Chapter 24 Carbohydrates

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Chapter 24Carbohydrates
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Carbohydrates

• Sugars and their derivatives are classified as
  carbohydrates
• Examples Glucose, Sucrose, Starch, Glycogen
• Molecular formulas fit a hydrate of carbon
  pattern Cn(H2O)m
• Sucrose C6H12O6 C6(H2O)6

24.1 Properties and Classification of
Carbohydrates
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Monosaccharides

• Simplest carbohydrates
• Do not break down into other carbohydrates
• Examples glucose (dextrose), fructose,
  galactose, xylose, ribose
• Usually colorless and water soluble
• Cyclic and open chain versions

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Classification of Monosaccharides

• Classification by functional group
• Either aldehydes or ketones
• If ketone ketose
• If aldehyde aldose

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Classification of Monosaccharides

• Classification by carbon chain length
• Chains contain 3-8 carbons
• Triose 3 carbon sugar
• Tetrose 4 carbon sugar
• Pentose 5 carbon sugar
• Hexose 6 carbon sugar
• Etc.

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Classifying Monosaccharides

• Functional group and chain length classifications
  can be combined
• Examples
• Aldehyde 5 carbons aldopentose
• Ketone 6 carbons ketohexose

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Problems

• Classify the following monosaccharides by both
  the number of carbons and functional group each
  contains.

Erythrulose
Glyceraldehyde
Sedoheptulose
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Fischer Projections

• Convenient 2D representation of 3D carbohydrate
  molecules
• Carbon chain written vertically
• Most oxidized carbon toward top
• All bonds depicted horizontally and vertically
• Carbons are represented by crossing lines

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• Vertical bonds go back
• Horizontal bonds come forward

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Manipulating Fischer Projections

1. A Fischer projection may be turned 180 in the
   plane of the paper

24.2 Fischer Projections
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1. A Fischer projection may not be turned 90 in the
   plane of the page
2. A Fischer projetion may not be lifted from the
   plane of the paper and turned over.

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1. A Fischer projection can be held steady while the
   groups at either end rotate in either a clockwise
   or a counterclockwise direction

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• An interchange of any two of the groups bound to
  an asymmetric carbon changes the configuration of
  that carbon
• Meso compounds are a possibility
• Will have a line of symmetry

24.2 Fischer Projections
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Problems

• Assign R or S stereochemistry to each chiral
  carbon

24.2 Fischer Projections
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Fischer Projections More Complex

• Based on an eclipsed molecular conformation

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Problem

• Assign R or S stereochemistry to each chiral
  carbon in the following monosaccharide

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The D,L System

• D-Glyceraldehyde rotates the plane of polarized
  light in a clockwise direction Dextrarotatory
  ( or D)
• L-Glyceraldehyde rotates the plane of polarized
  light in a counterclockwise direction
  Levorotatory (- or L)

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• Almost all naturally occurring monosaccharides
  have the same R stereochemical configuration as
  D-glyceraldehyde at chiral center furthest from
  carbonyl group
• When furthest chiral center has an OH drawn to
  the right, the sugar is D, when the chiral center
  has its OH drawn to the left, the sugar is L

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• D and L notation have no relation to the
  direction in which a given sugar rotates
  plane-polarized light except for glyceraldehyde
• D and L can be either dextrorotatory or
  levorotatory

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Problems

• Classify the following sugars as D or L

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Cyclic Structures of the Monosaccharides

• g- and d-hydroxy aldehydes exist predominantly as
  cyclic hemiacetals
• 5 and 6 membered rings are very stable

24.3 Structures of the Monosaccharides
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Fischer Projections
Haworth Structures
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Drawing Haworth Structures

1. Flip the sugar to the right 90
2. Fold the chain into a hexagon (or pentagon)

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• Form the hemiacetals
• 2 versions, a and ß
• Anomers

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Problems

• Draw the cyclic structures for the following
  sugars

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Monosaccharide Anomers Mutarotation

• The two anomers of D-glucopyranose can be
  crystallized and purified
• ?-D-glucopyranose melts at 146 and its specific
  rotation, ?D 112.2
• b-D-glucopyranose melts at 148155C with a
  specific rotation of ?D 18.7
• Rotation of solutions of either pure anomer
  slowly changes due to slow conversion of the pure
  anomers into a 3763 equilibrium mixture of ?b
  with a ?D 52.6
• called mutarotation

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Conformational Representations of Pyranoses

• Convert the Haworth form to a chair

24.3 Structures of the Monosaccharides
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Oxidation and Reduction of Carbohydrates

• The aldehydes of aldoses may be reduced or
  selectively oxidized without impacting the other
  alcohols
• Selective oxidation of the primary alcohol group
  may also be realized

24.8 Oxidation and Reduction Reactions of
Carbohydrates
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Common Oxidation and Reduction Products
24.8 Oxidation and Reduction Reactions of
Carbohydrates
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Disaccharides

• Disaccharides consist of two monosaccharides

24.11 Disaccharides and Polysaccharides
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Disaccharides

• Note that the glycosidic linkage is an acetal and
  can be hydrolyzed with aqueous acid

24.11 Disaccharides and Polysaccharides
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Disaccharides

• C-1 of the glucose residue can be oxidized
  however, C-1 of the galactose residue cannot
• Reducing sugars Carbohydrates that be oxidized
  (they reduce the oxidizing agent)

24.11 Disaccharides and Polysaccharides
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Disaccharides

• Another important disaccharide is ()-sucrose
• ()-Sucrose is a nonreducing sugar as it cannot
  be oxidized with bromine water
• It also cannot undergo mutarotation

24.11 Disaccharides and Polysaccharides
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Polysaccharides

• Sugars with many monosaccharide residues
  connected by glycosidic linkages are called
  polysaccharides
• Cellulose is polymer of glucose

24.11 Disaccharides and Polysaccharides
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Polysaccharides

• Starch is a glucose polymer
• It consists of two different types of glucose
  polymer

24.11 Disaccharides and Polysaccharides
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Polysaccharides

• Chitin is a polysaccharide that occurs widely in
  nature (e.g., shells of lobsters and crabs)

24.11 Disaccharides and Polysaccharides