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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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Converting to Proper Structures

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Converting to Proper Structures

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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.