klein2e lecture ch18 - university of...

Chapter 18Aromatic Compounds

Organic ChemistrySecond Edition

David Klein

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

18.1IntroductiontoAromaticCompounds

• Aromatic compoundsorarenes includebenzeneandbenzenederivatives

• Manyaromaticcompoundswereoriginallyisolatedfromfragrantoils

• However,manyaromaticcompoundsareodorless• Aromaticcompoundsarequitecommon

Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 18-2 Klein, Organic Chemistry 2e


• 8ofthe10best-sellingdrugshavearomaticmoieties



• Coalcontainsaromaticringsfusedtogetherandjoinedbynonromanticmoieties


18.2NomenclatureofBenzeneDerivatives

• Benzeneisgenerallytheparentnameformonosubstituted derivatives



• Manybenzenederivativeshavecommonnames.• Forsomecompounds,thecommonnamebecomestheparentname.



• Ifthesubstituentislargerthanthering,thesubstituentbecomestheparentchain

• AromaticringsareoftenrepresentedwithaPh(forphenyl)orwithaφ (phi)symbol



• Thecommonnamefordimethyl benzenederivativesisxylene

• Whatdoortho,meta,andparamean?



1. Identifytheparentchain(thelongestconsecutivechainofcarbons)

2. IdentifyandNamethesubstituents3. Numbertheparentchainandassignalocant(and

prefixifnecessary)toeachsubstituent– Givethefirstsubstituentthelowestnumberpossible

4. Listthenumberedsubstituentsbeforetheparentnameinalphabeticalorder

– Ignoreprefixes(exceptiso)whenorderingalphabetically



• Locants arerequiredforringswithmorethan2substituents

1. Identifytheparentchain(generallythearomaticring)– Oftenacommonname

canbetheparentchain

2. IdentifyandNamethesubstituents


3. Numbertheparentchainandassignalocant(andprefixifnecessary)toeachsubstituent

– AsubstituentthatispartoftheparentnamemustbeassignedlocantNUMBER1

4. Listthenumberedsubstituentsbeforetheparentnameinalphabeticalorder

– Ignoreprefixes(exceptiso)whenorderingalphabetically– Completethenameforthemoleculeabove

• PracticewithSkillBuilder 18.1




• Namethefollowingmolecules


18.3StructureofBenzene• In1866,AugustKekuléproposedthatbenzeneisaring

comprisedofalternatingdoubleandsinglebonds

• Kekulésuggestedthattheexchangeofdoubleandsinglebondswasanequilibriumprocess


18.3StructureofBenzene• Wenowknowthatthearomaticstructuresare

resonancecontributorsratherthaninequilibrium

• HOWisresonancedifferentfromequilibrium?• Sometimestheringisrepresentedwithacircleinit• WHY?


18.4StabilityofBenzene• Thestabilitythatresultsfromaringbeingaromaticis

striking• Recallthatingeneral,alkenesreadilyundergoaddition

reactions

• Aromaticringsarestableenoughthattheydonotundergosuchreactions


18.4StabilityofBenzene• Heatsofhydrogenationcanbeusedtoquantify

aromaticstability.Practicewithconceptualcheckpoints18.6and18.7


18.4StabilityofBenzene• MOtheorycanhelpusexplainaromaticstability• The6atomicp-orbitals

ofbenzeneoverlaptomake6molecularorbitals


18.4StabilityofBenzene• ThelocationsofnodesintheMOsdeterminestheir

shapesbasedonhigh-levelmathematicalcalculations


18.4StabilityofBenzene• Thedelocalizationofthe6pielectronsinthethree

bondingmolecularorbitalsaccountsforthestabilityofbenzene


• Doeseveryfullyconjugatedcycliccompoundhavearomaticstability?NO

• Somefullyconjugatedcycliccompoundsarereactiveratherthanbeingstablelikebenzene

18.4StabilityofBenzene


• AROMATICcompoundsfulfilltwocriteria1. Afullyconjugatedringwithoverlappingp-orbitals2. MeetsHückel’srule: anODDnumberofe- pairsor4n+2

totalπ electronswheren=0,1,2,3,4,etc.

• ShowhowthemoleculesbelowdoNOTmeetthecriteria



• WecanexplainHückel’sruleusingMOtheory• Let’sconsidertheMOsforcyclobutadiene

• Theinstabilityoftheunpairedelectrons(similartofreeradicals)makesthisantiaromatic



• AsimilarMOanalysisforcyclooctatetraenesuggeststhatitisalsoantiaromatic



• However,ifthestructureadoptsatub-shapedconformation,itcanavoidtheantiaromatic instability

• Theconjugationdoesnotextendaroundtheentirering,sothesystemisneitheraromaticnorantiaromatic



• PredictingtheshapesandenergiesofMOsrequiressophisticatedmathematics,butwecanuseFrostcircles topredicttherelativeMOenergies



• Isthecompoundbelowaromaticorantiaromatic?HOW?

Theconjugationaroundtheoutsideofthemoleculeprovides14pielectron,anaromaticnumber.Thecenterpielectronswillnotbeinvolvedintheresonance• Practicewithconceptualcheckpoint18.8



• UsetheFrostcircles belowtoexplainthe4n+2rule

• Notethatthenumberofbondingorbitalsisalwaysanoddnumber- aromaticcompoundswillalwayshaveanoddnumberofelectronpairs

• Practicewithconceptualcheckpoint18.9



• AROMATICcompoundsfulfilltwocriteria1. Afullyconjugatedringwithoverlappingp-orbitals2. MeetsHückel’s rule: anODDnumberofe- pairsor4n+2


• ANTIAROMATICcompoundsfulfilltwocriteria1. Afullyconjugatedringwithoverlappingp-orbitals2. AnEVENnumberofelectronpairsor4n totalπ

electronswheren=0,1,2,3,4,etc.

18.5AromaticCompoundsOtherThanBenzene


• Annulenes areringsthatarefullyconjugated

• Someannulenes arearomatic,whileothersareantiaromatic

• [10]Annuleneisneither.WHY?• Practicewithconceptualcheckpoint18.10



• Someringsmustcarryaformalchargetobearomatic• Considera5-memberedring

• If6pielectronsarepresent,drawtheresonancecontributorsforthestructure



• ThepKa valueforcyclopentadiene ismuchlowerthantypicalC-Hbonds.WHY?

vs.



• Considera7-memberedring

• If6pielectronsarepresent,whatchargewillbenecessary?

• Drawtheresonancecontributorsforthestructure• PracticewithSkillBuilder 18.2



• Considera7-memberedring

• If6pielectronsarepresent,apositivechargewillbenecessary?



• Heteroatoms (atomsotherthanCorH)canalsobepartofanaromaticring



• Iftheheteroatom’slonepairisnecessary,itwillbeincludedintheHückelnumberofpielectrons



• Ifthelonepairisnecessarytomakeitaromatic,theelectronswillnotbeasbasic


pKa=5.2

pKa=0.4


• Thedifferenceinelectrondensitycanalsobeobservedbyviewingtheelectrostaticpotentialmaps

• PracticewithSkillBuilder 18.3



• Manypolycycliccompoundsarealsoaromatic

• Suchcompoundsareshowntobearomaticusingheatsofhydrogenation.HOW?



• Willthecompoundsbelowbearomatic,antiaromatic,ornonaromatic?

Antiaromatic neither aromatic antiaromatic4npie- lonepairs 10pie- unlessit

willavoid canavoidantiaromatic beingflat



• Showthatthemoleculesbelowmeetthecriteriaforaromaticity1. Afullyconjugatedringwithoverlappingp-orbitals2. MeetsHückel’srule: anODDnumberofe- pairsor4n+2




• Acarbonthatisattachedtoabenzeneringisbenzylic

• Recallthataromaticringsandalkylgroupsarenoteasilyoxidized

18.6ReactionsattheBenzylicPosition


• Ingeneral,benzylicpositionscanreadilybefullyoxidized

• Thebenzylicpositionneedstohaveatleast1protonattachedtoundergooxidation



• Permanganatecanalsobeusedasanoxidizingreagent

• Practicewithconceptualcheckpoint18.19



18.7ReductionoftheAromaticMoiety• Underforcefulconditions,benzenecanbereducedto

cyclohexane

• Istheprocessendothermicorexothermic?WHY?

• WHYareforcefulconditionsrequired?


18.7ReductionoftheAromaticMoiety• Vinylsidegroupscanbeselectivelyreduced

• ΔHisjustslightlylessthantheexpected-120kJ/molexpectedforaC=Cà C-Cconversion

• WHYarelessforcefulconditionsrequired?


Graphite,Buckyballs,andNanotubes• Graphiteconsistsoflayersofsheetsoffusedaromatic

rings


Graphite,Buckyballs,andNanotubes• BuckyballsareC60 spheresmadeofinterlocking

aromaticrings

• FullerenescomeinothersizessuchasC70

• HowareBuckyballsaromaticwhentheyarenotFLAT?


Graphite,Buckyballs,andNanotubes• Fullerenescanalsobemadeintotubes(cylinders)

• Single,double,andmulti-walledcarbonnanotubeshavemanyapplications:– ConductivePlastics,EnergyStorage,ConductiveAdhesives,MolecularElectronics,ThermalMaterials,FibresandFabrics,CatalystSupports,BiomedicalApplications


klein2e lecture ch18 - university of...

Documents