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Chapter 23Carbohydrates and Nucleic
Acids
Jo BlackburnRichland College, Dallas, TX
Dallas County Community College District2003,Prentice Hall
Organic Chemistry, 5th EditionL. G. Wade, Jr.
Chapter 23 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 23 3
Classification of Carbohydrates
• Monosaccharides or simple sugarspolyhydroxyaldehydes or aldosespolyhydroxyketones or ketoses
• Disaccharides can be hydrolyzed to two monosaccharides.
• Polysaccharides hydrolyze to many monosaccharide units. E.g., starch and cellulose have > 1000 glucose units. =>
Chapter 23 4
Monosaccharides• Classified by:
aldose or ketosenumber of carbons in chainconfiguration of chiral carbon farthest from
the carbonyl group
glucose, a D-aldohexose
fructose, a D-ketohexose =>
Chapter 23 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.
=>
Chapter 23 6
The D Aldose Family
=>
Chapter 23 7
Erythro and Threo• Terms used for diastereomers with two
adjacent chiral C’s, without symmetric ends.
• For symmetric molecules, use meso or d,l.
=>
Chapter 23 8
EpimersSugars that differ only in their
stereochemistry at a single carbon.
=>
Chapter 23 9
Cyclic Structure for Glucose
Glucose cyclic hemiacetal formed by reaction of -CHO with -OH on C5.
=>
D-glucopyranose
Chapter 23 10
Cyclic Structure for FructoseCyclic hemiacetal formed by reaction of
C=O at C2 with -OH at C5.
=>
D-fructofuranose
Chapter 23 11
Anomers
=>
Chapter 23 12
Mutarotation
=>Glucose also called dextrose; dextrorotatory.
Chapter 23 13
EpimerizationIn base, H on C2 may be removed to form
enolate ion. Reprotonation may change the stereochemistry of C2.
=>
Chapter 23 14
Enediol RearrangementIn base, the position of the C=O can shift.
Chemists use acidic or neutral solutionsof sugars to preserve their identity.
=>
Chapter 23 15
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. =>
Chapter 23 16
Oxidation by Bromine
Bromine water oxidizes aldehyde, but not ketone or alcohol; forms aldonic acid.
=>
Chapter 23 17
Oxidation by Nitric AcidNitric acid oxidizes the aldehyde and the
terminal alcohol; forms aldaric acid.
=>
Chapter 23 18
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.
=>
Chapter 23 19
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 20
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 21
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 22
Ester FormationAcetic anhydride with pyridine catalyst
converts all the oxygens to acetate esters.
=>
Chapter 23 23
Osazone FormationBoth C1 and C2 react with phenylhydrazine.
=>
Chapter 23 24
Ruff DegradationAldose chain is shortened by oxidizing the
aldehyde to -COOH, then decarboxylation.
=>
Chapter 23 25
Kiliani-Fischer Synthesis• This process lengthens the aldose chain.
• A mixture of C2 epimers is formed.
=>
Chapter 23 26
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! =>
Chapter 23 27
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.
O
H
OH
H
HO
HO
H
OH
H
C
H
H2OHexcess CH3I
Ag2O O
H
OCH3
H
CH3O
CH3O
H
O
HH
C
CH3
H2OCH3H3O
+
O
H
OH
H
CH3O
CH3O
H
O
HH
C
CH3
H2OCH3
=>
Chapter 23 28
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 29
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. =>
Chapter 23 30
Cellobiose
• Two glucose units linked 1-4’.
• Disaccharide of cellulose.
• A mutarotating, reducing sugar.
=>
Chapter 23 31
MaltoseTwo glucose units linked 1-4’.
=>
Chapter 23 32
Lactose• Galactose + glucose linked 1-4’.
• “Milk sugar.”
=>
Chapter 23 33
Gentiobiose• Two glucose units linked 1-6’.
• Rare for disaccharides, but commonly seen as branch point in carbohydrates.
=>
Chapter 23 34
Sucrose• Glucose + fructose, linked 1-1’
• Nonreducing sugar
=>
Chapter 23 35
Cellulose
• Polymer of D-glucose, found in plants.
• Mammals lack the -glycosidase enzyme.
=>
Chapter 23 36
Amylose• Soluble starch, polymer of D-glucose.
• Starch-iodide complex, deep blue.
=>
Chapter 23 37
AmylopectinBranched, insoluble fraction of starch.
=>
Chapter 23 38
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.
=>
Chapter 23 39
Chitin• Polymer of N-acetylglucosamine.
• Exoskeleton of insects.
=>
Chapter 23 40
Nucleic Acids
• Polymer of ribofuranoside rings linked by phosphate ester groups.
• Each ribose is bonded to a base.
• Ribonucleic acid (RNA)• Deoxyribonucleic acid
(DNA)=>
Chapter 23 41
RibonucleosidesA -D-ribofuranoside bonded to a
heterocyclic base at the anomeric carbon.
=>
Chapter 23 42
Ribonucleotides
Add phosphate at 5’ carbon.
Chapter 23 43
Structure of RNA
=>
Chapter 23 44
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.
=>
Chapter 23 45
Base Pairings
=>
Chapter 23 46
Double Helix of DNA
• Two complementary polynucleotide chains are coiled into a helix.
• Described by Watson and Crick, 1953.
=>
Chapter 23 47
DNA Replication
=>
Chapter 23 48
Additional Nucleotides
• Adenosine monophosphate (AMP), a regulatory hormone.
• Nicotinamide adenine dinucleotide (NAD), a coenzyme.
• Adenosine triphosphate (ATP), an energy source.
=>
Chapter 23 49
End of Chapter 23