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Chapter 13: Carbohydrates
Chem 20
El Camino College
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Carbohydrates
Carbohydrates have the formulas of Cn(H2O)n
Glucose is formed by photosynthesis:
6 CO2 + 6 H2O → C6 (H2O)6 + 6O2
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D and L The first carbon (C #1) is the carbon on the top In the L version, the OH on the last chiral carbon is on
the left In the D version, the OH on the last chiral carbon is on
the right
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Examples of D and L Isomers of Monosaccharides
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Cyclic Structure of Glucose The carbonyl group of D-glucose can react with the
–OH on carbon 5 to form a ring of six atoms.
It has one more chiral center (carbon 1) than the open chain.
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Drawing the Cyclic (Haworth) Structure
1. Starting with the aldehyde carbon, number the carbons
2. Draw the structure laying on the right sideMake sure you don’t change the locations of the
OH groups
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H
OHH
OH
C
H H
OH OH
C C CH
O
CHOCH2
Drawing the Cyclic Structure for Glucose
STEP 1 Number the carbon chain and turn on its side to the right
HHO
H
CH2OH
OHC
H
H
OH
OH
C
C
C
OH
C1
2
3
4
5
6
6 5 4 3 2 1
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Cyclic Structures
3. Draw the following cyclic structureNote that there is an oxygen in the ringAlways draw the structure in the exact same way
O
1
23
4
5
6
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Cyclic Structures 4. In your straight chain drawing, note if the OH is
down or up on carbons 2, 3, and 4Put those OH groups down or up on the ringCH2OH is up, OH on C1 can go up or down Fill in the rest with hydrogens
HOH2C
H
OH
OHH
OH
OH
H
OH
H
1
2345
6
downup
down
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Cyclic Structures
HOH2C
H
OH
OHH
OH
OH
H
OH
H
1
2345
6
downup
down
O
OH
OH
OH
1
23
4
5
6
O
OH
H
H
OH
OH
H
H
OH
CH2OH
H
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Cyclic Structure of Glucose In the Haworth structure, we suppose that the ring
is flat. the group -CH2OH is above the ring. The –OH group on carbon 1 is written down in the
alpha form and up in the beta form.
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Cyclic Structure of Glucose Solid glucose is α-D-glucose. In aqueous solution, there is equilibrium of three forms: 36%
α-D-glucose, 64% β-D-glucose and trace of open chain.
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Cyclic Structure, D-Galactose
CH2OH
HO
H OH
HHO
HHO
OHH
HOH2C
H
OH
OHH
OH
H
OH
OH
H
1
2345
6
downupup
O
OH
H
H
OH
OH
H
OH
H
CH2OH
H 1
23
4
5
6
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Cyclic Structure, D-Galactose
O
OH
H
H
OH
OH
H
OH
H
CH2OH
H 1
23
4
5
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The cyclic structure of galactose is similar to glucose, except that the –OH on carbon 4 is up.
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Cyclic Structure D-Galactose
The cyclic D-galactose can be in alpha and beta forms
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Cyclic Structure
CH2OH
HO
H OH
OHH
OHH
OHH
HOH2C
H
OH
OHOH
H
OH
H
OH
H
down down down
1
2345
6
OH
OH
H
OH
H
OHOH
H
CH2OH
H
23
4
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Cyclic Structure Fructose
The hydroxyl group on carbon 5 reacts with the ketone group on carbon 2 to form a five-atom ring. The cyclic D-fructose can be in alpha and beta forms
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Chemical Properties
Reduction of monosaccharides: The carbonyl group (-C=O) can be reduced to –OH group. For example D-glucose is reduced to sorbitol (D-glucitol)
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Oxidation of Monosaccharides
Aldoses are called “reducing sugars” The aldehyde group (-CHO) in aldose can be
oxidized by silver ammonia ion Ag(NH3)2+ of Tollens’
reagent to form silver In the Benedict test, the aldehyde group is oxidized
to a carboxylic acid group by Cu(II) citrate complex and a brick-red ppt (Cu2O) forms
Fructose is also a reducing sugar since it is converted to glucose in basic solution.
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+ Cu2O(s)
D-gluconic acidD-glucose
+ Cu2+
H OH
H OH
HHO
H OH
O
OH
CH2OH
C
C
C
C
C
H OH
H OH
HHO
H OH
O
H
CH2OH
C
C
C
C
C
Benedict Test of D-Glucose
[O]
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Disaccharides Disaccharides can be broken down into two
monosaccharidesMaltose + H+ + H2O glucose + glucose
Lactose + H+ + H2O glucose + galactose
Sucrose + H+ + H2O glucose + fructose
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Alpha and Beta Linkages There are different types of linkages in
disaccharidesIn -linkages, both OH groups involved in
bonding are pointing downIn -linkages, one OH involved in bonding is up
and the other is down.
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Formation of alpha lactose from glucose + galactose (formation of ether bridge)
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Formation of Beta Lactose
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Formation of Alpha Maltose
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Sucrose from glucose + fructose
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Polysaccharides Long chains of monosaccharides are called
polysaccharides Amylose and amylopectin are plant starches Glycogen is animal starch—it provides storage
for glucose in muscles
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Polysaccharides Cellulose (in wood and cotton) is a structural
material for plants
People don’t have the enzymes to digest the -linkages in cellulose, but cows do!
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