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Resonance:DrawingResonanceStructures

ExerciseSolutions

a.Astructurewithapositivechargeonacarbonimmediatelyadjacenttoanalkeneistermedanallyliccarbocation.Shiftingtheelectronsgivesustheonlyotherreasonableresonancestructure.

b.Thismoleculeistoluene.Thefunctionalgroupisabenzenering.Shiftingallthreedoublebondsprovidestheonlyotherreasonableresonancestructure.Thisresonancehasaprofoundinfluenceontheextrastabilityassociatedwiththebenzenering(Chapter9ofthetext).Thedirectionoftheelectronflow(clockwiseorcounterclockwise)doesnotmatter,asitgetsustothesameplace.

c.Astructurewithapositivechargeonacarbonimmediatelyadjacenttoabenzeneringistermedabenzyliccarbocation.Ithasseveralresonanceformsthatarisefromshiftingthepositionofthepositivechargeaswellasshiftingthepositionofthedoublebondswithinthebenzeneringasinthepreviousproblem.

d.Justbecauseamoleculeisneutraldoesnotmeanitdoesnothaveresonancestructures.WeseethiswithCO(carbonmonoxide)aswellaswithtoluene(questionb).

e.Whendrawingresonancestructures,alonepaircanbemovedinthesamewayasabond.Inthiscase,thecarbonbearingthelonepairandnegativechargeisthedonatoratom,andthecarbonatomofthecarbonylgroup(C=O)istheacceptoratom.Anmoleculethatbearsanegativechargenexttoacarbonylgroupistermedanenolate.Resonanceinvolvingalonepairofanelectronsorapositivechargenexttoabondoccursinmanystructures.

f.Thisisanothercaseoflonepairbondresonanceinteraction.Thelonepairdoesnothavetobeon

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anatomthatbearsaformalnegativechargeasinthepreviousquestion.

g.Phosphorousisathirdrowelement,soitcanexpanditsoctettocontaintenvalenceelectrons.(Phosphorusistheonlyatomcommonlyencounteredinorganicchemistrythatroutinelyexpandsitsoctettotenvalenceelectrons.)Theoxygenatomscaneachloanalonepairtothephosphorus,soseveralresonancestructuresresult.Theresonanceofphosphateionhasimportantbiochemicalconsequences.

h.Anyionwiththreebondstoanoxygenatomthatbearsapositivechargeiscalledanoxoniumion.Thisoxoniumionhasnootherresonancestructuresbecauseitdoesnothaveadjacentdonorandacceptoratoms.

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