Studies on Organic Magnetic Resonance Contrast Agents for ...
Drawing Resonance Structures-- organic chemistry
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Chemists commonly use the following types of arrows: • A double‐headed arrow links two resonance structures • Two half‐headed arrows indicate an equilibrium • A curved arrow indicates the movement of an electron pair in the direction of the arrowhead • A curved half‐headed arrow indicates the movement of a single electron in the direction of the arrowhead RESONANCE: sometimes all the properties of a molecule can’t be represented by a single structure but by two or more structures. These contributing structures are known as canonical forms or resonating structures. The actual molecule is represented by a hybrid of these canonical forms. Resonating structures are not the structures for the actual molecule op ion; they exist only on paper. Resonance is a hypothetical concept used to explain the extra stability of some molecules or ions. Rules for writing the resonance structures: • All the canonical forms must be proper Lewis structures. For instance, none of them may have a carbon with five bonds. • The positions of the nuclei must be the same in all the structures. We are allowed only to move electrons and no atom should be displaced from its position. • All atoms taking part in the resonance, that is, covered by delocalized electrons, must lie in a plane or nearly so. This, of course, does not apply to atoms that have the same www.canvas4u.org
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Transcript of Drawing Resonance Structures-- organic chemistry
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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