Chapter · PDF fileChapter 15 Dienes, Resonance, and Aromacity Organic Chemistry, 5th ed....
Transcript of Chapter · PDF fileChapter 15 Dienes, Resonance, and Aromacity Organic Chemistry, 5th ed....
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Chapter15Dienes,Resonance,andAroma0city
Organic Chemistry, 5th ed. Marc Loudon
Eric J. Kantorowski California Polytechnic State University San Luis Obispo, CA
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Chapter15Overview
• 15.1StructureandStabilityofDienes• 15.2Ultraviolet‐VisibleSpectroscopy• 15.3TheDiels‐AlderReacAon• 15.4Review• 15.5Free• 15.6RelaAonship• 15.7Free
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Classifica3onofDienes
• Dienes
• Cumulenes
• Non‐conjugated(“ordinary”)dienes
315.1StructureandStabilityofDienes
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HeatsofForma3on
415.1StructureandStabilityofDienes
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OriginofConjugatedDieneStability
• σbondswithmores‐characterarestronger
• ThesebondingdifferencesareesAmatedtoaccountfor5‐6kJmol‐1ofthestabilizaAon
• TheremainingstabilizaAonisaTributedtotheπsystem
515.1StructureandStabilityofDienes
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MOPictureof1,3‐Butadiene
615.1StructureandStabilityofDienes
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Delocaliza3on
• Theπ1MOshowsthattheelectrondensityisspreadacrosstheen0remolecule
• Theelectronsaresaidtobedelocalized
715.1StructureandStabilityofDienes
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StructureofConjugatedDienes
• Thesp2‐sp2bondshortensthelengthofC2‐C3• Compare:sp3‐sp3=1.54Åandsp2‐sp3=1.50Å
• Increasings‐characterdecreasesbondlength
815.1StructureandStabilityofDienes
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Conforma3onsofDienes
915.1StructureandStabilityofDienes
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Conforma3onsofDienes
• NewmanprojecAons:
1015.1StructureandStabilityofDienes
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CumulatedDienes
• Thecentralcarbonissp‐hybridized• Thetwopibondsaremutuallyperpendicular
• TheC=CstretchingabsorpAonis1950cm‐1,notfarfromthatofalkynes
1115.1StructureandStabilityofDienes
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CumulatedDienes
1215.1StructureandStabilityofDienes
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CumulatedDienes
• Cumulateddienesmaybechiraleventhoughtheylackanasymmetriccarbon
1315.1StructureandStabilityofDienes
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UV‐VisSpectroscopy
• DienesandordinaryalkenesdisplaysimilarIRandNMRspectra
• UV‐visiblelightcanprobetheextentofconjugaAoninconjugatedsystems
• Nearultraviolet(200–400nm)andvisiblelight(400–750nm)arethemostusefulregionsoftheEMspectrum
1415.2Ultraviolet‐VisibleSpectroscopy
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TheUV‐VisSpectrum
• Anabsorp0onspectrumisproducedinUV‐visspectroscopy
1515.2Ultraviolet‐VisibleSpectroscopy
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Absorbance
• Absorbance(oropAcaldensity,OD)measurestheincidentradiaAonabsorbedbyasample
• λmax:ThewavelengthatthemaximumofanabsorpAonpeak
1615.2Ultraviolet‐VisibleSpectroscopy
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Absorbance
• Beer‐LambertLaw(orBeer’sLaw):
• εisthemolarex3nc3oncoefficientormolarabsorp3vity
• ExAncAoncoefficient’sof104–105M‐1cm‐1arecommonforconjugatedmolecules
1715.2Ultraviolet‐VisibleSpectroscopy
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• UV‐visisbasedonexcitaAonofπelectrons
PhysicalBasisofUV‐VisSpectroscopy
1815.2Ultraviolet‐VisibleSpectroscopy
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UV‐VisandConjuga3on
• UV‐visismostusefulforconjugatedsystems
• Chromophore:ThestructuralfeatureofamoleculeresponsibleforitsUV‐visabsorpAon
1915.2Ultraviolet‐VisibleSpectroscopy
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UV‐VisSpectroscopyofConjugatedSystems
• λmaxisthemostimportantpartoftheUV‐visspectrum
• Thelongertheconjugatedπ‐electronsystem,thehigherthewavelengthofabsorp0on
• π‐electronpromoAonoccursfromthehighest‐occupiedMOtothelowest‐unoccupiedMO(HOMO→LUMO)
2015.2Ultraviolet‐VisibleSpectroscopy
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UV‐VisSpectroscopyofConjugatedSystems
2115.2Ultraviolet‐VisibleSpectroscopy
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• Longerπ‐system→higherλmax
UV‐VisSpectroscopyofConjugatedSystems
2215.2Ultraviolet‐VisibleSpectroscopy
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UV‐VisSpectroscopyofConjugatedSystems
2315.2Ultraviolet‐VisibleSpectroscopy
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Es3ma3ngλmax
• ConformaAonaleffects:
• SubsAtuenteffects:
2415.2Ultraviolet‐VisibleSpectroscopy
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TheDiels‐AlderReac3on
• Conjugateddienescanreactwithalkenes
• DiscoveredbyOToDielsandKurtAlderin1928(sharedNobelPrizeinchemistry1950)
2515.3TheDiels‐AlderReac0on
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TheDiels‐AlderReac3on
• Cycloaddi3onreac3on:AnaddiAonreacAonthatresultsinformaAonofaring
• TheD‐AreacAoncanalsobeclassifiedasa1,4‐addi3onreac3onoraconjugateaddi3on
2615.3TheDiels‐AlderReac0on
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PericyclicReac3ons
• AconcertedreacAonthatinvolvesacyclicflowofelectrons
• TheD‐AreacAoncanalsobeclassifiedasapericyclicreac3on
2715.3TheDiels‐AlderReac0on
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OtherDienophilesandDienes
• Alkynescanalsoserveasthedienophile
• Cyclicdienescanleadtobicycliccompounds
2815.3TheDiels‐AlderReac0on
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EffectofDieneConforma3on
2915.3TheDiels‐AlderReac0on
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Conforma3onandReac3vity
3015.3TheDiels‐AlderReac0on
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Transi3onStateoftheDiels‐AlderReac3on
3115.3TheDiels‐AlderReac0on
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StereochemistryoftheDiels‐AlderReac3on
• TheconcertedtransiAonstateresultsinsyn‐addiAonofthedienetothedienophile
3215.3TheDiels‐AlderReac0on
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StereochemistryoftheDiels‐AlderReac3on
• Syn‐addiAonisalsorevealedwithsubsAtuentsattheterminalendsofthediene
3315.3TheDiels‐AlderReac0on
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StereochemistryoftheDiels‐AlderReac3on
• AlsonotethedifferentreacAoncondiAons
3415.3TheDiels‐AlderReac0on
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StereochemistryoftheDiels‐AlderReac3on
• ThepreviousreacAonsalsohaveaddiAonalstereochemistryattheringjuncAon
• Thisissueariseswhenboththeterminalcarbonsofthedieneandthecarbonsofthedienophilearestereocenters
3515.3TheDiels‐AlderReac0on
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EndoandExoProducts
• Endoproduct:ThealkenesubsAtuentsarecistotheouterdienesubsAtuents(Ro)
• Exoproduct:ThealkenesubsAtuentsaretranstotheouterdienesubsAtuents(Ro)
• Theendoproductgenerallyformsfaster
3615.3TheDiels‐AlderReac0on
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EndoandExoProducts
• Endovsexoproductsarealsoobservedwhencyclicdienesareused
3715.3TheDiels‐AlderReac0on
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1,2‐and1,4‐Addi3ons
• ConjugateddienesreactwithH‐Xlikenormalalkenes,buttwotypesofaddiAoncanoccur
• 1,2‐addi3on:Occursatadjacentcarbons• 1,4‐addi3on:Occurstocarbonsthathavea1,4‐relaAonship
3815.4Addi0onofHydrogenHalidestoConjugatedDienes
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1,2‐and1,4‐Addi3ons
3915.4Addi0onofHydrogenHalidestoConjugatedDienes
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1,2‐and1,4‐Addi3ons
4015.4Addi0onofHydrogenHalidestoConjugatedDienes
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AllylicCarboca3ons
4115.4Addi0onofHydrogenHalidestoConjugatedDienes
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MODiagramoftheAllylCa3on
4215.4Addi0onofHydrogenHalidestoConjugatedDienes
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Delocaliza3onintheAllylCarboca3on
• Theπ1MOshowsthatπ electronsarespreadacrossallthreecarbons
• LewisnotaAon:
• Theπ2MOshowsthatelectrondeficiencyissplitbetweenC1andC3
4315.4Addi0onofHydrogenHalidestoConjugatedDienes
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Kine3candThermodynamicControl
• AtlowT,thelessstable1,2‐productdominates
• ThemajorproductcanbeidenAfiedunderequilibraAngcondiAons
4415.4Addi0onofHydrogenHalidestoConjugatedDienes
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Kine3candThermodynamicControl
• Kine3ccontrol:WhenproductsdonotcomeintoequilibriumunderthereacAoncondiAons
• Thermodynamiccontrol:WhenproductsdocomeintoequilibriumunderthereacAoncondiAons
4515.4Addi0onofHydrogenHalidestoConjugatedDienes
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Kine3candThermodynamicControl
4615.4Addi0onofHydrogenHalidestoConjugatedDienes
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Kine3candThermodynamicControl
• Anionpairexplainswhythe1,2‐productisformedmorerapidly
4715.4Addi0onofHydrogenHalidestoConjugatedDienes
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DienePolymers
• 1,3‐ButadieneisoneofthemostimportantrawmaterialsofthesyntheAcrubberindustry
• Itcanbepolymerized:
4815.5DienePolymers
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DienePolymers
• Acopolymercanalsobeformedifstyreneisadded
4915.5DienePolymers
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DienePolymers
• Naturalrubberisanotherdienepolymer
5015.5DienePolymers
• Vulcaniza0oncanbeusedtoincreasethestrengthandrigidityofthepolymer
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Resonance
• ResonancestructuresdescribeamoleculewhenasingleLewisstructureisinadequate
• Moleculeswithresonancehybridsareresonance‐stabilized
• Resonancestructureshelpusunderstandmolecularstructure,stability,andreac0vity
5115.6Resonance
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DrawingResonanceStructures
• Atomsarenotmovedinresonancestructures• CurvedarrownotaAonisuseful
5215.6Resonance
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Rela3veImportanceofResonanceStructures
• IdenAcalstructuresareequallyimportant
• CompleAngoctets
5315.6Resonance
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Rela3veImportanceofResonanceStructures
• PosiAvechargesandelectronegaAveatoms
5415.6Resonance
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Rela3veImportanceofResonanceStructures
• StructurallimitaAons
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ResonanceandMolecularGeometry
• ThelemstructureimpliesthatNissp3andtetrahedral
• …buttherightstructureimpliessp2andtrigonalplanar
• ThehybridizaAonopAonthatmaximizesanatom’sabilitytointeractmosteffec0velywithitsneighborsisbest–inthiscase,Nissp2
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UseofResonanceStructures
• Allotherthingsbeingequal,themoleculewiththegreaternumberofresonancestructuresismorestable
• Thiswasencounteredinthe1,2‐vs1,4‐addiAonofH‐Xtoadiene:
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Aroma3cCompounds
• ManyfragrantcompoundsarederivaAvesofbenzene
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Benzene,aPuzzling“Alkene”
• Kekuléproposedthecurrentstructureforbenzenein1865(derivedfromadream)
• TheKekuléstructureportraysbenzeneasacyclictriene
• However,benzene’sreacAvitydoesnotparallelthatofalkenes
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Benzene,aPuzzling“Alkene”
• Cyclohexeneaddsbrominereadily
• …asdoes1,3,5,7‐cyclooctatetraene(COT)
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StructureofBenzene
• Kekulé’s“triene”vsbenzeneandresonance
• Benzeneisonecompound
• IthasonetypeofC‐Cbondthatissomewherebetweenasingleanddoublebond
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ComparisonofStructures
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TheBenzeneπSystem
• Eachcarboninbenzeneistrigonalplanarandsp2hybridized
• Hence,alloftheporbitalsinbenzeneareparallelandreadilyoverlap
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TheCyclooctatetraeneπSystem
• COTisnotplanar,buttub‐shaped
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StabilityofBenzene
• ReacAvity(orlackofit)isacrudewaytomeasureenergycontent
• Amoreprecisewayisviastandardheatsofforma0on,ΔH°f
• Benzene:ΔH°fperCHgroupis13.8kJmol‐1
• COT:ΔH°fperCHgroupis37.3kJmol‐1
• Theempiricalresonanceenergyinbenzeneis141kJmol‐1[(37.3–13.8)x6CHgroups]
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CriteriaforAroma3city
1.AromaAccompoundscontainoneormoreringswithacyclicarrangementofporbitals
2.EveryatominanaromaAcringhasaporbital
3.AromaAcringsareplanar(orverynearlyso)
4.Theseporbitalsmustcontain4n+2πelectrons(n=0,1,2,…)
• ThesecriteriaarecollecAvelycalledHückel’s4n+2ruleorthe4n+2rule
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FrostCircle
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Summaryofthe4n+2Rule
• Cyclicconjugatedmoleculesandionswith4n+2electronsfillthebondingMOs
• ThebondingMOsinthesecompoundshaveverylowenergy
• Asaresult,thesemoleculeshavealargeresonanceenergy
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Aroma3cHeterocycles
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Aroma3cIons
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Aroma3cPolycyclicCompounds
• TheHückel4n+2ruleappliesstrictlytosingleringcompounds
• However,manyfusedbicyclicandpolycycliccompoundsarealsoaromaAc
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Aroma3cOrganometallicCompounds
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An3aroma3cCompounds
• An3aroma3ccompounds:Containplanar,con0nuousringsof4nelectrons
• AnAaromaAccompoundsareespeciallyunstable
• TheFrostcirclewouldshowtwohalf‐filleddegenerateorbitals(π2andπ3)
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Non‐Aroma3cCompounds
• COTcontainsaconAnuouscycleof4nelectrons
• However,COTisnotplanartherebyavoidinganAaromaAccharacter
• ItisesAmatedthatplanarCOT(whichisanAaromaAc)is58kJmol‐1lessstablethanthetubconformaAon
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