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.”

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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.

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Chapter 23 6

The D Aldose Family

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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.

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Chapter 23 8

EpimersSugars that differ only in their

stereochemistry at a single carbon.

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Chapter 23 9

Cyclic Structure for GlucoseGlucose cyclic hemiacetal formed by

reaction of -CHO with -OH on C5.

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D-glucopyranose

Chapter 23 10

Cyclic Structure for FructoseCyclic hemiacetal formed by reaction of

C=O at C2 with -OH at C5.

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D-fructofuranose

Chapter 23 11

Anomers

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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.

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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.

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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 BromineBromine water oxidizes aldehyde, but not

ketone or alcohol; forms aldonic acid.

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Chapter 23 17

Oxidation by Nitric AcidNitric acid oxidizes the aldehyde and the

terminal alcohol; forms aldaric acid.

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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.

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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.

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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.

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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.

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Chapter 23 22

Ester FormationAcetic anhydride with pyridine catalyst

converts all the oxygens to acetate esters.

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Chapter 23 23

Osazone FormationBoth C1 and C2 react with phenylhydrazine.

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Chapter 23 24

Ruff DegradationAldose chain is shortened by oxidizing the

aldehyde to -COOH, then decarboxylation.

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Chapter 23 25

Kiliani-Fischer Synthesis• This process lengthens the aldose chain.• A mixture of C2 epimers is formed.

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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

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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.

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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.

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Chapter 23 31

MaltoseTwo glucose units linked 1-4’.

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Chapter 23 32

Lactose• Galactose + glucose linked 1-4’.• “Milk sugar.”

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Chapter 23 33

Gentiobiose• Two glucose units linked 1-6’.• Rare for disaccharides, but commonly

seen as branch point in carbohydrates.

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Chapter 23 34

Sucrose• Glucose + fructose, linked 1-1’• Nonreducing sugar

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Chapter 23 35

Cellulose• Polymer of D-glucose, found in plants.• Mammals lack the -glycosidase enzyme.

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Chapter 23 36

Amylose• Soluble starch, polymer of D-glucose.• Starch-iodide complex, deep blue.

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Chapter 23 37

AmylopectinBranched, insoluble fraction of starch.

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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.

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Chapter 23 39

Chitin• Polymer of N-acetylglucosamine.• Exoskeleton of insects.

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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.

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Chapter 23 42

Ribonucleotides

Add phosphate at 5’ carbon.

Chapter 23 43

Structure of RNA

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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.

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Chapter 23 45

Base Pairings

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Chapter 23 46

Double Helix of DNA

• Two complementary polynucleotide chains are coiled into a helix.

• Described by Watson and Crick, 1953.

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Chapter 23 47

DNA Replication

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Chapter 23 48

Additional Nucleotides

• Adenosine monophosphate (AMP), a regulatory hormone.

• Nicotinamide adenine dinucleotide (NAD), a coenzyme.

• Adenosine triphosphate (ATP), an energy source.

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Chapter 23 49

End of Chapter 23