chapter-1-structure-and-教學講解課件1chapter-1-structure-and-教學講解課件2ChapterIStructure&Bonding1-1ElectronConfigurationsofAtoms1-2ChemicalBonding&Valence1-3ChargeDistributioninMolecules1-4TheShapeofMolecules1-5Isomers1-6Resonance1-71-8AtomicandMolecularOrbitalsThestudyoforganicchemistrymustatsomepointextendtothemolecularlevel,forthephysicalandchemicalpropertiesofasubstanceareultimatelyexplainedintermsofthestructureandbondingofmolecules.Thismodule模塊introducessomebasicfactsandprinciplesthatareneededforadiscussionoforganicmolecules.ChapterIStructure&Bonding31-1ElectronicConfigurationsElectronConfigurationsinthePeriodicTableTheperiodictableshownhereisseverelytruncated.Thereare,ofcourse,overeightyotherelements.1-1ElectronicConfigurations4Thehalogensareoneelectronshortofavalenceshelloctet,andareamongthemostreactiveoftheelements.Intheirchemicalreactionshalogenatomsachieveavalenceshelloctetbycapturingorborrowingtheeighthelectronfromanotheratomormolecule.Thealkalimetalsarealsoexceptionallyreactive,butfortheoppositereason.Theseatomshaveonlyoneelectroninthevalenceshell,andonlosingthiselectronarriveatthelowershellvalenceoctet.Asaconsequenceofthiselectronloss,theseelementsarecommonlyencounteredascations(positivelychargedatoms).

Thehalogensareonee5chapter-1-structure-and-教學講解課件61-2-1IonicBondingWhensodiumisburnedinachlorineatmosphere,itproducessodiumchloride.Thishasahighmeltingpoint(800oC)anddissolvesinwatertogiveaconductingsolution.Sodiumchlorideisanioniccompound,inwhichanelectronofsodiumatomwastransferredtoachlorineatomandgeneratesasodiumcationandachlorideanion.

Electrostaticattractionresultsintheseoppositelychargedionspackingtogetherinalattice.Theattractiveforcesholdingtheionsinplacecanbereferredtoasionicbonds.1-2ChemicalBondingandValence

1-2-1IonicBonding1-2Chemic71-2-2CovalentBondingWaterisaliquidatroomtemperature;carbondioxideandcarbontetrafluoridearegases.Noneofthesecompoundsiscomposedofions.Adifferentattractiveinteractionbetweenatoms,calledcovalentbonding,isinvolvedhere.Covalentbondingoccursbysharingofvalenceelectrons,ratherthananoutrightelectrontransfer.Similaritiesinphysicalproperties(theyareallgases)suggestthatthediatomicelementsH2,N2,O2,F2&Cl2alsohavecovalentbonds.

Carbondioxideisnotablebecauseitisacaseinwhichtwopairsofelectronsaresharedbythesametwoatoms.Thisisanexampleofadoublecovalentbond.1-2-2CovalentBonding81-2-3ValenceThenumberofelectronsanatomgainorlosetoachieveavalenceoctetiscalledvalence.Thevalenceshererepresentthemostcommonform普通形式inorganiccompounds.Manyelements,suchaschlorine,bromineandiodine,areknowntoexistinseveralvalencestatesindifferentinorganiccompounds.1-2-3ValenceThevalen91-3ChargeDistributionIftheelectronpairsincovalentbondsweresharedabsolutelyevenlytherewouldbenofixedlocalchargeswithinamolecule.AlthoughthisistruefordiatomicelementssuchasH2,N2andO2,mostcovalentcompoundsshowsomedegreeoflocalchargeseparation,resultinginbondand/ormoleculardipoles.1-3-1electronegativityDifferentatomshavedifferentaffinitiesfornearbyelectrons.Theabilityofanelementtoattractorholdontoelectronsiscalledelectronegativity.1-3ChargeDistribution10Fluorinehasthegreatestelectronegativityofalltheelements,andtheheavieralkalimetalssuchaspotassium,rubidiumandcesiumhavethelowestelectronegativities.Itshouldbenotedthatcarbonisaboutinthemiddleoftheelectronegativityrange,andisslightlymoreelectronegativethanhydrogen.Fluorinehasthegre11Whentwodifferentatomsarebondedcovalently,thesharedelectronsareattractedtothemoreelectronegativeatomofthebond,resultinginashiftofelectrondensitytowardthemoreelectronegativeatom.Suchacovalentbondispolar,andwillhaveadipole.

Thedegreeofpolarityandthemagnitudeofthebonddipolewillbeproportionaltothedifferenceinelectronegativityofthebondedatoms.ThusaO–HbondismorepolarthanaC–Hbond,withthehydrogenatomoftheformerbeingmorepositivethanthehydrogenbondedtocarbon.

Likewise,C–ClandC–Libondsarebothpolar,butthecarbonendispositiveintheformerandnegativeinthelatter.Thedipolarnatureofthesebondsisoftenindicatedbyapartialchargenotation(δ+/–)orbyanarrowpointingtothenegativeendofthebond.1-3-2PolarCovalentBondsWhentwodifferentat12Theshiftofelectrondensityinacovalentbondtowardthemoreelectronegativeatomorgroupcanbeobservedinseveralways.Forbondstohydrogen,acidityisonecriterion.Ifthebondingelectronpairmovesawayfromthehydrogennucleustheprotonwillbemoreeasilytransferedtoabase(itwillbemoreacidic).Methaneisalmostnon-acidic,sincetheC–Hbondisnearlynon-polar.TheO–Hbondofwaterispolar,anditisatleast25powersoftenmoreacidicthanmethane.H–Fisover12powersoftenmoreacidicthanwaterasaconsequenceofthegreaterelectronegativitydifferenceinitsatoms.

Electronegativitydifferencesmaybetransmittedthroughconnectingcovalentbondsbyaninductiveeffect.Thisinductivetransferofpolaritytapersoffasthenumberoftransmittingbondsincreases,andthepresenceofmorethanonehighlyelectronegativeatomhasacumulativeeffect.Forexample,trifluoroethanol,CF3CH2–O–Hisabouttenthousandtimesmoreacidicthanethanol,CH3CH2–O–H.Theshiftofelectronde131-3-2FunctionalGroupsFunctionalgroupsareatomsorsmallgroupsofatoms(twotofour)thatexhibitacharacteristicreactivitywhentreatedwithcertainreagents.Aparticularfunctionalgroupwillalmostalwaysdisplayitscharacteristicchemicalbehaviorwhenitispresentinacompound.

Becauseoftheirimportanceinunderstandingorganicchemistry,functionalgroupshavecharacteristicnamesthatoftencarryoverinthenamingofindividualcompoundsincorporatingspecificgroups.chapter-1-structure-and-教學講解課件14chapter-1-structure-and-教學講解課件15chapter-1-structure-and-教學講解課件16chapter-1-structure-and-教學講解課件171-4TheShapeofMoleculesThethreedimensionalshapeorconfigurationofamoleculeisanimportantcharacteristic.Threedimensionalconfigurationsarebestviewedwiththeaidofmodels.Inordertorepresentsuchconfigurationsonatwo-dimensionalsurface(paper,blackboardorscreen),weoftenuseperspectivedrawingsinwhichthedirectionofabondisspecifiedbythelineconnectingthebondedatoms.1-4TheShapeofMolecules18Asimplestraightlinerepresentsabondlyingapproximatelyinthesurfaceplane.ThetwobondstosubstituentsAinthestructureontheleftareofthiskind.Awedgeshapedbondisdirectedinfrontofthisplane(thickendtowardtheviewer),asshownbythebondtosubstituentB;andahatchedbondisdirectedinbackoftheplane(awayfromtheviewer),asshownbythebondtosubstituentD.

Asimplestraightline19Thefollowingexamplesmakeuseofthisnotation,andalsoillustratetheimportanceofincludingnon-bondingvalenceshellelectronpairs(coloredblue)whenviewingsuchconfigurations.Thefollowingexamplesm20Bondingconfigurationsarereadilypredictedbyvalence-shellelectron-pairrepulsiontheory,commonlyreferredtoasVSEPRinmostintroductorychemistrytexts.Thissimplemodelisbasedonthefactthatelectronsrepeleachother,andthatitisreasonabletoexpectthatthebondsandnon-bondingvalenceelectronpairswillprefertobeasfarapartaspossible.Thebondingconfigurationsofcarbonareeasytoremember,sincethereareonlythreecategories.Bondingconfigurations21Inthethreeexamplesshownabove,thecentralatom(carbon)doesnothaveanynon-bondingvalenceelectrons;consequentlytheconfigurationmaybeestimatedfromthenumberofbondingpartnersalone.Formoleculesofwaterandammonia,however,thenon-bondingelectronsmustbeincludedinthecalculation.Ineachcasetherearefourregionsofelectrondensityassociatedwiththevalenceshellsothatatetrahedralbondangleisexpected.Themeasuredbondanglesofthesecompounds(H2O104.5o&NH3107.3o)showthattheyareclosertobeingtetrahedralthantrigonalorlinear.Ofcourse,itistheconfigurationofatoms(notelectrons)thatdefinesthetheshapeofamolecule,andinthissenseammoniaissaidtobepyramidal(nottetrahedral).Thecompoundborontrifluoride,BF3,doesnothavenon-bondingvalenceelectronsandtheconfigurationofitsatomsistrigonal.Inthethreeexamples221-5Isomers1-5-1StructuralFormulas

Itisnecessarytodrawstructuralformulasfororganiccompoundsbecauseinmostcasesamolecularformuladoesnotuniquelyrepresentasinglecompound.Differentcompoundshavingthesamemolecularformulaarecalledisomers.

Whenthegroupofatomsthatmakeupthemoleculesofdifferentisomersarebondedtogetherinfundamentallydifferentways,werefertosuchcompoundsasconstitutionalisomers.TherearesevenconstitutionalisomersofC4H10O,andstructuralformulasforthesearedrawninthefollowingtable.TheseformulasrepresentallknownandpossibleC4H10Ocompounds,anddisplayacommonstructuralfeature.Therearenodoubleortriplebondsandnoringsinanyofthesestructures.1-5Isomers23chapter-1-structure-and-教學講解課件24Simplificationofstructuralformulasmaybeachievedwithoutanylossoftheinformationtheyconvey.Incondensedstructuralformulasthebondstoeachcarbonareomitted,buteachdistinctstructuralunit(group)iswrittenwithsubscriptnumbersdesignatingmultiplesubstituents,includingthehydrogens.

Shorthand(line)formulasomitthesymbolsforcarbonandhydrogenentirely.Eachstraightlinesegmentrepresentsabond,theendsandintersectionsofthelinesarecarbonatoms,andthecorrectnumberofhydrogensiscalculatedfromthetetravalencyofcarbon.Non-bondingvalenceshellelectronsareomittedintheseformulas.

Simplificationofstru251-5-2DistinguishingCarbonAtoms

Whendiscussingstructuralformulas,itisoftenusefultodistinguishdifferentgroupsofcarbonatomsbytheirstructuralcharacteristics.Aprimarycarbon(1o)isonethatisbondedtonomorethanoneothercarbonatom.Asecondarycarbon(2o)isbondedtotwoothercarbonatoms,andtertiary(3o)andquaternary(4o)carbonatomsarebondedrespectivelytothreeandfourothercarbons.ThethreeC5H12isomersshownbelowillustratetheseterms.

1-5-2DistinguishingCarbonAt261-6Resonance(self-study)Kekuléstructuralformulasareessentialtoolsforunderstandingorganicchemistry.However,thestructuresofsomecompoundsandionscannotberepresentedbyasingleformula.Forexample,sulfurdioxide(SO2)andnitricacid(HNO3)mayeachbedescribedbytwoequivalentformulas(equations1&2).Forclaritythetwoambiguousbondstooxygenaregivendifferentcolorsintheseformulas.1-6Resonance(self-study)27Ifonlyoneformulaforsulfurdioxidewascorrectandaccurate,thenthedoublebondtooxygenwouldbeshorterandstrongerthanthesinglebond.Sinceexperimentalevidenceindicatesthatthismoleculeisbent(bondangle120o)andhasequallengthsulfur:oxygenbonds(1.432?),asingleformulaisinadequate,andtheactualstructureresemblesanaverageofthetwoformulas.

Thisaveragingofelectrondistributionovertwoormorehypotheticalcontributingstructures(canonicalforms)toproduceahybridelectronicstructureiscalledresonance.

Likewise,thestructureofnitricacidisbestdescribedasaresonancehybridoftwostructures,thedoubleheadedarrowbeingtheuniquesymbolforresonance.Theaboveexamplesrepresentoneextremeintheapplicationofresonance.Here,twostructurallyandenergeticallyequivalentelectronicstructuresforastablecompoundcanbewritten,butnosinglestructureprovidesanaccurateorevenanadequaterepresentationofthetruemolecule.

Incasessuchasthese,theelectrondelocalizationdescribedbyresonanceenhancesthestabilityofthemolecules,andcompoundsorionscomposedofsuchmoleculesoftenshowexceptionalstability.Ifonlyoneformulafo28Theelectronicstructuresofmostcovalentcompoundsdonotsuffertheinadequacynotedabove.Thus,completelysatisfactoryKekuléformulasmaybedrawnforwater(H2O),methane(CH4)andacetyleneC2H2).Nevertheless,theprinciplesofresonanceareveryusefulinrationalizingthechemicalbehaviorofmanysuchcompounds.Forexample,thecarbonylgroupofformaldehyde(thecarbon-oxygendoublebond)reactsreadilytogiveadditionproducts.Thecourseofthesereactionscanbeexplainedbyasmallcontributionofadipolarresonancecontributor,asshowninequation3.Here,thefirstcontributor(ontheleft)isclearlythebestrepresentationofthismolecularunit,sincethereisnochargeseparationandboththecarbonandoxygenatomshaveachievedvalenceshellneon-likeconfigurationsbycovalentelectronsharing.Ifthedoublebondisbrokenheterolytically,formalchargepairsresult,asshownintheothertwostructures.Thepreferredchargedistributionwillhavethepositivechargeonthelesselectronegativeatom(carbon)andthenegativechargeonthemoreelectronegativeatom(oxygen).Thereforethemiddleformularepresentsamorereasonableandstablestructurethantheoneontheright.Theelectronicstructu29Theapplicationofresonancetothiscaserequiresaweightedaveragingofthesecanonicalstructures.Thedoublebondedstructureisregardedasthemajorcontributor,themiddlestructureaminorcontributorandtherighthandstructureanon-contributor.Sincethemiddle,charge-separatedcontributorhasanelectrondeficientcarbonatom,thisexplainsthetendencyofelectrondonors(nucleophiles)tobondatthissite.Thebasicprinciplesoftheresonancemethodmaynowbesummarized.

Foragivencompound,asetofLewis/Kekuléstructuresarewritten,keepingtherelativepositionsofallthecomponentatomsthesame.Thesearethecanonicalformstobeconsidered,andallmusthavethesamenumberofpairedandunpairedelectrons.

Thefollowingfactorsareimportantinevaluatingthecontributioneachofthesecanonicalstructuresmakestotheactualmolecule.1.Thenumberofcovalentbondsinastructure.(Thegreaterthebonding,themoreimportantandstablethecontributingstructure.)2.Formalchargeseparation.(Otherfactorsaside,chargeseparationdecreasesthestabilityandimportanceofthecontributingstructure.)3.Electronegativityofchargebearingatomsandchargedensity.(Highchargedensityisdestabilizing.Positivechargeisbestaccommodatedonatomsoflowelectronegativity,andnegativechargeonhighelectronegativeatoms.)

Theapplicationofreso30Thestabilityofaresonancehybridisalwaysgreaterthanthestabilityofanycanonicalcontributor.Consequently,ifonecanonicalformhasamuchgreaterstabilitythanallothers,thehybridwillcloselyresembleitelectronicallyandenergetically.Thisisthecaseforthecarbonylgroup(eq.3).ThelefthandC=Ostructurehasmuchgreatertotalbondingthaneithercharge-separatedstructure,soitdescribesthisfunctionalgroupratherwell.Ontheotherhand,iftwoormorecanonicalformshaveidenticallowenergystructures,theresonancehybridwillhaveexceptionalstabilizationanduniqueproperties.Thisisthecaseforsulfurdioxide(eq.1)andnitricacid(eq.2).Toillustratetheseprincipleswes