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Case Western Reserve University

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1 Case Western Reserve University
Chapter 22 Carbohydrates Organic Chemistry 4th Edition Paula Yurkanis Bruice Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall

2 Carbohydrates They have the molecular formulas Cn(H2O)n
Compounds that can be hydrolyzed to polyhydroxy aldehydes or polyhydroxy ketones are also classified as carbohydrates

3 Simple Carbohydrates Are Monosacharides
Complex carbohydrates contain two or more sugar units linked together disaccharides oligosaccharides polysaccharides Polyhydroxy aldehydes are aldoses Polyhydroxy ketones are ketoses

4 D and L notations are used to describe the configurations
of carbohydrates


6 Configurations of Aldoses
Aldotetroses have two asymmetric carbons and four stereoisomers

7 Diastereomers that differ in configuration at only one
asymmetric carbon are called epimers

8 A ketose has one less asysmmetric carbon than aldoses, therefore they have fewer stereoisomers

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


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

12 In a basic solution, ketoses are converted into aldoses

13 A strong oxidizing agent such as HNO3 can oxidize the
aldehyde and the alcohol groups

14 Osazone Formation Aldoses and ketoses react with three equivalents of

15 The C-2 epimers of aldoses form identical osazones

16 Reaction of Ketoses with Phenylhydrazine

17 The carbon chain of an aldose can be increased by one
carbon in a Kiliani–Fischer synthesis

18 The Ruff degradation shortens an aldose chain by one

19 Preparation of the Calcium D-Gluconate for the Ruff Degradation

20 Cyclic Structure of Monosaccharides
Hemiacetal Formation anomer anomer The specific rotation of pure a-D-glucose or b-D-glucose changes over time to reach an equilibrium (mutarotation)

21 Note … If an aldose can form a five- or six-membered ring, it will exist predominantly as a cyclic hemiacetal Six-membered rings are called pyranoses Five-membered rings are called furanoses A sugar with an aldehyde, a ketone, a hemiacetal, or a hemiacetal group is a reducing sugar

22 The structures of cyclic sugars are best represented by
the Haworth projections Haworth projections allow us to see the relative orientation of the OH groups in the ring

23 Ketoses also exist predominantly in cyclic forms

24 b-D-Glucose Is More Stable
b-D-glucose is the predominant form at equilibrium

25 Acylation of Monosaccharides
The OH groups of monosaccharides show the chemistry of typical alcohols

26 Alkylation of the OH Groups

27 Formation of Glycosides
The acetal (or ketal) of a sugar is called a glycoside

28 Mechanism of Glycoside Formation

29 Formation of an N-Glycoside

30 The Anomeric Effect The formation of a glycoside favors the a-glucoside product: the anomeric effect

31 Determination of Ring Size
Approach 1 The size of the ring can be determined from the structure of the open-chain form

32 Determination of Ring Size
Approach 2 An acetal of the monosaccharide is oxidized with excess HIO4 The a-hydroxyaldehyde formed from HIO4 oxidation is further oxidized to formic acid and another aldehyde

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 Lactose is a reducing sugar

36 The most common disaccharide is sucrose
Sucrose is not a reducing sugar

37 Polysaccharides Amylose is a component of starch

38 Amylopectin is another polysaccharide component of
starch that has a branched structure


40 An example of a naturally occurring product derived from

41 Blood type is determined by the nature of the sugar
bound to the protein on the surface of red blood cells

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