Organic Chemistry, 5th Edition
L. G. Wade, Jr.
Chapter 23
Carbohydrates and Nucleic
Acids
Jo Blackburn
Richland College, Dallas, TX
Dallas County Community College District
2003, Prentice Hall
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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2
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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3
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
Chapter 23
=>
4
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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5
=>
The D Aldose Family
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6
Erythro and Threo
• Terms used for diastereomers with two
adjacent chiral C’s, without symmetric ends.
• For symmetric molecules, use meso or d,l.
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7
Epimers
Sugars that differ only in their
stereochemistry at a single carbon.
=>
Chapter 23
8
Cyclic Structure for Glucose
Glucose cyclic hemiacetal formed by
reaction of -CHO with -OH on C5.
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Chapter 23
D-glucopyranose
9
Cyclic Structure for Fructose
Cyclic hemiacetal formed by reaction of
C=O at C2 with -OH at C5.
D-fructofuranose
Chapter 23
10
=>
Anomers
Chapter 23
11
=>
Mutarotation
Glucose also called
dextrose; dextrorotatory.
Chapter 23
=>
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
Enediol Rearrangement
In base, the position of the C=O can shift.
Chemists use acidic or neutral solutions
of sugars to preserve their identity.
=>
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14
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 23
Oxidation by Bromine
Bromine water oxidizes aldehyde, but not
ketone or alcohol; forms aldonic acid.
=>
Chapter 23
16
Oxidation by Nitric Acid
Nitric acid oxidizes the aldehyde and the
terminal alcohol; forms aldaric acid.
Chapter 23
17
=>
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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18
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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19
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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20
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
21
=>
Ester Formation
Acetic anhydride with pyridine catalyst
converts all the oxygens to acetate esters.
=>
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22
Osazone Formation
Both C1 and C2 react with phenylhydrazine.
=>
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23
Ruff Degradation
Aldose chain is shortened by oxidizing the
aldehyde to -COOH, then decarboxylation.
=>
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24
Kiliani-Fischer Synthesis
• This process lengthens the aldose chain.
• A mixture of C2 epimers is formed.
Chapter 23
25
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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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Determination of Ring Size
• Haworth determined the pyranose
structure of glucose in 1926.
• The anomeric carbon can be found by
methylation of the -OH’s, then hydrolysis.
H
HO
excess CH3I
Ag2O
CH2OHO
H H
HO
CH3O
OCH3
CH3O
H
H
+
CH3O
H H
CH3O
H
H3O
CH2OCH3
O
OH
OH
H
H
H
CH2OCH3
O
H H
CH3O
OH
CH3O
H
H
=>
Chapter 23
27
Periodic Acid Cleavage
• Periodic acid cleaves vicinal diols to
give two carbonyl compounds.
• Separation and identification of the
products determine the size of the ring.
Chapter 23
28
=>
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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29
Cellobiose
• Two glucose units linked 1-4’.
• Disaccharide of cellulose.
• A mutarotating, reducing sugar.
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30
=>
Maltose
Two glucose units linked 1-4’.
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Lactose
• Galactose + glucose linked 1-4’.
• “Milk sugar.”
=>
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Gentiobiose
• Two glucose units linked 1-6’.
• Rare for disaccharides, but commonly
seen as branch point in carbohydrates.
=>
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33
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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37
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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Chitin
• Polymer of N-acetylglucosamine.
• Exoskeleton of insects.
=>
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39
Nucleic Acids
• Polymer of ribofuranoside
rings linked by phosphate
ester groups.
• Each ribose is bonded to
a base.
• Ribonucleic acid (RNA)
• Deoxyribonucleic acid
(DNA)
=>
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40
Ribonucleosides
A -D-ribofuranoside bonded to a
heterocyclic base at the anomeric
carbon.
=>
Chapter 23
41
Ribonucleotides
Add phosphate at 5’ carbon.
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42
Structure of RNA
Chapter 23
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43
Structure of DNA
• -D-2-deoxyribofuranose is the sugar.
• Heterocyclic bases are cytosine,
thymine (instead of uracil), adenine, and
guanine.
• Linked by phosphate ester groups to
form the primary structure.
=>
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44
Base Pairings
Chapter 23
45
=>
Double Helix of DNA
• Two complementary
polynucleotide
chains are coiled
into a helix.
• Described by
Watson and Crick,
1953.
=>
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46
DNA Replication
=>
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47
Additional Nucleotides
• Adenosine monophosphate (AMP), a
regulatory hormone.
• Nicotinamide adenine dinucleotide
(NAD), a coenzyme.
• Adenosine triphosphate (ATP), an
energy source.
=>
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48
End of Chapter 23
Chapter 23
49