Carbohydrates - Monosaccharides and its qualitative tests - Part 1

Discusses about monosaccharides definition, classification, structure and reactions of glucose, galactose, and fructose. Qualitative tests for carbohydrates with reaction scheme. Terminologies in carbohydrates such as epimeris, anomers and mutarotation.

1. Dr. S.M.Y. Mohamed mukthar Ali Department of chemistry Sadakathullah appa college, tirunelveli Emai l id: [email protected] CARBOHYDRATES PART 1

2. Defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis. General formula C m (H 2 O) n . Polyhydrates of carbons. Functions: Most abundant dietary source of energy (4 Cal/g). Precursors for many organic compounds i.e. fats, amino aicds . Participate in the structure of cell membrane and cellular functions. They are the structural compounds as cellulose of plants, exoskeleton of some insects and cell wall of microorganisms. Serve as a storage of energy (as glycogen) for immediate energy demand of the body. Carbohydrates definition

3. Carbohydrates are referred as saccharides. They are classified based on the number of sugar units. Mainly classified into monosaccharides , oligosaccharides and polysaccharides. Carbohydrates C lassification

4. Monosaccharides : They are the simplest group of sugars or carbohydrates. They cannot be further hydrolysed. General formula C n (H 2 O) n . They are further classified based on functional groups and number of carbon atoms present. Based on functional group classification: Aldoses – contain aldehyde function groups e.g. glyceraldehyde, glucose. Ketoses – contain keto function groups e.g. dihydroxy acetone, fructose. Based on number of carbon atoms: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C) e.g. glucose is aldohexose while fructose is ketohexose. Carbohydrates C lassification

5. Oligosaccharides: They contain 2 to 10 units of monosaccharide molecules. Oligosaccharides on hydrolysis produces monosaccharides . Oligosaccharides can be further subdivided into disaccharides, trisaccharides etc. based on the number of monosaccharides . Polysaccharides : They contain many number of monosaccharide unit of molecules . They have very high molecular weight ( upto million). On hydrolysis, produces monosaccharides and oligosaccharides. Polysaccharides are of two types – homopolysaccharides and heteropolysaccharides . Carbohydrates C lassification

6. Molecular formula of glucose is C 6 H 12 O 6 . It is an aldohexose i.e. 6C system with aldehyde functional group. It is a reducing sugar. It has four secondary hydroxyl groups and one primary hydroxyl group. It has two enantiomers i.e. L-Glucose and D-Glucose. Enantiomers are mirror image compounds of a molecule. Glucose can exist in cyclic and acyclic forms. Structure of Monosaccharide - Glucose Reference compounds

7. In cyclic form, it can exist in two different forms namely -D- Glucopyranose and - D- Glucofuranose . Pyranose is 6 membered ring structure while furanose is 5 membered ring structure. The name pyranose is derived from pyran , while furanose is derived from furan. The structure is depicted using Haworth projection formulae. Of these two cyclic forms, pyranose based structure is most stable. Structure of Monosaccharide - Glucose

8. There are two different cyclic forms of Glucose. The  and  cyclic forms of D-glucose are known as anomers . They differ from each other configuration only around C 1 is known as anomeric carbon. In - anomer , the –OH group on the anomeric carbon is placed on the opposite side of the –CH 2 OH group. In - anomer , the –OH group on the anomeric carbon is placed on the same side of the –CH 2 OH group . Anomers of Glucose -D- Glucopyranose -D- Glucopyranose

9. Tautomerization or Enolization : The process of shifting a hydrogen from one carbon atom to another to produce enediols is known as tautomerization . Anomeric carbon of glucose undergo tautomerization in alkaline solutions. This reaction is known as Lobry de Bruyn -von Ekenstein transformation. Reactions of Monosaccharide - Glucose

10. Reducing property of glucose: Sugars are classified as reducing or non-reducing sugars . Glucose is a reducing sugar. Reducing property depends on the free aldehyde or free keto group of anomeric carbon. Reducing sugars can be differentiated using Benedict’s test, Fehling’s test and Barfoed test. CuSO 4 is reduced to Cu 2 O (red ppt ) by the reducing sugar. Reactions of Monosaccharide - Glucose

11. Oxidation: Depends on the types of oxidizing agent either aldehyde group or terminal CH 2 OH group or both can be oxidized. Aldehyde group oxidation lead to the formation of gluconic acid. Terminal CH 2 OH oxidation lead to the formation of glucuronic acid . Reactions of Monosaccharide - Glucose

12. Reduction: In the presence of Na-amalgam, D-glucose is reduced to D-sorbitol. During the process, aldehyde group is reduced to alcohol. Reactions of Monosaccharide - Glucose

13. Dehydration: On treatment with conc. H 2 SO 4, glucose undergo dehydration with an elimination of 3 water molecules to produce hydroxymethyl furfural. This hydroxymethyl furfural on reaction with - napthol produces coloured compounds in Molisch test. Reactions of Monosaccharide - Glucose

14. Osazone formation: Phenylhydrazine in acetic acid on reaction with glucose produces glucosazone . Reactions of Monosaccharide - Glucose

15. Ester formation: Alcoholic groups of glucose can be esterified by enzymatic or non-enzymatic reactions. Esterification of glucose with phosphoric acid is a common reaction in metabolism. Reactions of Monosaccharide - Glucose

16. Molecular formula of galactcose is C 6 H 12 O 6 . Found in milk as disaccharide, lactose. It is an aldohexose i.e. 6C system with aldehyde functional group. It is a reducing sugar. It has four secondary hydroxyl groups and one primary hydroxyl group. It has two enantiomers i.e. L- Galactose and D- Galactcose . Enantiomers are mirror image compounds of a molecule. Glucose and galactose are epimers . Galactose can exist in cyclic and acyclic forms also. Structure of Monosaccharide - Galactose Epimers ?

17. Structure of Monosaccharide - Galactose In cyclic form, it can exist in two different forms namely -D- Galactopyranose and -D- Galactofuranose . Pyranose is 6 membered ring structure while furanose is 5 membered ring structure. The structure is depicted using Haworth projection formulae. Of these two cyclic forms, pyranose based structure is most stable.

18. Find the differences D-Glucose D- Galactose

19. Tautomerization : Galactose undergoes Keto-enediol tautomerization . Reactions of Monosaccharide - Galactose

20. Reactions of Monosaccharide - Galactose Oxidation: Reduction :

21. Dehydration Reactions of Monosaccharide - Galactose Osazone formation:

22. Molecular formula of frutcose is C 6 H 12 O 6 . Found in honey and fruit juices. It is an ketoohexose i.e. 6C system with keto functional group. It is a reducing sugar. It has three secondary hydroxyl groups and two primary hydroxyl group. It has two enantiomers i.e. L-fructose and D- fructcose . Fructose can exist in cyclic and acyclic forms also. Structure of Monosaccharide - Fructose

23. Structure of Monosaccharide - Fructose In cyclic form, it can exist in five membered ring structure such as -D- fructofuranose and -D- fructofuranose . The structure is depicted using Haworth projection formulae.

24. Tautomerization : Fructose undergoes Keto-enediol tautomerism . Reactions of Monosaccharide - Fructose

25. Reactions of Monosaccharide - Fructose Oxidation Reduction

26. Reactions of Monosaccharide - Fructose Fructose undergoes dehydration to form hydroxymethyl furfural similar to glucose and galactose . It also undergoes ester formation and fructosazone formation

27. Mutarotation - Glucose Mutarotation is defined as the change in the specific optical rotation representing the interconversion of  and  forms of the sugars to an equilibrium mixture The  and  form of fresh glucose show different optical rotations. Specific optical rotation of freshly prepared -D-glucose in water is +112.2° while the - D-glucose +18.7°. Due to mutarotation , the value gradually reaches an equilibrium value of +52.7°. Mutarotation occurs faster in alkali solution. In D-Glucose, Equilibrium mixture contains, 63% of  form, 36% of  form and 1% of open form.

28. Epimerisation If two monosaccharides differ from each other in their configuration around a single specific carbon (other than anomeric carbon) is referred to as epimers to each other. The interconversion of epimers is known as epimerisation. Epimerase enzyme catalyses the epimerisation in biological systems.

29. Relationship between the D-Aldoses

30. C onversion of D-Arabinose to D-Glucose During the conversion of D-Arabinose to D-Glucose, one carbon chain length is increased. The conversion is known as Kiliani -Fischer synthesis. It involves three steps namely, 1. Nucleophilic attack by CN - ion, 2. Hydrogenation and 3. Hydrolysis.

31. C onversion of D-Glucose to D-Arabinose During the conversion of D-Glucose to D-Arabinose , one carbon chain length is decreased. The conversion is known as Ruff Degradation. It involves two steps namely, 1. Oxidation with bromine water, 2. Loss of CO 2 .

32. Interconversion of D-Glucose and D-Fructose Interconversion of D-Glucose and D-Fructose involves enediol formation followed by rearrangement. Both the steps are reversible. The steps involve proton abstraction and rearrangement.

33. Qualitative test for monosaccharides https://www.biologydiscussion.com/carbohydrates/test/qualitative-and-quantitative-tests-for-carbohydrates/13042

34. Molisch Test: To the aqueous solution of carbohydrates, conc. H 2 SO 4 is added to get the dehydrated 5-hydroxymethyl furfural. Alcoholic alpha - naphthol with 5-hydroxymethyl furfural forms the furfural derivatives. This compound forms a reddish-violet coloured ring at the junction of the two liquids. Molisch’s reagent is 5% solution of alpha naphthol in alcohol . This confirms the presence of carbohydrates. Qualitative test for monosaccharides

35. Seliwanoff’s Test: It distinguishes aldoses and ketoses. Seliwanoff reagent is 0.5% resorcinol in con. HCl . To the sugar solution, 2 ml of Seliwanoff reagent is added and the mixture is heated . Strong colour change indicates ketoses while slower colour change indicates aldoses. Qualitative test for monosaccharides

36. Free aldehydic or free ketonic groups can be used for reducing the various metallic ions. Fehling’s Test: It contains two freshly prepared stock solutions A and B. Solution A – 6.93 g of CuSO 4 .5H 2 O in 100 ml. Solution B – 20 g of KOH and 34.6 g of sodium potassium tartarate in 100 ml. To the sugar solution, 5 ml of Fehling’s solution are added and the mixture is heated. Appearance of yellow or red precipitate confirms the fructose, glucose, galactose . Qualitative tests for monosaccharides – reduction Tests

37. Benedict’s Test: It is a modified Fehling’s test. Benedict solution is prepared by dissolving 173 g of sodium citrate and 100 g of sodium carbonate in 100 ml. To this solution, CuSO 4 ( 17.3 g in 100 ml) solution is added. To the sugar solution, Benedict solution is added and heated in water bath. Appearance of red precipitate confirms the fructose, glucose, galactose . Qualitative tests for monosaccharides – reduction Tests

38. Barfoed’s Test: It is also copper reduction test but in the presence of acid medium. Barfoed reagent is CuSO 4 .5H 2 O in acetic acid medium. To the sugar solution, Barfoed solution is added and heated in water bath. Appearance of red precipitate confirms the fructose, glucose, galactose . Qualitative tests for monosaccharides – reduction Tests

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