1. Chapter 12 Carbohydrates

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.”
Chapter 13

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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4. Monosaccharides Classified by: aldose or ketose
number of carbons in chain
configuration of chiral carbon farthest from the carbonyl group
fructose, a
D-ketohexose
=>
glucose, a
D-aldohexose
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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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6. The D Aldose Family
Chapter 23

7. Epimers
Sugars that differ only in their stereochemistry at a single carbon.
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8. Cyclic Structure for Glucose
Glucose cyclic hemiacetal formed by reaction of -CHO with -OH on C5.
D-glucopyranose
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9. Cyclic Structure for Fructose
Cyclic hemiacetal formed by reaction of C=O at C2 with -OH at C5.
D-fructofuranose
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10. Anomers
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11. Mutarotation
Glucose also called
dextrose; dextrorotatory.
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12. Epimerization
In base, H on C2 may be removed to form enolate ion. Reprotonation may change the stereochemistry of C2.
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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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14. Oxidation by Nitric Acid
Nitric acid oxidizes the aldehyde and the terminal alcohol; forms aldaric acid.
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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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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.
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.
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.
Chapter 23

19. Ester Formation
Acetic anhydride with pyridine catalyst converts all the oxygens to acetate esters.
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20. Osazone Formation Both C1 and C2 react with phenylhydrazine.
Chapter 23

21. Kiliani-Fischer Synthesis
This process lengthens the aldose chain.
A mixture of C2 epimers is formed.
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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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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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24. Cellobiose Two glucose units linked 1-4’. Disaccharide of cellulose.
A mutarotating, reducing sugar.
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25. Maltose
Two glucose units linked 1-4’.
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26. Lactose
Galactose + glucose linked 1-4’.
“Milk sugar.”
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27. Sucrose
Glucose + fructose, linked 1-1’
Nonreducing sugar
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28. Cellulose Polymer of D-glucose, found in plants.
Mammals lack the -glycosidase enzyme.
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29. Amylose Soluble starch, polymer of D-glucose.
Starch-iodide complex, deep blue.
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30. Amylopectin
Branched, insoluble fraction of starch.
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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