Chapter 12 Carbohydrates

1. Chapter 12Carbohydrates

2. Carbohydrates • Synthesized by plants using sunlight to convert CO2 and H2O to glucose and O2. • Polymers include starch and cellulose. • Starch is storage unit for solar energy. • Most sugars have formula Cn(H2O)n, “hydrate of carbon.” Chapter 13

3. Classification of Carbohydrates • Monosaccharides or simple sugars • polyhydroxyaldehydes or aldoses • polyhydroxyketones or ketoses • Disaccharides can be hydrolyzed to two monosaccharides. • Polysaccharides hydrolyze to many monosaccharide units. E.g., starch and cellulose have > 1000 glucose units. Chapter 12

4. glucose, a D-aldohexose fructose, a D-ketohexose => Monosaccharides • Classified by: • aldose or ketose • number of carbons in chain • configuration of chiral carbon farthest from the carbonyl group Chapter 12

5. D and L Sugars • D sugars can be degraded to the dextrorotatory (+) form of glyceraldehyde. • L sugars can be degraded to the levorotatory (-) form of glyceraldehyde. Chapter 12

6. The D Aldose Family Chapter 23

7. Epimers Sugars that differ only in their stereochemistry at a single carbon. Chapter 12

8. D-glucopyranose Cyclic Structure for Glucose Glucose cyclic hemiacetal formed by reaction of -CHO with -OH on C5. Chapter 12

9. D-fructofuranose Cyclic Structure for Fructose Cyclic hemiacetal formed by reaction of C=O at C2 with -OH at C5. Chapter 12

10. Anomers Chapter 12

11. Glucose also called dextrose; dextrorotatory. Mutarotation Chapter 12

12. Epimerization In base, H on C2 may be removed to form enolate ion. Reprotonation may change the stereochemistry of C2. Chapter 12

13. Reduction of Simple Sugars • C=O of aldoses or ketoses can be reduced to C-OH by NaBH4 or H2/Ni. • Name the sugar alcohol by adding -itol to the root name of the sugar. • Reduction of D-glucose produces D-glucitol, commonly called D-sorbitol. • Reduction of D-fructose produces a mixture of D-glucitol and D-mannitol. Chapter 12

14. Oxidation by Nitric Acid Nitric acid oxidizes the aldehyde and the terminal alcohol; forms aldaric acid. Chapter 12

15. Oxidation by Tollens Reagent • Tollens reagent reacts with aldehyde, but the base promotes enediol rearrangements, so ketoses react too. • Sugars that give a silver mirror with Tollens are called reducing sugars. Chapter 12

16. Nonreducing Sugars • Glycosides are acetals, stable in base, so they do not react with Tollens reagent. • Disaccharides and polysaccharides are also acetals, nonreducing sugars. Chapter 23

17. Formation of Glycosides • React the sugar with alcohol in acid. • Since the open chain sugar is in equilibrium with its - and -hemiacetal, both anomers of the acetal are formed. • Aglycone is the term used for the group bonded to the anomeric carbon. Chapter 23

18. Ether Formation • Sugars are difficult to recrystallize from water because of their high solubility. • Convert all -OH groups to -OR, using a modified Williamson synthesis, after converting sugar to acetal, stable in base. Chapter 23

19. Ester Formation Acetic anhydride with pyridine catalyst converts all the oxygens to acetate esters. Chapter 12

20. Osazone Formation Both C1 and C2 react with phenylhydrazine. Chapter 23

21. Kiliani-Fischer Synthesis • This process lengthens the aldose chain. • A mixture of C2 epimers is formed. Chapter 12

22. Fischer’s Proof • Emil Fischer determined the configuration around each chiral carbon in D-glucose in 1891, using Ruff degradation and oxidation reactions. • He assumed that the -OH is on the right in the Fischer projection for D-glyceraldehyde. • This guess turned out to be correct! Chapter 12

23. Disaccharides • Three naturally occurring glycosidic linkages: • 1-4’ link: The anomeric carbon is bonded to oxygen on C4 of second sugar. • 1-6’ link: The anomeric carbon is bonded to oxygen on C6 of second sugar. • 1-1’ link: The anomeric carbons of the two sugars are bonded through an oxygen. Chapter 12

24. Cellobiose • Two glucose units linked 1-4’. • Disaccharide of cellulose. • A mutarotating, reducing sugar. Chapter 12

25. Maltose Two glucose units linked 1-4’. Chapter 12

26. Lactose • Galactose + glucose linked 1-4’. • “Milk sugar.” Chapter 12

27. Sucrose • Glucose + fructose, linked 1-1’ • Nonreducing sugar Chapter 12

28. Cellulose • Polymer of D-glucose, found in plants. • Mammals lack the -glycosidase enzyme. Chapter 12

29. Amylose • Soluble starch, polymer of D-glucose. • Starch-iodide complex, deep blue. Chapter 12

30. Amylopectin Branched, insoluble fraction of starch. Chapter 12

31. Glycogen • Glucose polymer, similar to amylopectin, but even more highly branched. • Energy storage in muscle tissue and liver. • The many branched ends provide a quick means of putting glucose into the blood. Chapter 12

32. REVIEW

33. In the Fischer projection of D-(+)-glyceraldehyde, the hydroxyl group on the asymmetric carbon center is: A) on the bottom B) on the top C) at the left D) at the right E) present as a hemiacetal

34. In the Fischer projection of D-(+)-glyceraldehyde, the hydroxyl group on the asymmetric carbon center is: A) on the bottom B) on the top C) at the left D) at the right E) present as a hemiacetal

35. All naturally occurring sugars can be degraded to: A) D-(-)-glyceraldehyde B) D-(+)-glyceraldehyde C) L-(-)-glyceraldehyde D) L-(+)-glyceraldehyde E) none of the above

36. All naturally occurring sugars can be degraded to: A) D-(-)-glyceraldehyde B) D-(+)-glyceraldehyde C) L-(-)-glyceraldehyde D) L-(+)-glyceraldehyde E) none of the above

37. All chiral D-sugars rotate plane-polarized light: A) clockwise. B) counterclockwise. C) +20.0°. D) in a direction that cannot be predicted but must be determined experimentally. E) since they are optically inactive.

38. All chiral D-sugars rotate plane-polarized light: A) clockwise. B) counterclockwise. C) +20.0°. D) in a direction that cannot be predicted but must be determined experimentally. E) since they are optically inactive.

39. The structure of D-arabinose is shown below. Which of the following correctly describes the configurations of the asymmetric carbons in D-arabinose? A) 2S, 3R, 4R B) 2R, 3S, 4S C) 2S, 3R, 4S D) 2R, 3S, 4R E) 2S, 3S, 4R

40. The structure of D-arabinose is shown below. Which of the following correctly describes the configurations of the asymmetric carbons in D-arabinose? A) 2S, 3R, 4R B) 2R, 3S, 4S C) 2S, 3R, 4S D) 2R, 3S, 4R E) 2S, 3S, 4R

41. Stereoisomeric aldohexoses that differ in configuration at only a single carbon are: A) meso compounds. B) threo sugars. C) enantiomers. D) constitutional isomers. E) epimers.

42. Stereoisomeric aldohexoses that differ in configuration at only a single carbon are: A) meso compounds. B) threo sugars. C) enantiomers. D) constitutional isomers. E) epimers.

43. Draw the Haworth structure of β-D-glucopyranose.

44. Draw the Haworth structure of β-D-glucopyranose. Answer:

45. Anomers of D-glucopyranose differ in their stereochemistry at: A) C5 B) C4 C) C3 D) C2 E) C1

46. Anomers of D-glucopyranose differ in their stereochemistry at: A) C5 B) C4 C) C3 D) C2 E) C1

47. The α and β anomers of a D-glucopyranose are related as: A) enantiomers. B) meso compounds. C) structural isomers. D) epimers. E) disaccharides.

48. The α and β anomers of a D-glucopyranose are related as: A) enantiomers. B) meso compounds. C) structural isomers. D) epimers. E) disaccharides.

49. Name . a. a-D-Fructopyranose b. b-D-Fructopyranose c. a-D-Fructofuranose d. b-D-Fructofuranose

50. a. a-D-Fructopyranose b. b-D-Fructopyranose c. a-D-Fructofuranose d. b-D-Fructofuranose The cyclic structure comes from D-fructose and has a five-membered ring (furan). The OH on the far-right carbon is pointed down (a).