Chapter 24 Carbohydrates

1. Chapter 24Carbohydrates

2. Carbohydrates • Sugars and their derivatives are classified as carbohydrates • Examples: Glucose, Sucrose, Starch, Glycogen • Molecular formulas fit a hydrate of carbon pattern: Cn(H2O)m • Sucrose: C6H12O6 = C6(H2O)6 24.1 Properties and Classification of Carbohydrates

4. Monosaccharides • Simplest carbohydrates • Do not break down into other carbohydrates • Examples: glucose (dextrose), fructose, galactose, xylose, ribose • Usually colorless and water soluble • Cyclic and open chain versions

5. Classification of Monosaccharides • Classification by functional group • Either aldehydes or ketones • If ketone = ketose • If aldehyde = aldose

6. Classification of Monosaccharides • Classification by carbon chain length • Chains contain 3-8 carbons • Triose = 3 carbon sugar • Tetrose = 4 carbon sugar • Pentose = 5 carbon sugar • Hexose = 6 carbon sugar • Etc.

7. Classifying Monosaccharides • Functional group and chain length classifications can be combined • Examples: • Aldehyde + 5 carbons = aldopentose • Ketone + 6 carbons = ketohexose

8. Problems • Classify the following monosaccharides by both the number of carbons and functional group each contains. Glyceraldehyde Erythrulose Sedoheptulose

9. Fischer Projections • Convenient 2D representation of 3D carbohydrate molecules • Carbon chain written vertically • Most oxidized carbon toward top • All bonds depicted horizontally and vertically • Carbons are represented by crossing lines

10. Vertical bonds go back • Horizontal bonds come forward

11. Manipulating Fischer Projections • A Fischer projection may be turned 180° in the plane of the paper 24.2 Fischer Projections

12. A Fischer projection may not be turned 90° in the plane of the page • A Fischer projetion may not be lifted from the plane of the paper and turned over.

13. A Fischer projection can be held steady while the groups at either end rotate in either a clockwise or a counterclockwise direction

14. An interchange of any two of the groups bound to an asymmetric carbon changes the configuration of that carbon • Meso compounds are a possibility • Will have a line of symmetry 24.2 Fischer Projections

15. Problems • Assign R or S stereochemistry to each chiral carbon 24.2 Fischer Projections

16. Fischer Projections – More Complex • Based on an eclipsed molecular conformation

17. Problem • Assign R or S stereochemistry to each chiral carbon in the following monosaccharide:

18. The D,L System • D-Glyceraldehyde rotates the plane of polarized light in a clockwise direction – Dextrarotatory (+ or D) • L-Glyceraldehyde rotates the plane of polarized light in a counterclockwise direction – Levorotatory (- or L)

19. Almost all naturally occurring monosaccharides have the same R stereochemical configuration as D-glyceraldehyde at chiral center furthest from carbonyl group • When furthest chiral center has an OH drawn to the right, the sugar is D, when the chiral center has its OH drawn to the left, the sugar is L

20. D and L notation have no relation to the direction in which a given sugar rotates plane-polarized light except for glyceraldehyde • D and L can be either dextrorotatory or levorotatory

21. Problems • Classify the following sugars as D or L

22. Cyclic Structures of the Monosaccharides • g- and d-hydroxy aldehydes exist predominantly as cyclic hemiacetals • 5 and 6 membered rings are very stable 24.3 Structures of the Monosaccharides

23. Fischer Projections Haworth Structures

24. Drawing Haworth Structures • Flip the sugar to the right 90° • Fold the chain into a hexagon (or pentagon)

25. Form the hemiacetals • 2 versions, α and β • Anomers

26. Problems • Draw the cyclic structures for the following sugars

28. Monosaccharide Anomers: Mutarotation • The two anomers of D-glucopyranose can be crystallized and purified • -D-glucopyranose melts at 146° and its specific rotation, []D = +112.2° • b-D-glucopyranose melts at 148–155°C with a specific rotation of []D =+18.7° • Rotation of solutions of either pure anomer slowly changes due to slow conversion of the pure anomers into a 37:63 equilibrium mixture of :b with a []D =+52.6° • called mutarotation

29. Conformational Representations of Pyranoses • Convert the Haworth form to a chair: 24.3 Structures of the Monosaccharides

30. Oxidation and Reduction of Carbohydrates • The aldehydes of aldoses may be reduced or selectively oxidized without impacting the other alcohols • Selective oxidation of the primary alcohol group may also be realized 24.8 Oxidation and Reduction Reactions of Carbohydrates

31. Common Oxidation and Reduction Products 24.8 Oxidation and Reduction Reactions of Carbohydrates

32. Disaccharides • Disaccharides consist of two monosaccharides 24.11 Disaccharides and Polysaccharides

33. Disaccharides • Note that the glycosidic linkage is an acetal and can be hydrolyzed with aqueous acid 24.11 Disaccharides and Polysaccharides

34. Disaccharides • C-1 of the glucose residue can be oxidized; however, C-1 of the galactose residue cannot • Reducing sugars: Carbohydrates that be oxidized (they reduce the oxidizing agent) 24.11 Disaccharides and Polysaccharides

35. Disaccharides • Another important disaccharide is (+)-sucrose • (+)-Sucrose is a nonreducing sugar as it cannot be oxidized with bromine water • It also cannot undergo mutarotation 24.11 Disaccharides and Polysaccharides

36. Polysaccharides • Sugars with many monosaccharide residues connected by glycosidic linkages are called polysaccharides • Cellulose is polymer of glucose 24.11 Disaccharides and Polysaccharides

37. Polysaccharides • Starch is a glucose polymer • It consists of two different types of glucose polymer 24.11 Disaccharides and Polysaccharides

38. Polysaccharides • Chitin is a polysaccharide that occurs widely in nature (e.g., shells of lobsters and crabs) 24.11 Disaccharides and Polysaccharides