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Chapter 25
Importance of Carbohydrates
• Distributedwidelyinnature• Keyintermediatesofmetabolism(sugars)• Structuralcomponentsofplants(cellulose)• Centraltomaterialsofindustrialproducts:paper,lumber,fibers
• Keycomponentoffoodsources:sugars,flour,vegetablefiber
• ContainOHgroupsonmostcarbonsinlinearchainsorinrings
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Chemical Formula and Name
• CarbohydrateshaveroughlyasmanyO’sasC’s(highlyoxidized)
• SinceH’sareaboutconnectedtoeachHandOtheempiricalformulasareroughly(C(H2O))n• Appears to be “carbon hydrate” from formula
• Currentterminology:naturalmaterialsthatcontainmanyhydroxylsandotheroxygen-containinggroups
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Chemical Formula and Name
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Sources
• GlucoseisproducedinplantsthroughphotosynthesisfromCO2 andH2O
• Glucoseisconvertedinplantstoothersmallsugarsandpolymers(cellulose,starch)
• Dietarycarbohydratesprovidethemajorsourceofenergyrequiredbyorganisms
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Why this Chapter?
• Toseewhatthestructuresand1˚biologicalfunctionsofcarbohydratesare
• Tohaveanintroductiononhowcarbohydratesarebiosynthesizedanddegradedinorganisms
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25.1 Classification of Carbohydrates
• Simplesugars(monosaccharides)can'tbeconvertedintosmallersugarsbyhydrolysis.
• Carbohydratesaremadeoftwoormoresimplesugarsconnectedasacetals (aldehydeandalcohol),oligosaccharidesandpolysaccharides
• Sucrose(tablesugar):disaccharidefromtwomonosaccharides(glucoselinkedtofructose),
• Celluloseisapolysaccharideofseveralthousandglucoseunitsconnectedbyacetal linkages(aldehydeandalcohol)
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25.1 Classification of Carbohydrates
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Aldoses and Ketoses
• aldo- andketo- prefixesidentifythenatureofthecarbonylgroup
• -ose suffixdesignatesacarbohydrate• NumberofC’sinthemonosaccharideindicatedbyroot(-tri-,tetra-,penta-,hexa-)
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Aldoses and Ketoses
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25.2 Depicting Carbohydrate Stereochemistry: Fischer Projections
• Carbohydrateshavemultiplechiralitycentersandcommonsetsofatoms
• AchiralitycenterCisprojectedintotheplaneofthepaperandothergroupsarehorizontalorverticallines
• Groupsforwardfrompaperarealwaysinhorizontalline.Theoxidizedendofthemoleculeisalwayshigheronthepage(“up”)
• The“projection”canbeseenwithmolecularmodels
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25.2 Depicting Carbohydrate Stereochemistry: Fischer Projections
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Stereochemical Reference
• Thereferencecompoundsarethetwoenantiomersofglyceraldehyde,C3H6O3
• Acompoundis“D”ifthehydroxylgroupatthechiralitycenterfarthestfromtheoxidizedendofthesugarisontherightor“L”ifitisontheleft.
• D-glyceraldehydeis(R)-2,3-dihydroxypropanal• L-glyceraldehydeis(S)-2,3-dihydroxypropanal
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Stereochemical Reference
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Working With Fischer Projections
• Ifgroupsarenotincorrespondingpositions,theycanbeexchangedthreeatatimeinrotation– workwithmolecularmodelstoseehowthisisdone
• Theentirestructuremayonlyberotatedby180°
• WhileR,SdesignationscanbededucedfromFischerprojections(withpractice),itisbesttomakemolecularmodelsfromtheprojectedstructureandworkwiththemodel
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Working With Fischer Projections
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25.3 D, L Sugars
• Glyceraldehydeexistsastwoenantiomers,firstidentifiedbytheiroppositerotationofplanepolarizedlight
• Naturallyoccurringglyceraldehyderotatesplane-polarizedlightinaclockwisedirection,denoted(+)andisdesignated“(+)-glyceraldehyde”
• Theenantiomergivestheoppositerotationandhasa(-)or“l”(levorotatory)prefix
• Thedirectionofrotationoflightdoesnotcorrelatetoanystructuralfeature
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NaturallyOccurringDSugars
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25.5 Cyclic Structures of Monosaccharides: Anomers
• Alcoholsaddreversiblytoaldehydesandketones,forminghemiacetals
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Internal Hemiacetals of Sugars
• Intramolecular nucleophilic additioncreatescyclichemiacetals insugars
• Five- andsix-memberedcyclichemiacetals areparticularlystable
• Five-memberedringsarefuranoses.Six-memberedarepyranoses
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Internal Hemiacetals of Sugars
• Formationofthethecyclichemiacetal createsanadditionalchiralitycentergivingtwodiasteromeric forms,designateda andb
• Thesediastereomers arecalledanomers• Thedesignationa indicatesthattheOHattheanomeric centerisonthesamesideoftheFischerprojectionstructureashydroxylthatdesignateswhetherthestructureusDorL
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Converting to Proper Structures
• TheFischerprojectionstructuresmustberedrawntoconsiderrealbondlengths,andyoualsoseethe“Pyran”form
• Pyranoseringshaveachair-likegeometrywithaxialandequatorialsubstituents
• Ringsareusuallydrawnplacingthehemiacetaloxygenatomattherightrear
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25.6 Reactions of Monosaccharides
• ¾OHgroupscanbeconvertedintoestersandethers,whichareofteneasiertoworkwiththanthefreesugarsandaresolubleinorganicsolvents.• Esterification by treating with an acid chloride
or acid anhydride in the presence of a base• All ¾OH groups react
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25.6 Reactions of Monosaccharides
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Ethers
• Treatmentwithanalkylhalideinthepresenceofbase—theWilliamsonethersynthesis
• Usesilveroxideasacatalystwithbase-sensitivecompounds
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Glycoside Formation
• Treatmentofamonosaccharidehemiacetalwithanalcoholandanacidcatalystyieldsanacetal inwhichtheanomeric¾OHhasbeenreplacedbyan¾ORgroup• β-D-glucopyranose with methanol and acid
gives a mixture of a and β methyl D-glucopyranosides
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Reduction of Monosaccharides
• Treatmentofanaldoseorketose withNaBH4reducesittoapolyalcohol(alditol)
• Reactionviatheopen-chainforminthealdehyde/ketonehemiacetal equilibrium
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Oxidation of Monosaccharides
• Aldosesareeasilyoxidizedtocarboxylicacidsby:Tollens'reagent(Ag+,NH3),Fehling'sreagent(Cu2+,sodiumtartrate),Benedict`sreagent(Cu2+,sodiumcitrate)• Oxidations generate metal mirrors; serve as
tests for “reducing” sugars (produce metallic mirrors)
• Ketosesarereducingsugarsiftheycanisomerizetoaldoses
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Oxidation of Monosaccharides
• Aldosesareeasilyoxidizedtocarboxylicacidsby:Tollens'reagent(Ag+,NH3),Fehling'sreagent(Cu2+,sodiumtartrate),Benedict`sreagent(Cu2+sodiumcitrate)• Oxidations generate metal mirrors; serve as
tests for “reducing” sugars (produce metallic mirrors)
• Ketosesarereducingsugarsiftheycanisomerizetoaldoses
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Oxidation of Monosaccharideswith Bromine
• Br2 inwaterisaneffectiveoxidizingreagentforconvertingaldosestocarboxylicacid,calledaldonic acids(themetalreagentsareforanalysisonly)
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Formation of Dicarboxylic Acids
• WarmdiluteHNO3 oxidizesaldosestodicarboxylic acids,calledaldaric acids
• The¾CHOgroupandtheterminal¾CH2OHgroupareoxidizedtoCOOH
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Summary
• Weonlycoveredmonosaccharidesandtheirreactions.
• Highlyrecommendreadingthischapterespeciallyifyouplantotakebiochemistryinthefuture.
• Wecovered25.1,25.2,25.3,25.5,25.6(throughoxidationpg.853).
• DonotneedtoknowKoenigs-Knorrreaction(pg.850-851)orBiologicalEsterFormation(Pg.851-852).
• Forfinalexam,practiceproblemsfromtextbook:1-7,11-20.