Transcript Chapter 1 Chemical Bonding and Chemical Structure

Chapter 24
Carbohydrates
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.
Glyceraldehyde
Erythrulose
Sedoheptulose
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Fischer Projections
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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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2) A Fischer projection may not be turned 90° in the plane of
the page
3) A Fischer projetion may not be lifted from the plane of the
paper and turned over.
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4) 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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5) An interchange of any two of the groups bound to an
asymmetric carbon changes the configuration of that carbon
6) 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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3) Form the hemiacetals
• 2 versions, α and β
• 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 148–155°C 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 37:63 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
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