1. Organic Chemistry Second Edition Chapter 24 David Klein Carbohydrates
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Klein, Organic Chemistry 2e

2. 24.1 Introduction to Carbohydrates
Carbohydrates (sugars) are abundant in nature
High energy biomolecules
Provide structural rigidity for organisms (plants, crustaceans, etc.)
The polymer backbone on which DNA and RNA are assembled
Expressed on cells so they can recognize one another
The term, carbohydrate, evolved to describe the formula for such molecules: Cx(H2O)x
Carbohydrates are NOT true hydrates. WHY?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

3. 24.1 Introduction to Carbohydrates
Carbohydrates (sugars) are polyhydroxy aldehydes or ketones
Consider glucose, which is made by plants
Describe the potential energy change that occurs during glucose photosynthesis
Is glucose a polyhydroxy aldehyde or ketone?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

4. 24.2 Classification of Monosaccharides
Saccharides have multiple chiral centers, and they are often expressed as Fischer projections
checkpoints 24.1 and 24.2
What does the suffix, “ose” mean?
Define the following terms:
Aldose and ketose
Pentose and hexose
Practice conceptual
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Klein, Organic Chemistry 2e

5. Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

6. 24.2 Classification of Monosaccharides
Glyceraldehyde is a monosaccharide with one chirality center
Natural glyceraldehyde is dextrorotatory (D) – it rotates plane polarized light in the clockwise direction
Does the direction plane polarized light is rotated necessarily tell us whether it is (R) or (S)?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

7. 24.2 Classification of Monosaccharides
Naturally occurring larger sugars can be broken down into glyceraldehyde by degradation
Such sugars are often called D sugars
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Klein, Organic Chemistry 2e

8. 24.2 Classification of Monosaccharides
Recall that dextrorotatory versus levorotatory rotation can not be predicted by the R or S configuration
Practice conceptual
checkpoints 24.3 through 24.6
Here, D no longer refers to dextrorotatory. Rather it refers to the R configuration at the chiral carbon farthest from the carbonyl
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

9. Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

10. 24.3 Configuration of Aldoses
There are four aldotetroses. Two are shown below
What are the other two structures?
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Klein, Organic Chemistry 2e

11. 24.3 Configuration of Aldoses
Aldopentoses have three chirality centers. The number of isomers will be 23
Recall the 2n rule from section 5.5
The D sugars are naturally occurring
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

12. 24.3 Configuration of Aldoses
Ribose is a key building block of RNA
WHAT is RNA? More detail to come in section 24.10
Arabinose is found in plants
Xylose is found in wood
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

13. 24.3 Configuration of Aldoses
Based on the 2n rule, how many aldohexoses are there?
How many of the aldohexoses are D isomers
Glucose is the most common aldohexose
Manose and Galactose are also common
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

14. 24.4 Configuration of Ketoses
Relevant ketoses have between 3 and 6 carbons
For each naturally occurring D isomer, there is an L enantiomer
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Klein, Organic Chemistry 2e

15. 24.4 Configuration of Ketoses
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Klein, Organic Chemistry 2e

16. 24.5 Cyclic Structures of Monosaccharides
Recall from section 20.5 that carbonyls can be attacked by alcohols to form hemiacetals
The intermolecular reaction is not favored. WHY?
The intramolecular reaction is generally favored for 5 and 6-membered rings. WHY?
Practice with SkillBuilder 24.1
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

17. Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

18. 24.5 Cyclic Structures of Monosaccharides
Monosaccharides like glucose can also undergo ring-closing hemiacetal formation
The equilibrium greatly favors the closed form called pyranose
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

19. 24.5 Cyclic Structures of Monosaccharides
What is the relationship between alpha and beta forms?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

20. 24.5 Cyclic Structures of Monosaccharides
Distinguish between the α and β anomers
[a]D = ˚
[a]D = +18.7˚
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

21. 24.5 Cyclic Structures of Monosaccharides
You should be able to draw monosaccharide structures in their various forms
Practice with SkillBuilder 24.3
Draw the chair form
D-Galactose
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Klein, Organic Chemistry 2e

22. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

23. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

24. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

25. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

26. 24.5 Cyclic Structures of Monosaccharides
Ketoses form both furanose (5-membered) and pyranose (6-membered) rings
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Klein, Organic Chemistry 2e

27. 24.5 Cyclic Structures of Monosaccharides
The furanose form takes part in most biochemical reactions
Practice with conceptual checkpoints through 24.25
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

28. 24.6 Reactions of Monosaccharides
Monosaccharides are generally soluble in water. WHY?
To improve their solubility in organic solvents, the hydroxyl groups can be acetylated
WHY is pyridine added to the reaction?
How might acetylation help in purification efforts?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

29. 24.6 Reactions of Monosaccharides
Monosaccharides can also be converted to ethers via the Williamson ether synthesis
Ether linkages are more robust than ester linkages. WHY?
Practice with conceptual checkpoints and 24.27
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

30. 24.6 Reactions of Monosaccharides
When treated with an excess of an alcohol, the hemiacetal equilibrium can be shifted to give an acetal
When a sugar is used, alpha and beta
glycosides are formed
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

31. 24.6 Reactions of Monosaccharides
The mechanism of glycoside formation is analogous to the acetal formation mechanism
Only the anomeric hydroxyl group is replaced
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

32. 24.6 Reactions of Monosaccharides
The mechanism of glycoside formation is analogous to the acetal formation mechanism
Practice with conceptual checkpoints and 24.29
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

33. Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e