chapter 13 carbohydrates
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1
Carbohydrates
ClassificationMonosaccharides
Chiral Carbon AtomsStructures 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
• Disaccharides
Contain 2 monosacchride units
• Polysacchrides
Contain many monosacchride units
4
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.
6
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
7
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
8
Chiral Objects
• Chiral compounds have the same number of atoms arranged differently in space.
• A chiral carbon atom has four different groups attached
9
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?
10
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
11
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
12
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.
13
D notation
C
C
CH2OH
OHH
OHH
CO
H
Right = D
14
Glucose
C
C
CH2OH
H OHC
OHH C
HHO
H OH
C OH
D-Glucose
15
Fructose
C
C
CH2OH
H OHC
OHH C
HHO
O
CH2OH
D-Fructose
16
Galactose
D-galactose
C
C
C
C
C
CH2OH
OH
OHH
HO H
HHO
OHH
17
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
18
Haworth Structure for D-Isomers
The cyclic structure of a D-isomer has the final CH2OH group located above the ring.
oCH2OH
19
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
20
Haworth Structure for D-Glucose
oCH2OH
OHOH
OH
OH
oCH2OH
OHOH
OH
OH
-D-Glucose -D-Glucose
21
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%)
22
Learning Check C3
Write the cyclic form of -D-galactose
C
C
C
C
C
CH2OH
OH
OHH
HO H
HHO
OHH
23
Solution C3
o
OH
OH
CH2OH
OH
OH
-D-galactose
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