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1
CarbohydratesClassification
MonosaccharidesChiral Carbon Atoms
Structures of Important MonosaccharidesCyclic Structures
2
Carbohydrates
• Major source of energy from our diet
• Composed of the elements C, H and
O
• Produced by photosynthesis in
plants
3
Types of Carbohydrates• Monosacchrides
• DisaccharidesContain 2 monosacchride units
• PolysacchridesContain many monosacchride units
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Monosacchrides
• Three Carbons = Triose
• Four Carbons = Tetrose
• Five Carbons = Pentose
• Six Carbons = Hexose
5
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 C1Identify each as tetrose, pentose or hexose, and as aldose or ketose
A B
CC
CH2OHH OHC
OHH COHH
H OHC O
H
CC
CH2OHOHH CHHOO
CH2OH
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Solution C1
A Baldose, hexose ketose, pentose
CC
CH2OHH OHC
OHH COHH
H OHC O
H
CC
CH2OHOHH CHHOO
CH2OH
8
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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Tetrahedral carbon atombonded to four different groups
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ARE THERE CHIRAL C-ATOMS IN THESE MOLECULES?
CH3CHClCH2CH3
A
CH2ClCH3
B
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Learning Check C2
Determine if there is a chiral carbon in each compound.
A B
CCl
CH3
CH2CH3
H CCl
CH3
HH
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Solution C2
A Yes, 4 different B No, thegroups are attached 2 H atomsto the second C atom are identical
CCl
CH3
CH2CH3
H CCl
CH3
HH
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15
Glyceraldehyde
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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
CCCH2OH
OHHOHH
CO
H
Right = D
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Glucose
CC
CH2OHH OHC
OHH CHHO
H OHC O
H
D-Glucose
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Fructose
CC
CH2OHH OHC
OHH CHHOO
CH2OH
D-Fructose
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Galactose
D-galactose
CCCCCCH2OH
OH
OHHHO H
HHOOHH
21
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 α anomer, up for β anomer
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Cyclization of glucose
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Haworth Structure for D-Glucose
oCH2OH
OHOH
OH
OH
oCH2OH
OHOH
OH
OH
α-D-Glucose β-D-Glucose
α
β
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d-fructose
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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 .
α-D-glucose D-glucose (open) β-D-glucose(36%) (trace) (64%)
28
Learning Check C3
Write the cyclic form of α-D-galactose
CCCCCCH2OH
OH
OHHHO H
HHOOHH
29
Solution C3
o
OH
OH
CH2OH
OH
OH
α-D-galactose