Chapter 23Carbohydrates and Nucleic AcidsJo BlackburnRichland College, Dallas, TXDallas County Community College District 2003, Prentice HallOrganic Chemistry, 5th Edition L. G. Wade, Jr.

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CarbohydratesSynthesized 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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Classification of CarbohydratesMonosaccharides or simple sugarspolyhydroxyaldehydes or aldosespolyhydroxyketones or ketosesDisaccharides 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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MonosaccharidesClassified by:aldose or ketosenumber of carbons in chainconfiguration of chiral carbon farthest from the carbonyl group

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D and L SugarsD 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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The D Aldose Family=>

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Erythro and ThreoTerms used for diastereomers with two adjacent chiral Cs, without symmetric ends.For symmetric molecules, use meso or d,l.

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EpimersSugars that differ only in their stereochemistry at a single carbon.

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Cyclic Structure for GlucoseGlucose cyclic hemiacetal formed by reaction of -CHO with -OH on C5.

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Cyclic Structure for FructoseCyclic hemiacetal formed by reaction of C=O at C2 with -OH at C5.

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Anomers

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Mutarotation

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EpimerizationIn base, H on C2 may be removed to form enolate ion. Reprotonation may change the stereochemistry of C2.

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Enediol RearrangementIn 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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Reduction of Simple SugarsC=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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Oxidation by BromineBromine water oxidizes aldehyde, but not ketone or alcohol; forms aldonic acid.=>

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Oxidation by Nitric AcidNitric acid oxidizes the aldehyde and the terminal alcohol; forms aldaric acid.

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Oxidation by Tollens ReagentTollens 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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Nonreducing SugarsGlycosides 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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Formation of GlycosidesReact 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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Ether FormationSugars 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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Ester FormationAcetic anhydride with pyridine catalyst converts all the oxygens to acetate esters.

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Osazone FormationBoth C1 and C2 react with phenylhydrazine.

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Ruff DegradationAldose chain is shortened by oxidizing the aldehyde to -COOH, then decarboxylation.

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

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Fischers ProofEmil 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 SizeHaworth determined the pyranose structure of glucose in 1926.The anomeric carbon can be found by methylation of the -OHs, then hydrolysis.

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Periodic Acid CleavagePeriodic 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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DisaccharidesThree 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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CellobioseTwo glucose units linked 1-4.Disaccharide of cellulose.A mutarotating, reducing sugar.

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MaltoseTwo glucose units linked 1-4.

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LactoseGalactose + glucose linked 1-4.Milk sugar.

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GentiobioseTwo glucose units linked 1-6.Rare for disaccharides, but commonly seen as branch point in carbohydrates.

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SucroseGlucose + fructose, linked 1-1Nonreducing sugar

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CellulosePolymer of D-glucose, found in plants.Mammals lack the -glycosidase enzyme.

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AmyloseSoluble starch, polymer of D-glucose.Starch-iodide complex, deep blue.

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AmylopectinBranched, insoluble fraction of starch.=>

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GlycogenGlucose 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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ChitinPolymer of N-acetylglucosamine.Exoskeleton of insects.

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Nucleic AcidsPolymer 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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RibonucleosidesA -D-ribofuranoside bonded to a heterocyclic base at the anomeric carbon.

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RibonucleotidesAdd phosphate at 5 carbon.

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Structure of RNA=>

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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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Base Pairings=>

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Double Helix of DNATwo complementary polynucleotide chains are coiled into a helix.Described by Watson and Crick, 1953.

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DNA Replication=>

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Additional NucleotidesAdenosine monophosphate (AMP), a regulatory hormone.Nicotinamide adenine dinucleotide (NAD), a coenzyme.Adenosine triphosphate (ATP), an energy source. =>

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End of Chapter 23

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