Chapter 13 carbohydrates
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Transcript of Chapter 13 carbohydrates
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Carbohydrates
ClassificationMonosaccharides
Chiral Carbon AtomsStructures of Important
MonosaccharidesCyclic Structures
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Carbohydrates
• Major source of energy from our diet
• Composed of the elements C, H and
O
• Produced by photosynthesis in
plants
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Types of Carbohydrates
• Monosacchrides
• Disaccharides
Contain 2 monosacchride units
• Polysacchrides
Contain many monosacchride units
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Monosacchrides
• Three Carbons = Triose
• Four Carbons = Tetrose
• Five Carbons = Pentose
• Six Carbons = Hexose
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Monosacchrides
• Aldoses are monosacchrides with an aldehyde group and many hydroxyl (-OH) groups.
• Ketoses are monosacchrides with a ketone group and many hydroxyl (-OH) groups.
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Learning Check C1
Identify each as tetrose, pentose or hexose, and as aldose or ketose
A B
C
C
CH2OH
H OHC
OHH C
OHH
H OH
C OH
C
C
CH2OH
OHH C
HHO
O
CH2OH
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Solution C1
A B
aldose, hexose ketose, pentose
C
C
CH2OH
H OHC
OHH C
OHH
H OH
C OH
C
C
CH2OH
OHH C
HHO
O
CH2OH
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Chiral Objects
• Chiral compounds have the same number of atoms arranged differently in space.
• A chiral carbon atom has four different groups attached
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Mirror Images• The three-dimensional structure of a chiral
compound has a mirror image.
• Your hands are chiral. Try to superimpose your thumbs, palms, back of hands, and little fingers. Is it possible? Why or why not?
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Learning Check C2
Determine if there is a chiral carbon in each compound.
A B
C
Cl
CH3
CH2CH3
H C
Cl
CH3
H
H
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Solution C2
A Yes, 4 different B No, the
groups are attached 2 H atoms
to the second C atom are identical
C
Cl
CH3
CH2CH3
H C
Cl
CH3
H
H
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D and L Notation
• D,L tells which of the two chiral isomers we are referring to.
• If the –OH group on the next to the bottom carbon atom points to the right , the isomer is a D-isomer; if it points left, the isomer is L.
• The D form is usually the isomer found in nature.
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D notation
C
C
CH2OH
OHH
OHH
CO
H
Right = D
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Glucose
C
C
CH2OH
H OHC
OHH C
HHO
H OH
C OH
D-Glucose
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Fructose
C
C
CH2OH
H OHC
OHH C
HHO
O
CH2OH
D-Fructose
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Galactose
D-galactose
C
C
C
C
C
CH2OH
OH
OHH
HO H
HHO
OHH
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Cyclic Structures
• Monosaccharides with 5-6 carbon atoms form cyclic structures
• The hydroxyl group on C-5 reacts with the aldehyde group or ketone group
o O
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Haworth Structure for D-Isomers
The cyclic structure of a D-isomer has the final CH2OH group located above the ring.
oCH2OH
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Haworth Structure for D-Glucose
• Write –OH groups on the right (C2, C4) up
• Write –OH groups on the left (C3) down
• The new –OH on C1 has two possibilites: down for anomer, up for anomer
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Haworth Structure for D-Glucose
oCH2OH
OHOH
OH
OH
oCH2OH
OHOH
OH
OH
-D-Glucose -D-Glucose
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Mutarotation
• Mutarotation: A small amount of open chain is in equilibrium with the cyclic forms.
• The most stable form of glucose is β-D-glucose .
-D-glucose D-glucose (open) β-D-glucose
(36%) (trace) (64%)
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Learning Check C3
Write the cyclic form of -D-galactose
C
C
C
C
C
CH2OH
OH
OHH
HO H
HHO
OHH
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Solution C3
o
OH
OH
CH2OH
OH
OH
-D-galactose