chapter 12 carbohydrates. chapter 132 carbohydrates synthesized by plants using sunlight to convert...
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Chapter 12Carbohydrates
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Chapter 13 2
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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Chapter 12 3
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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Chapter 12 4
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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Chapter 12 5
D and L Sugars• D sugars can be degraded to the
dextrorotatory (+) form of glyceraldehyde.• L sugars can be degraded to the levorotatory
(-) form of glyceraldehyde.
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Chapter 23 6
The D Aldose Family
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Chapter 12 7
EpimersSugars that differ only in their
stereochemistry at a single carbon.
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Chapter 12 8
Cyclic Structure for GlucoseGlucose cyclic hemiacetal formed by reaction
of -CHO with -OH on C5.
D-glucopyranose
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Chapter 12 9
Cyclic Structure for FructoseCyclic hemiacetal formed by reaction of C=O
at C2 with -OH at C5.
D-fructofuranose
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Chapter 12 10
Anomers
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Chapter 12 11
Mutarotation
Glucose also called dextrose; dextrorotatory.
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Chapter 12 12
EpimerizationIn base, H on C2 may be removed to form
enolate ion. Reprotonation may change the stereochemistry of C2.
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Chapter 12 13
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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Chapter 12 14
Oxidation by Nitric AcidNitric acid oxidizes the aldehyde and the
terminal alcohol; forms aldaric acid.
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Chapter 12 15
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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Chapter 23 16
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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Chapter 23 17
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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Chapter 23 18
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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Chapter 12 19
Ester Formation
Acetic anhydride with pyridine catalyst converts all the oxygens to acetate esters.
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Chapter 23 20
Osazone FormationBoth C1 and C2 react with phenylhydrazine.
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Chapter 12 21
Kiliani-Fischer Synthesis• This process lengthens the aldose chain.• A mixture of C2 epimers is formed.
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Chapter 12 22
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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Chapter 12 23
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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Chapter 12 24
Cellobiose• Two glucose units linked 1-4’.• Disaccharide of cellulose.• A mutarotating, reducing sugar.
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Chapter 12 25
MaltoseTwo glucose units linked 1-4’.
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Chapter 12 26
Lactose• Galactose + glucose linked 1-4’.• “Milk sugar.”
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Chapter 12 27
Sucrose• Glucose + fructose, linked 1-1’• Nonreducing sugar
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Chapter 12 28
Cellulose
• Polymer of D-glucose, found in plants.• Mammals lack the -glycosidase enzyme.
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Chapter 12 29
Amylose• Soluble starch, polymer of D-glucose.• Starch-iodide complex, deep blue.
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Chapter 12 30
AmylopectinBranched, insoluble fraction of starch.
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Chapter 12 31
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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32
End of Chapter 12