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Organic Chemistry Second Edition Chapter 24 David Klein Carbohydrates

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1 Organic Chemistry Second Edition Chapter 24 David Klein Carbohydrates
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

15 24.4 Configuration of Ketoses
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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

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

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

25 24.5 Cyclic Structures of Monosaccharides
Ketoses form both furanose (5-membered) and pyranose (6-membered) rings Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

26 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

27 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

28 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

29 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

30 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

31 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

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

33 24.6 Reactions of Monosaccharides
Under strongly basic conditions, glucose and mannose interconvert Mannose and glucose are epimers, because they only differ in the configuration of one carbon center Practice with conceptual checkpoint 24.30 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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

35 24.6 Reactions of Monosaccharides
Monosaccharides can be reduced to alditols shifting the equilibrium to the right. HOW? D-sorbitol or D-glucitol are sugar substitutes Practice with checkpoints24.31 through 24.33 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

36 24.6 Reactions of Monosaccharides
A monosaccharide can also be oxidized using a variety of oxidizing agents including Br2 to form a aldonic acid A mild oxidizing agent must be used to avoid oxidation of the alcohol groups Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

37 24.6 Reactions of Monosaccharides
Practice with SkillBuilder 24.4 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

38 24.6 Reactions of Monosaccharides
HCN can react with sugars to produce cyanohydrins In the original Kiliani-Fischer synthesis, the cyanohydrin is subsequently converted to an acid and then an aldehyde extending the carbon chain by one unit Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

39 24.6 Reactions of Monosaccharides
A more recent version of the Kiliani-Fischer synthesis converts the cyanohydrin to the aldehyde in one step Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

40 24.6 Reactions of Monosaccharides
A more recent version of the Kiliani-Fischer synthesis converts the cyanohydrin to the aldehyde in one step Practice with conceptual checkpoints 24.37 and 24.38 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

41 24.6 Reactions of Monosaccharides
Although it is not very high yielding, the Wohl synthesis achieves the reverse of the Kiliani-Fischer synthesis Practice with conceptual checkpoints 24.39 through 24.41 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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

43 24.7 Disaccharides Disaccharides form when two sugars connect through a glycosidic linkage Because the anomeric position of the bottom ring is a HEMIacetal rather than an acetal, it is in equilibrium with the open form. Thus, maltose is a reducing sugar Practice with SkillBuilder 24.5 The 1  4 glycosidic linkage is most common The bottom ring is capable of maturotation at its anomeric position Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

44 24.7 Disaccharides Cellobiose is similar to maltose. WHAT are the differences? Will cellobiose be a reducing sugar? Practice conceptual checkpoint 24.44 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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

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

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

48 24.7 Disaccharides Lactose is another disaccharide
Some people have trouble digesting lactose. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

49 24.7 Disaccharides Sucrose (table sugar) is also a disaccharide
Honey bees can convert sucrose into a mixture of sucrose, fructose, and glucose Fructose is very sweet Sucrose is not a reducing sugar. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

50 24.8 Polysaccharides Cellulose is a polysaccharide containing glucose units connected through glycosidic bonds How is cellulose capable of giving plants like trees their rigidity and strength? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

51 24.8 Polysaccharides Starch is a major components of grains and other foods like potatoes Starch is made of amylose and amylopectin What is the difference between molecules of starch and molecules of cellulose? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

52 24.8 Polysaccharides Amylopectin has some 16-α-glycoside branches
We can eat corn and potatoes, but not grass or trees. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

53 24.9 Amino Sugars Amino sugars like glucosamine are important biomolecules Acetylated glucosamine can form an important polysaccharide called chitin Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

54 24.9 Amino Sugars The carbonyl groups in chitin allow for even stronger H-bonding between neighboring chains Chitin is used in insect and arthropod exoskeletons. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

55 24.10 N-Glycosides N-glycosides can be formed when sugars are treated with an amine and an acid catalyst RNA and DNA incorporate important N-glycosides called nucleosides Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

56 24.10 N-Glycosides Ribose forms ribonucleosides in RNA
Deoxyribose forms deoxyribonucleosides in DNA Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

57 24.10 N-Glycosides There are 4 different heterocyclic amines that attach to deoxyribose molecules to form DNA nucleosides Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

58 24.10 N-Glycosides In DNA, the nucleosides are attached to phosphate groups forming nucleotides Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

59 24.10 N-Glycosides The phosphate groups of the nucleotides are connected together to make the DNA strand or polynucleotide Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

60 24.10 N-Glycosides The nucleotides in DNA can attract one another through H-bonding of the DNA base pairs Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

61 24.10 N-Glycosides WHY does DNA form a double helix?
Consider both H-bonding and pi stacking of the base pairs Consider ion-dipole interactions with the solvent Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

62 24.10 N-Glycosides RNA is structurally different from DNA
The sugar in RNA is ribose. WHAT is the sugar in DNA? RNA contains the uracil instead of thymine RNA translates the information stored in DNA into working molecules (proteins and enzymes) Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

63 24.10 N-Glycosides RNA strands generally do not form the double helix like DNA RNA strands can fold into many different shapes, and some even act as catalysts called ribozymes It is possible that RNA evolved self-replication as an early step in the evolution of life from small molecules Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

64 Additional Practice Problems
How does the potential energy of carbohydrates and O2 compare with the potential energy state of CO2 and H2O, and how do carbohydrates provide an energy source? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

65 Additional Practice Problems
Explain why the equilibrium for ring formation in pentoses and hexoses favors the ring rather than the open form. Is your argument kinetic or thermodynamic? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

66 Additional Practice Problems
Predict the product of a sugar reacting with an amine and give the mechanism for formation. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

67 Additional Practice Problems
Which of the molecules discussed in this chapter could accurately be classified as polymers? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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