chapter · pdf filechapter 15 dienes, resonance, and aromacity organic chemistry, 5th ed....

Chapter15Dienes,Resonance,andAroma0city

Organic Chemistry, 5th ed. Marc Loudon

Eric J. Kantorowski California Polytechnic State University San Luis Obispo, CA

Chapter15Overview

•  15.1StructureandStabilityofDienes•  15.2Ultraviolet‐VisibleSpectroscopy•  15.3TheDiels‐AlderReacAon•  15.4Review•  15.5Free•  15.6RelaAonship•  15.7Free

2

Classifica3onofDienes

•  Dienes

•  Cumulenes

•  Non‐conjugated(“ordinary”)dienes

315.1StructureandStabilityofDienes

HeatsofForma3on


OriginofConjugatedDieneStability

•  σbondswithmores‐characterarestronger

•  ThesebondingdifferencesareesAmatedtoaccountfor5‐6kJmol‐1ofthestabilizaAon

•  TheremainingstabilizaAonisaTributedtotheπsystem


MOPictureof1,3‐Butadiene


Delocaliza3on

•  Theπ1MOshowsthattheelectrondensityisspreadacrosstheen0remolecule

•  Theelectronsaresaidtobedelocalized


StructureofConjugatedDienes

•  Thesp2‐sp2bondshortensthelengthofC2‐C3•  Compare:sp3‐sp3=1.54Åandsp2‐sp3=1.50Å

•  Increasings‐characterdecreasesbondlength


Conforma3onsofDienes


Conforma3onsofDienes

•  NewmanprojecAons:


CumulatedDienes

•  Thecentralcarbonissp‐hybridized•  Thetwopibondsaremutuallyperpendicular

•  TheC=CstretchingabsorpAonis1950cm‐1,notfarfromthatofalkynes


CumulatedDienes


CumulatedDienes

•  Cumulateddienesmaybechiraleventhoughtheylackanasymmetriccarbon


UV‐VisSpectroscopy

•  DienesandordinaryalkenesdisplaysimilarIRandNMRspectra

•  UV‐visiblelightcanprobetheextentofconjugaAoninconjugatedsystems

•  Nearultraviolet(200–400nm)andvisiblelight(400–750nm)arethemostusefulregionsoftheEMspectrum

1415.2Ultraviolet‐VisibleSpectroscopy

TheUV‐VisSpectrum

•  Anabsorp0onspectrumisproducedinUV‐visspectroscopy


Absorbance

•  Absorbance(oropAcaldensity,OD)measurestheincidentradiaAonabsorbedbyasample

•  λmax:ThewavelengthatthemaximumofanabsorpAonpeak


Absorbance

•  Beer‐LambertLaw(orBeer’sLaw):

•  εisthemolarex3nc3oncoefficientormolarabsorp3vity

•  ExAncAoncoefficient’sof104–105M‐1cm‐1arecommonforconjugatedmolecules


•  UV‐visisbasedonexcitaAonofπelectrons

PhysicalBasisofUV‐VisSpectroscopy


UV‐VisandConjuga3on

•  UV‐visismostusefulforconjugatedsystems

•  Chromophore:ThestructuralfeatureofamoleculeresponsibleforitsUV‐visabsorpAon


UV‐VisSpectroscopyofConjugatedSystems

•  λmaxisthemostimportantpartoftheUV‐visspectrum

•  Thelongertheconjugatedπ‐electronsystem,thehigherthewavelengthofabsorp0on

•  π‐electronpromoAonoccursfromthehighest‐occupiedMOtothelowest‐unoccupiedMO(HOMO→LUMO)


•  Longerπ‐system→higherλmax



Es3ma3ngλmax

•  ConformaAonaleffects:

•  SubsAtuenteffects:


TheDiels‐AlderReac3on

•  Conjugateddienescanreactwithalkenes

•  DiscoveredbyOToDielsandKurtAlderin1928(sharedNobelPrizeinchemistry1950)

2515.3TheDiels‐AlderReac0on

TheDiels‐AlderReac3on

•  Cycloaddi3onreac3on:AnaddiAonreacAonthatresultsinformaAonofaring

•  TheD‐AreacAoncanalsobeclassifiedasa1,4‐addi3onreac3onoraconjugateaddi3on


PericyclicReac3ons

•  AconcertedreacAonthatinvolvesacyclicflowofelectrons

•  TheD‐AreacAoncanalsobeclassifiedasapericyclicreac3on


OtherDienophilesandDienes

•  Alkynescanalsoserveasthedienophile

•  Cyclicdienescanleadtobicycliccompounds


EffectofDieneConforma3on


Conforma3onandReac3vity


Transi3onStateoftheDiels‐AlderReac3on


StereochemistryoftheDiels‐AlderReac3on

•  TheconcertedtransiAonstateresultsinsyn‐addiAonofthedienetothedienophile



•  Syn‐addiAonisalsorevealedwithsubsAtuentsattheterminalendsofthediene



•  AlsonotethedifferentreacAoncondiAons



•  ThepreviousreacAonsalsohaveaddiAonalstereochemistryattheringjuncAon

•  Thisissueariseswhenboththeterminalcarbonsofthedieneandthecarbonsofthedienophilearestereocenters


EndoandExoProducts

•  Endoproduct:ThealkenesubsAtuentsarecistotheouterdienesubsAtuents(Ro)

•  Exoproduct:ThealkenesubsAtuentsaretranstotheouterdienesubsAtuents(Ro)

•  Theendoproductgenerallyformsfaster


EndoandExoProducts

•  Endovsexoproductsarealsoobservedwhencyclicdienesareused


1,2‐and1,4‐Addi3ons

•  ConjugateddienesreactwithH‐Xlikenormalalkenes,buttwotypesofaddiAoncanoccur

•  1,2‐addi3on:Occursatadjacentcarbons•  1,4‐addi3on:Occurstocarbonsthathavea1,4‐relaAonship

3815.4Addi0onofHydrogenHalidestoConjugatedDienes

AllylicCarboca3ons


MODiagramoftheAllylCa3on


Delocaliza3onintheAllylCarboca3on

•  Theπ1MOshowsthatπ electronsarespreadacrossallthreecarbons

•  LewisnotaAon:

•  Theπ2MOshowsthatelectrondeficiencyissplitbetweenC1andC3


Kine3candThermodynamicControl

•  AtlowT,thelessstable1,2‐productdominates

•  ThemajorproductcanbeidenAfiedunderequilibraAngcondiAons



•  Kine3ccontrol:WhenproductsdonotcomeintoequilibriumunderthereacAoncondiAons

•  Thermodynamiccontrol:WhenproductsdocomeintoequilibriumunderthereacAoncondiAons



•  Anionpairexplainswhythe1,2‐productisformedmorerapidly


DienePolymers

•  1,3‐ButadieneisoneofthemostimportantrawmaterialsofthesyntheAcrubberindustry

•  Itcanbepolymerized:

4815.5DienePolymers

DienePolymers

•  Acopolymercanalsobeformedifstyreneisadded

4915.5DienePolymers

DienePolymers

•  Naturalrubberisanotherdienepolymer

5015.5DienePolymers

•  Vulcaniza0oncanbeusedtoincreasethestrengthandrigidityofthepolymer

Resonance

•  ResonancestructuresdescribeamoleculewhenasingleLewisstructureisinadequate

•  Moleculeswithresonancehybridsareresonance‐stabilized

•  Resonancestructureshelpusunderstandmolecularstructure,stability,andreac0vity

5115.6Resonance

DrawingResonanceStructures

•  Atomsarenotmovedinresonancestructures•  CurvedarrownotaAonisuseful

5215.6Resonance

Rela3veImportanceofResonanceStructures

•  IdenAcalstructuresareequallyimportant

•  CompleAngoctets

5315.6Resonance


•  PosiAvechargesandelectronegaAveatoms

5415.6Resonance


•  StructurallimitaAons

5515.6Resonance

ResonanceandMolecularGeometry

•  ThelemstructureimpliesthatNissp3andtetrahedral

•  …buttherightstructureimpliessp2andtrigonalplanar

•  ThehybridizaAonopAonthatmaximizesanatom’sabilitytointeractmosteffec0velywithitsneighborsisbest–inthiscase,Nissp2

5615.6Resonance

UseofResonanceStructures

•  Allotherthingsbeingequal,themoleculewiththegreaternumberofresonancestructuresismorestable

•  Thiswasencounteredinthe1,2‐vs1,4‐addiAonofH‐Xtoadiene:

5715.6Resonance

Aroma3cCompounds

•  ManyfragrantcompoundsarederivaAvesofbenzene

5815.7Introduc0ontoAroma0cCompounds

Benzene,aPuzzling“Alkene”

•  Kekuléproposedthecurrentstructureforbenzenein1865(derivedfromadream)

•  TheKekuléstructureportraysbenzeneasacyclictriene

•  However,benzene’sreacAvitydoesnotparallelthatofalkenes


Benzene,aPuzzling“Alkene”

•  Cyclohexeneaddsbrominereadily

•  …asdoes1,3,5,7‐cyclooctatetraene(COT)


StructureofBenzene

•  Kekulé’s“triene”vsbenzeneandresonance

•  Benzeneisonecompound

•  IthasonetypeofC‐Cbondthatissomewherebetweenasingleanddoublebond


ComparisonofStructures


TheBenzeneπSystem

•  Eachcarboninbenzeneistrigonalplanarandsp2hybridized

•  Hence,alloftheporbitalsinbenzeneareparallelandreadilyoverlap


TheCyclooctatetraeneπSystem

•  COTisnotplanar,buttub‐shaped


StabilityofBenzene

•  ReacAvity(orlackofit)isacrudewaytomeasureenergycontent

•  Amoreprecisewayisviastandardheatsofforma0on,ΔH°f

•  Benzene:ΔH°fperCHgroupis13.8kJmol‐1

•  COT:ΔH°fperCHgroupis37.3kJmol‐1

•  Theempiricalresonanceenergyinbenzeneis141kJmol‐1[(37.3–13.8)x6CHgroups]


CriteriaforAroma3city

1.AromaAccompoundscontainoneormoreringswithacyclicarrangementofporbitals

2.EveryatominanaromaAcringhasaporbital

3.AromaAcringsareplanar(orverynearlyso)

4.Theseporbitalsmustcontain4n+2πelectrons(n=0,1,2,…)

•  ThesecriteriaarecollecAvelycalledHückel’s4n+2ruleorthe4n+2rule


FrostCircle


Summaryofthe4n+2Rule

•  Cyclicconjugatedmoleculesandionswith4n+2electronsfillthebondingMOs

•  ThebondingMOsinthesecompoundshaveverylowenergy

•  Asaresult,thesemoleculeshavealargeresonanceenergy


Aroma3cHeterocycles


Aroma3cIons


Aroma3cPolycyclicCompounds

•  TheHückel4n+2ruleappliesstrictlytosingleringcompounds

•  However,manyfusedbicyclicandpolycycliccompoundsarealsoaromaAc


Aroma3cOrganometallicCompounds


An3aroma3cCompounds

•  An3aroma3ccompounds:Containplanar,con0nuousringsof4nelectrons

•  AnAaromaAccompoundsareespeciallyunstable

•  TheFrostcirclewouldshowtwohalf‐filleddegenerateorbitals(π2andπ3)


Non‐Aroma3cCompounds

•  COTcontainsaconAnuouscycleof4nelectrons

•  However,COTisnotplanartherebyavoidinganAaromaAccharacter

•  ItisesAmatedthatplanarCOT(whichisanAaromaAc)is58kJmol‐1lessstablethanthetubconformaAon


chapter · pdf filechapter 15 dienes, resonance, and aromacity organic chemistry, 5th ed....

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