Organic Chemistry 6 th Edition Paula Yurkanis Bruice

1. Organic Chemistry 6th Edition Paula Yurkanis Bruice Chapter 22 The Organic Chemistry of Carbohydrates Introduction, Basic structural features and types of carbohydrates, Reactions and conversions, role in biological systems.

2. Carbohydrates • General molecular formula is Cn(H2O)n….’hydrates of carbon’ • Structurally, carbohydrades are polyhydroxy aldehydes or ketones. • The most abundant carbohydrate in nature is glucose. The carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis Exam. Sucrose: used in our home Lactose: present in milk

3. Carbohydrates are also called saccharides (Greek: sakcharon means sugar) Simple carbohydrates are monosaccharides: Cannot be hydrohyzed further; About 20 monosaccharides are known in nature…glucose, fructose, ribose etc Complex carbohydrates contain two or more sugar units linked together: • Disaccharides • Oligosaccharides • Polysaccharides Sucrose on hydrolysis gives glucose and fructose but maltose gives only two molecules of glucose Polysaccharides are not sweet and are called non-sugar

4. Polyhydroxy aldehydes are aldoses Polyhydroxy ketones are ketoses Position of carbonyl group at C1, carbonyl is an aldehyde: aldose at any other carbon, carbonyl is a ketone: ketose Number of carbons three carbons: triose six carbons: hexose four carbons: tetrose seven carbons: heptose five carbons: pentose etc.

5. Fischer Projections and the D, L Notation. Representation of a three-dimensional molecule as a flat structure (Ch. 7.7). Tetrahedral carbon represented by two crossed lines: horizontal line is coming out of the plane of the page (toward you) vertical line is going back behind the plane of the paper (away from you) substituent carbon (R)-(+)-glyceraldehyde (S)-(-)-glyceraldehyde

6. before the R/S convention, stereochemistry was related to (+)-glyceraldehyde D-glyceraldehyde L-glyceraldehyde R-(+)-glyceraldhyde S-(-)-glyceraldhyde (+)-rotation = dextrorotatory = d (-)-rotation = levorotatory = l D-carbohydrates have the -OH group of the highest numbered chiral carbon pointing to the right in the Fischer projection as in R-(+)-glyceraldhyde For carbohydrates, the convention is to arrange the Fischer projection with the carbonyl group at the top for aldoses and closest to the top for ketoses. The carbons are numbered from top to bottom.

7. Carbohydrates are designated as D- or L- according to the stereochemistry of the highest numbered chiral carbon of the Fischer projection. If the hydroxyl group of the highest numbered chiral carbon is pointing to the right, the sugar is designated as D (Dextro: Latin for on the right side). If the hydroxyl group is pointing to the left, the sugar is designated as L (Levo: Latin for on the left side). Most naturally occurring carbohydrates are of the D-configuration.

8. The Aldotetroses.Glyceraldehyde is the simplest carbohydrate (C3, aldotriose, 2,3-dihydroxypropanal). The next carbohydrate are aldotetroses (C4, 2,3,4-trihydroxybutanal). Aldose – polyhydroxyaldehyde, eg glucose Ketose – polyhydroxyketone, eg fructose Triose, tetrose, pentose, hexose, etc. – carbohydrates that contain three, four, five, six, etc. carbons per molecule (usually five or six); eg. Aldohexose, ketopentose, etc.

9. Aldopentoses and Aldohexoses. Aldopentoses: C5, three chiral carbons, eight stereoisomers Aldohexoses: C6, four chiral carbons, sixteen stereoisomers

10. A ketose has one fewer asymmetric center than an aldose. • Therefore, ketoses have fewer stereoisomers than aldoses. • Naturally occurring ketoses have the ketone group in the 2-position.

11. Cyclization of carbohydrates to the hemiacetal creates a new chiral center. The hemiacetal or hemiketal carbon of the cyclic form of carbohydrates is the anomeric carbon. Carbohydrate isomers that differ only in the stereochemistry of the anomeric carbon are calledanomers. An aldehyde can react with an alcohol to form a hemiacetal. A ketone can react with an alcohol to form a hemiketal.

12. D-Glucose exists in three different forms: anomer The specific rotation of pure a-D-glucose or b-D-glucose changes over time to reach an equilibrium (mutarotation) anomer

13. If an aldose can form a five- or six-membered ring, it will exist predominantly as a cyclic hemiacetal:

14. Six-membered rings are called pyranoses. • Five-membered rings are called furanoses. • The prefix a- indicates the configuration about the anomeric carbon.

15. Ketoses also exist predominantly in cyclic forms:

16. b-D-Glucose is the most stable of all aldohexoses and predominates at equilibrium:

17. Mechanism for the base-catalyzed enediol rearrangement of a monosaccharide:

18. Redox Reactions of Monosaccharides The carbonyl of aldoses and ketoses can be reduced by the carbonyl-group reducing agents:

20. Oxidation The aldehyde groups can be oxidized by Br2 Ketones and alcohols cannot be oxidized by Br2

21. Both aldoses and ketoses are oxidized to aldonic acids by Tollens reagent:

22. A strong oxidizing agent such as HNO3 can oxidize both the aldehyde and the alcohol groups: A primary alcohol is the one most easily oxidized

23. Osazone Formation Aldoses and ketoses react with three equivalents of phenylhydrazine to form osazones:

24. The C-2 epimers of aldoses form identical osazones… …because the configuration of the C-2 carbon is lost during osazone formation

25. The C-1 and C-2 carbons of ketoses also react with phenylhydrazine:

26. The carbon chain of an aldose can be increased by one carbon in a Kiliani–Fischer synthesis: This reaction leads to a pair of C-2 epimers

27. The Wohl degradation shortens an aldose chain by one carbon:

28. Formation of Glycosides The acetal (or ketal) of a sugar is called a glycoside:

29. Mechanism for glycoside formation

30. Formation of an N-Glycoside

31. The Anomeric Effect The preference of certain substituents bonded to the anomeric carbon for the axial position is called the anomeric effect:

32. Reducing and Nonreducing Sugars A sugar with an aldehyde, a ketone, a hemiacetal, or a hemiketal group is a reducing sugar A sugar without one of these groups is a nonreducing sugar Nonreducing sugar cannot reduce Ag+ or Br2

33. Disaccharides Composed of two monosaccharide subunits hooked together by an acetal linkage: In a-maltose, the OH group bonded to the anomeric carbon is axial Maltose is a reducing sugar

34. In cellobiose, the two subunits are hooked together by a b-1,4-glycosidic linkage Cellobiose is a reducing sugar

35. In lactose, the two different subunits are joined by a b-1,4-glycosidic linkage: The -1,4 glycosidic linkage is cleaved by the enzyme lactase Without lactase, lactose is turned into gases by flora in the lower bowel, producing flatulence and bloating Lactose is a reducing sugar and undergoes mutarotation

36. The most common disaccharide is sucrose Sucrose is not a reducing sugar and does not exhibit mutarotation

37. Galactose Glucose Fructose Cleaved by -galactosidase (-GAL) Sucrose How Does Beano Work? Raffinose trisaccharide is found in beans and a variety of grains and vegetables • -GAL is not in the human digestive tract. • Raffinose is not cleaved to sucrose and galactose. • Lower gut flora converts raffinose to gases; flatulence results. Beano contains fungal -GAL that cleaves raffinose and other polysaccharides

38. Polysaccharides Amylose is a component of starch:

39. Amylopectin is another polysaccharide component of starch that has a branched structure:

40. Cellulose is the structural material of higher plants: a-1,4-Glycosidic linkages are easier to hydrolyze than b-1,4-glycosidic linkages because of the anomeric effect

41. An example of a naturally occurring product derived from carbohydrates:

42. D-Ribose is the sugar component of ribonucleic acid D-2-Deoxyribose is the sugar component of deoxyribonucleic acid

43. Proteins bonded to oligosaccharides are called glycoproteins: