazone formation reaction was developed by famous German chemist Emil Fischer, who used the reaction as a test to identify monosaccharides whose stereochemistry differed by only one chiral carbon.

18. The reaction involves formation of a pair of phenylhydrazone functionalities, concomitant with the oxidation of the hydroxy group on the alpha carbon (carbon atom adjacent to the carbonyl center). This formation allows two sugars with closely related structures to give the same osazone . Glucosazone and fructosazone , for example, are identical. Since the reaction requires a free carbonyl group, only reducing sugars can form osazones . Sucrose , for example, is unable to form an osazone , because it is non-reducing.

19. The mechanism involves two steps, each of which forms one of the phenylhydrazone functionalities. In the first step, a molecule of phenylhydrazine reacts with the carbonyl group on the sugar to form a phenylhydrazone by elimination of a water molecule. The next step involves two molecules of phenylhydrazine . The first oxidizes the hydroxy group on the reactive alpha carbon to a carbonyl group, and the second then forms a hydrazone with it. The end product is stabilized by a hydrogen bond between the two phenylhydrazone groups.

20. Maltosazone (from maltose) forms petal-shaped crystals Lactosazone (from lactose) forms powder puff-shaped crystals Galactosazone (from galactose ) forms rhombic-plate shaped crystals Glucosazone (from glucose, fructose or mannose) forms broomstick or needle-shaped crystals Osazones are highly coloured and crystalline compounds and can be easily detected. Each different osazone has a characteristic crystal formation.

21. Properties of Polysaccharides They do not have sweet taste They are amorphous substances Most of them are insoluble in water and they give opalascence Some polysaccharides are soluble in water eg ., Glycogen They dissolve in boiling water forming colloids They do not exhibit any of the properties of aldehyde or ketone They are branched or unbranched The mucopolysaccharides have a mucous consistency

22. Determination of Ring size The ring size of the cyclic hemiacetal structure assumed by many monosaccharides was determined by oxidative cleavage of a permethylated derivative. Five and six- membered rings are favored over other ring sizes because of their low angle and eclipsing strain. The equations in the following diagram illustrate this approach for the aldohexose , glucose.

23. First, a pentamethyl derivative is prepared, as noted earlier. One of the methyl ether functions in this derivative is part of an acetal , and is therefore readily hydrolyzed by aqueous acid. The open chain form of this tetramethylglucose derivative is oxidized to a keto -acid intermediate by nitric acid treatment. The location of the ketone carbonyl reflects the size of the initial heterocyclic ring, C-4 for a pyranose ring and C-3 for a furanose ring. Further oxidation cleaves the carbon chain at bonds leading to the carbonyl group. This final oxidation produces a mixture of two dicarboxylic acids, differing in length by one carbon atom. Once these fragments have been identified, the location of the ketone function is established.

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