Chapter 12 Carbohydrates - Seattle Central College

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Chapter 12
Carbohydrates
Carbohydrates
• Synthesized by plants using sunlight to
convert CO2 and H2O to glucose and O2.
• Polymers include starch and cellulose.
• Starch is storage unit for solar energy.
• Most sugars have formula Cn(H2O)n, “hydrate
of carbon.”
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Classification of Carbohydrates
• Monosaccharides or simple sugars
– polyhydroxyaldehydes or aldoses
– polyhydroxyketones or ketoses
• Disaccharides can be hydrolyzed to two
monosaccharides.
• Polysaccharides hydrolyze to many
monosaccharide units. E.g., starch and
cellulose have > 1000 glucose units.
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Monosaccharides
• Classified by:
– aldose or ketose
– number of carbons in chain
– configuration of chiral carbon farthest from the
carbonyl group
glucose, a
D-aldohexose
fructose, a
D-ketohexose
=>
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D and L Sugars
sugars can be degraded to the
dextrorotatory (+) form of glyceraldehyde.
L sugars can be degraded to the levorotatory
(-) form of glyceraldehyde.
• D
•
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The D Aldose Family
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Epimers
Sugars that differ only in their
stereochemistry at a single carbon.
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Cyclic Structure for Glucose
Glucose cyclic hemiacetal formed by reaction
of -CHO with -OH on C5.
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D-glucopyranose
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Cyclic Structure for Fructose
Cyclic hemiacetal formed by reaction of C=O
at C2 with -OH at C5.
D-fructofuranose
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Anomers
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Mutarotation
Glucose also called
dextrose; dextrorotatory.
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Epimerization
In base, H on C2 may be removed to form
enolate ion. Reprotonation may change
the stereochemistry of C2.
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Reduction of Simple Sugars
• C=O of aldoses or ketoses can be reduced
to C-OH by NaBH4 or H2/Ni.
• Name the sugar alcohol by adding -itol to
the root name of the sugar.
• Reduction of D-glucose produces
D-glucitol, commonly called D-sorbitol.
• Reduction of D-fructose produces a mixture
of D-glucitol and D-mannitol.
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Oxidation by Nitric Acid
Nitric acid oxidizes the aldehyde and the
terminal alcohol; forms aldaric acid.
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Oxidation by Tollens Reagent
• Tollens reagent reacts with aldehyde, but
the base promotes enediol
rearrangements, so ketoses react too.
• Sugars that give a silver mirror with Tollens
are called reducing sugars.
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Nonreducing Sugars
• Glycosides are acetals, stable in base, so they
do not react with Tollens reagent.
• Disaccharides and polysaccharides are also
acetals, nonreducing sugars.
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Formation of Glycosides
• React the sugar with alcohol in acid.
• Since the open chain sugar is in equilibrium
with its - and -hemiacetal, both anomers
of the acetal are formed.
• Aglycone is the term used for the group
bonded to the anomeric carbon.
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Ether Formation
• Sugars are difficult to recrystallize from water
because of their high solubility.
• Convert all -OH groups to -OR, using a
modified Williamson synthesis, after
converting sugar to acetal, stable in base.
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Ester Formation
Acetic anhydride with pyridine catalyst converts
all the oxygens to acetate esters.
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Osazone Formation
Both C1 and C2 react with phenylhydrazine.
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Kiliani-Fischer Synthesis
• This process lengthens the aldose chain.
• A mixture of C2 epimers is formed.
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Fischer’s Proof
• Emil Fischer determined the configuration
around each chiral carbon in D-glucose in 1891,
using Ruff degradation and oxidation reactions.
• He assumed that the -OH is on the right in the
Fischer projection for D-glyceraldehyde.
• This guess turned out to be correct!
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Disaccharides
• Three naturally occurring glycosidic linkages:
• 1-4’ link: The anomeric carbon is bonded
to oxygen on C4 of second sugar.
• 1-6’ link: The anomeric carbon is bonded
to oxygen on C6 of second sugar.
• 1-1’ link: The anomeric carbons of the two
sugars are bonded through an oxygen.
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Cellobiose
• Two glucose units linked 1-4’.
• Disaccharide of cellulose.
• A mutarotating, reducing sugar.
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Maltose
Two glucose units linked 1-4’.
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Lactose
• Galactose + glucose linked 1-4’.
• “Milk sugar.”
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Sucrose
• Glucose + fructose, linked 1-1’
• Nonreducing sugar
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Cellulose
• Polymer of D-glucose, found in plants.
• Mammals lack the -glycosidase enzyme.
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Amylose
• Soluble starch, polymer of D-glucose.
• Starch-iodide complex, deep blue.
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Amylopectin
Branched, insoluble fraction of starch.
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Glycogen
• Glucose polymer, similar to amylopectin, but
even more highly branched.
• Energy storage in muscle tissue and liver.
• The many branched ends provide a quick
means of putting glucose into the blood.
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End of Chapter 12
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