Physical and chemical properties of carbohydrates

Carbohydrate physical properties - optical isomerism, optical activity, mutarotation. Carbohydrate chemical properties - reducing property, reactions with acids and alkali, oxidation , some review questions, chiral carbon, enantiomers, chirality

1. Physical and chemical properties of carbohydrates Presented by: Jasmine Juliet Biochemistry, ACRI, Madurai.

2. Physical properties of carbohydrates Optical isomerism : It is exhibited by compounds containing asymmetric carbon atoms. All monosaccharides except dihydroxy acetone have one or more asymmetric carbon atom and show optical activity . Monosaccharides which causes rotation of plane polarized light to the right are called dextrorotatory and is designated ( d ) or (+). Those which rotate the plane polarized light towards left are said to be leavorotatory , ( l ) or (-). These two forms (D (+) Glucose & L(-) Glucose) are non- superimposable mirror images of each other and are called enantiomers .

3. Enantiomers

4. Physical properties of carbohydrates Optical activity is the ability of a chiral molecule to rotate the plane of plane-polarised light in either a clockwise or anti-clockwise direction. All monosaccharides except dihydroxy acetone have one or more asymmetric carbon atom and are able to rotate the plane polarized light. Monosaccharides which causes rotation of plane polarized light to the right are called dextrorotatory and is designated ( d ) or (+). Those which rotate the plane polarized light towards left are said to be leavorotatory , ( l ) or (-). These two forms are non- superimposable and are termed enantiomers .

5. Physical properties of carbohydrates Mutarotation : Mutarotation refers to the change in optical rotation when an aqueous sugar solution is allowed to stand. Sugars having a potential free aldehyde or keto group exhibit mutarotation . A freshly prepared solution of  -D-glucose has a specific rotation of +113 o . If the solution of  -D-glucose is allowed to stand, the specific rotation changes to +52.2 o . Similarly a fresh solution of  -D-glucose has a specific rotation of +19 o which changes to +52.2 o on standing. This change in optical rotation is called mutarotation .

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7. Chemical properties of carbohydrates- Reducing property Sugars are classified as either reducing or non-reducing depending upon the presence of potentially free aldehyde or keto groups . The reducing property is mainly due to the ability of these sugars to reduce metal ions such as copper or silver under alkaline conditions . This reducing property is the basis of Fehling’s and Benedict’s tests . In these tests, cupric ion in copper sulphate is reduced to red cuprous oxide.

8. Chemical properties of carbohydrates Carbohydrate reacts with acids: Molisch’s test is a general test for all carbohydrates. In this test, carbohydrates when reacted with conc. H2SO4 get dehydrated to form furfural and its derivatives.

9. Chemical properties of carbohydrates - Oxidation When aldoses are oxidized under proper conditions with different types of oxidizing agents, three types of acids are produced, namely aldonic acids, uronic acids and aldaric or saccharic acids. Aldonic acids: Oxidation of an aldose with bromine water at neutral pH converts the aldehyde group to a carboxyl group. Ketoses are not readily oxidizd by bromine water. Uronic acids: When aldoses are oxidized with enzymes ,uronic acids are formed. In this reaction only the primary alcohol group is oxidized to carboxyl group, whereas the aldehyde group remains unchanged. Uronic acids are constituents of pectic polysaccharides.

10. Chemical properties of carbohydrates - Oxidation Aldaric acid: When aldoses are oxidized with nitric acid , aldaric acids are formed. Both aldehyde and primary alcohol groups are oxidized to carboxyl groups. Glucose on oxidation with nitric acid produces glucaric acid or glucosaccharic acid. The aldaric acid of galactose is called as mucic acid. Ketoses are split by the action of nitric acid to form a mixture of acids having fewer carbon atoms. Fructose on oxidation gives a mixture of glycolic acid and tartaric acid.

11. Chemical properties of carbohydrates – Reaction with alkali With dilute alkali and on standing, glucose undergoes Lobry de Bruyn –van Ekenstein transformation. Glucose is partly converted to fructose and mannose and hence, all the three sugars will exist at equilibrium. The same transformation occurs with fructose or mannose. This is due to formation of an enediol .

12. Review questions True or False: Cellulose is a homopolysaccharide . - True D-glucose is always dextrorotatory. - True The principal storage polysaccharide in plant cell is glycogen. - False The major polysaccharide present in cell wall is starch. - True Dihydroxyacetone is optically inactive. - True The repeating unit in chitin is N- acetylglucosamine . - True

13. Review questions A1. Isomerism is due to ------ carbon atom a)Symmetric b) Asymmetric c) functional d)nonfunctional A2. Glucose reacts with concentrated sulphuric acid and gives furfural b) Hydroxymethyl furfural c) Gluconic acid d) Glucouronic acid B1.Define enantiomer ? Two forms (D(+)Glucose & L(-)Glucose) are non- superimposable mirror images of each other and are called enantiomers . C1. Explain mutarotation of sugars with example. Mutarotation refers to the change in optical rotation when an aqueous sugar solution is allowed to stand. A freshly prepared solution of  -D-glucose has a specific rotation of +113 o . If the solution of  -D-glucose is allowed to stand, the specific rotation changes to +52.2 o . This change in optical rotation is called mutarotation .

14. Review questions C2. Write short notes on reducing property of sugars.(2 mark) The reducing property is mainly due to presence of free aldehyde or keto groups in sugars. Carbohydrate reduce metal ions such as copper or silver under alkaline conditions. Cupric ion in copper sulphate is reduced to red cuprous oxide. D1.Discuss the physical and chemical properties of monosaccharides . (5 mark)