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

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

•  NewmanprojecAons:

CumulatedDienes

•  Thecentralcarbonissp‐hybridized•  Thetwopibondsaremutuallyperpendicular

•  TheC=CstretchingabsorpAonis1950cm‐1,notfarfromthatofalkynes

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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