Chemistscommonlyusethefollowingtypesofarrows:

• Adouble‐headedarrowlinkstworesonancestructures

• Twohalf‐headedarrowsindicateanequilibrium

• Acurvedarrowindicatesthemovementofanelectronpairinthedirectionofthearrowhead

• Acurvedhalf‐headedarrowindicatesthemovementofasingleelectroninthedirectionofthearrowhead

RESONANCE:sometimesallthepropertiesofamoleculecan’tberepresentedbyasinglestructurebutbytwoormorestructures.Thesecontributingstructuresareknownas

canonicalformsorresonatingstructures.Theactualmoleculeisrepresentedbyahybridof

thesecanonicalforms.Resonatingstructuresarenotthestructuresfortheactualmoleculeop

ion;theyexistonlyonpaper.Resonanceisahypotheticalconceptusedtoexplaintheextra

stabilityofsomemoleculesorions.

Rulesforwritingtheresonancestructures:

• AllthecanonicalformsmustbeproperLewisstructures.Forinstance,noneofthem

mayhaveacarbonwithfivebonds.

• Thepositionsofthenucleimustbethesameinallthestructures.Weareallowedonly

tomoveelectronsandnoatomshouldbedisplacedfromitsposition.

• Allatomstakingpartintheresonance,thatis,coveredbydelocalizedelectrons,must

lieinaplaneornearlyso.This,ofcourse,doesnotapplytoatomsthathavethesame

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bondinginallthecanonicalforms.Thereasonforplanarityismaximumoverlapofthe

porbitals.

• Allcanonicalformsmusthavethesamenumberofunpairedelectrons.ThusCH2—

CH=CH—CH2isnotavalidcanonicalformforbutadiene.

• Theenergyoftheactualmoleculeislowerthanthatofanyform,obviously.Therefore,

delocalizationisastabilizingphenomenon.

• Allcanonicalformsdonotcontributeequallytothetruemolecule.Eachform

contributesinproportiontoitsstability,themoststableformcontributingmost.

Equivalentcanonicalforms,suchas1and2,contributeequally.Thegreaterthe

numberofsignificantstructuresthatcanbewrittenandthemorenearlyequalthey

are,thegreatertheresonanceenergy,otherthingsbeingequal.

• Structureswithmorecovalentbondsareordinarilymorestablethanthosewithfewer.

• Stabilityisdecreasedbyanincreaseinchargeseparation.Structureswithformal

chargesarelessstablethanunchargedstructures.Structureswithmorethantwo

formalchargesusuallycontributeverylittle.Anespeciallyunfavorabletypeof

structureisonewithtwolikechargesonadjacentatoms.

• Structuresthatcarryanegativechargeonamoreelectronegativeatomaremore

stablethanthoseinwhichthechargeisonalesselectronegativeatom.

DrawingResonanceStructures

SeeingCationsNexttoaDoubleBond,TripleBond,orLonePairOnepatternthatindicatesthatmorethanoneresonancestructureexistsforamoleculeisa

cationsituatednexttoadoublebond,atriplebond,oralonepair.Acationisorganic‐speak

forapositivelychargedatom;whenacationislocatedonacarbonatom,thechargeisoften

specificallyreferredtoasacarbocation.

Todrawalternativeresonancestructuresmatchingthispattern,followthesesteps:

1. Pushtheelectronsfromthedoublebond,triplebond,orlonepairtowardthe

chargedatom.

Drawanarrowfromtheelectronsyou’removingtowardthepositivelycharged

atom.

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2. Movethearrowscorrectly.

Arrowsrepresentthemovementoftwoelectronsfromwheretheystartedtowhere

they’regoing,somakesurethearrowstartsinoneoftheseplaces:

• Atthedoubleortriplebond,toindicatethemovementofthesepielectrons

• Fromthelonepairofelectrons

Thenmakethearrowpointbetweenthepositivelychargedatomandtheclosest

adjacentatomtoindicatethatthesetwoelectronsarebeingmovedhereinorderto

formanewdoublebond.

3. Drawtheresonancestructureshowingthenewdoublebond,andcheckyour

charges.

Thepositivechargeshould’vemovedeithertooneoftheoriginaldouble‐bondor

triple‐bondatoms(ifyoumovedadoubleortriplebond)ortotheatomthatoriginally

containedthelonepair(ifyoumovedalonepair).

Q.Drawtheotherresonancestructuresforthefollowingcationicmoleculebyarrowpushing.Afteryou’vedrawnalltheresonancestructures,drawwhattheactualstructureofthemoleculelookslike,includingcharges.

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Answer:

Q.Drawtheresonancestructureforthefollowingmoleculesbyusingarrow‐pushing.Afteryou’vedrawntheresonancestructure,sketchwhattheactualstructurelookslike,includingcharges.

(a) (b) (c) (d)

(e) (f)Answer:

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PushingLonePairsNexttoaDoubleorTripleBondAnotherpatternyouseethatindicatesthatamoleculecanbedescribedbymorethanone

Lewisstructureisanatomwithalonepairdirectlyattachedtoadoublebondortriplebond.

Todrawalternativeresonancestructuresmatchingthispattern,followthesesteps:

1. Pushtheelectronsfromthelonepairtowardthedoublebondortriplebond.

Toaccomplishthiselectronpushing,drawanarrowfromthelone‐pairelectrons

pointingtowardthecenteroftheadjacentbond.Thismoveindicatesthatyou’re

usingthelonepairtoformanewdoublebond.

2. Movethedouble­ortriplebondpielectronsontothefarthestatomasalone

pair.

Drawanarrowstartingfromthemiddleofthedoubleortriplebondandmovethe

electronsontothefarthestatominthedoubleortriplebondbypointingtheheadof

thearrowtothisatom.

3. Drawtheresonancestructureandcheckyourcharges.

Theatomwiththeinitiallonepairbecomesonechargemorepositiveinthe

resonancestructurethanwhatitstartedwith,andtheatomthatreceivedthe

double‐ortriplebondelectronsasalonepairbecomesonechargemorenegative

thanwhatitstartedwith.

Q.Drawtheotherresonancestructuresforthefollowingmoleculesbyarrow‐pushing.

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(a) (b) (c)

(d) (e)Answers:

PushingDoubleorTripleBondsContaininganElectronegativeAtomIfadoubleortriplebondcontainsanelectronegativeatom,suchasoxygenornitrogen,you

candrawaresonancestructureinwhichthedoublebondmovesontotheelectronegative

atomasalonepair.Forneutralmolecules,thisleadstoaresonancestructureinwhichthe

electronegativeatomhasanegativechargeandthelesselectronegativeatomisleftpositively

charged.

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Todrawalternativeresonancestructuresmatchingthispattern,followthesesteps:

1. Pushthepibondelectronsfromthedoubleortriplebondontothe

electronegativeatomasalonepair.

Startwithanarrowfromthemiddleofthedoubleortriplebondandpointittoward

theelectronegativeatom,showinghowthepibondelectronsarereassignedasa

lonepairontothemoreelectronegativeatom.

2. Drawtheresonancestructureandcheckthecharges.

Theelectronegativeatom,havingacceptedelectronsasalonepair,should’vebecome

onechargemorenegativeintheresonancestructurethanitbegan.Thus,if the

electronegativeatomstartedpositive,itshouldbecomeneutralintheresonance

structure.Likewise,themoreelectropositiveatomshould’vebecomeonecharge

unitmorepositiveintheresonancestructurethanitbegan.

*** Alonepairoremptyorbitalcannotinteractwithaπbondtowhichitisorthogonal

(perpendicular).E.g.

Q.Drawtheotherresonancestructureforthefollowingmoleculesbyarrow‐pushing.

(a) (b) (c) (d) (e)Answers:

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DrawingMultipleResonanceStructuresSometimesacompoundhasanumberofresonancestructures.Practicingonafewofthese

moleculesthatneedtobedescribedwithmultipleresonancestructuresisagoodideasoyou

cangetsomemorepracticeatspottingmoleculesthathaveresonancestructures.

Todrawmultipleresonancestructures,followthesesteps:

1. Lookforoneofthepatternstofindafirstalternativeresonancestructure.

Thefourpatternsareasfollows:

• Cationsnexttoadoublebond,triplebond,orlonepair

• Lonepairsnexttoadoubleortriplebond

• Doubleortriplebondscontaininganelectronegativeatom

• Doublebondsalternatingaroundaring

2. Afterdrawingthealternativeresonancestructure,lookforadditionalpatterns

inthenewresonancestructure.

Often,drawingaresonancestructureleadstoastructurethatmeetsoneofthe

precedingpatterns,inwhichcaseanadditionalresonancestructurecanbedrawn.

Repeatthisstepuntilyoucan’tfindanymorealternativeresonancestructures.

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Q.Drawalltheresonancestructuresforthefollowingmoleculesbyarrow‐pushing.

(a) (b) (c)

(d) (e)Answers:

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AssigningImportancetoResonanceStructuresAmoleculewithmorethanoneresonancestructurelookslikethehybridofalltheresonance

structures—butnotallresonancestructuresnecessarilycontributeequallytotheoverall

hybrid.Inotherwords,ifamoleculehastworesonancestructures,theactualstructuremay

lookmorelikeoneresonancestructurethantheother.

Generally,resonancestructuresthatcorrespondtostablestructurescontributemoretothe

overallhybridthandoresonancestructuresthatarelessstable.Sohowdoyoudetermine

stability?Herearesomegeneralguidelines:

• Resonancestructureswithfewerchargesaremorestablethanresonance

structureswithmorecharges.Forexample,aresonancestructurethathasno

chargescontributesmoretotheoverallhybridthanaresonancestructurethathas

two.Thisrulecomesasaresultoftheenergycostofseparatingcharges.

•

• Forthosemoleculesthatarecharged,themoststableresonancestructuresare

theonesthatplacethechargeonthebestatom.Negativechargesprefertoreston

electronegativeatoms(likeoxygenandnitrogen),andpositivechargesprefertorest

onelectropositiveatoms(likecarbon).

• Resonancestructuresthathaveallatomswithcompleteoctetsofelectronsare

morestablethanresonancestructuresthathaveatomswithincompleteoctets.

Thisrulecomesasaresultofthedesireofatomstoownacompleteoctetofelectrons.

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• Thedesireofamoleculetohaveallatomswithcompleteoctetstrumpsthe

desiretoputthechargeonthebestatoms.

Q.Predictwhichresonancestructureofthefollowingmoleculecontributesmoretotheoverallhybrid.

Answer:

Inthisexample,theright‐handresonancestructureismorestablethantheoneontheleft.In

therightresonancestructure,allatomsowncompleteoctetsofelectrons,whereasintheleft

structure,thecarbonistwoelectronsshortofanoctet.

Thus,eventhoughapositivechargepreferstorestoncarbonratherthanoxygen(because

carbonismoreelectropositivethanoxygen),thedesireofamoleculetohavefilledoctets

trumpsthedesiretoputthechargeonthebestatoms.Therefore,theactualstructureisa

mixtureofthetworesonancestructuresbutlooksmoreliketheresonancestructureonthe

rightthantheoneontheleft.

Q.Predictwhichresonancestructureofthefollowingmoleculescontributesmoretotheoverallhybrid.

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Answers:

Theneutralresonancestructurecontributesmoretotheoverallhybrid(theactualstructure)

becauseseparatingchargescostsenergy.

Theleft‐handresonancestructureinwhichthenegativechargerestsonoxygencontributes

moretotheoverallhybrid.

Theresonancestructurethatcontributesmosttotheoverallhybridistheright‐most

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structurebecausethisstructurehasallatomswithcompleteoctetsofelectrons.Recallthat

thedesiretohaveallatomswithcompleteoctetstrumpsthedesiretoputthechargesonto

themostdesirableatoms.

Trickquestion!Bothresonancestructuresareequallystable,sobothcontributeequallytothe

overallhybrid.

Q.Doyouthinkdelocalizationasshownbythefollowingresonancestructuresisimportant?Explainwhyorwhynot.

Answer:Theresonanceformswithapositivechargeonnitrogenoroxygendonotcontribute

significantlytostabilizationofthepositivecharge,becausethenitrogenandoxygendonot

haveanoctetofelectronswhentheybearthepositivecharge.Thefactthatoxygenor

nitrogendoesnothaveanoctetisacriticalpoint.Whenoxygenandnitrogendohaveanoctet,

theycancontributesignificantlytoaresonancehybridinwhichtheyarepositivelycharged.

Forexample,theadjacentoxygenstabilizesthe1‐methoxyethylcarbocationsignificantly:

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