有机化学教案.docx
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有机化学教案.docx
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有机化学教案
Chapter1:
ChemicalBonding
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Summary:
Understandingthestructureandbondingoforganicmoleculesisfundamentaltobeingabletounderstandtheirpropertiesandreactions. Thisisbecausethepropertiesandreactionsarecontrolledbytheinteractionsbetweenmoleculeswhichareinturndictatedbythetypesofbondswithinthosemolecules.
AlthoughyoumayfeelthatmostofthischapterisreviewfromyourGeneralChemistrycourse(s),manystudentsarenotasstrongastheyneedtobetosurvivetheadventurethatliesahead,somakesureyoustartyourorganiccourseingoodshape.
Asyouwillseelater,understandingthelocationofelectronsisespeciallyimportantasitisthearrangementoftheelectronsthatcreatesthebondsbetweentheatoms,andchemicalreactionsarejustareorganisationofthoseelectronstoformnewbonds.
Review:
WhatshouldIalreadyknowabout?
∙Atoms,ElectronsandOrbitals
∙Bondtypes:
Ionic,Covalent,PolarCovalent
∙LewisStructuresandFormalCharge
∙TheShapesofMolecules
∙Hybridisation
∙Morepracticeproblems?
Atoms:
Allmatteriscomposedofatoms. Atomsareacollectionofvarioussubatomicparticles.
Youshouldbefamiliarwiththesubatomicparticleshowtodeterminehowmanyofeachaspecificatomhasandwhattheatomicmassofanatomrepresents. Trysomequestions. Ifyouarenotcomfortableperhapsyouneedtolookabitmoreindepth.
Inorganicchemistry,asfarasthecompoundsareconcerned,theatomsofgreatestinterestarelocatedthefirstthreerowsoftheperiodictable. Laterwhenoneprobesdeeperintosyntheticmethods,oneneedstobefamiliarwithsomeoftheheaviermetalsaswell.
ElectronsandOrbtials:
Thesubatomicparticleofgreatestinterestwithrespecttochemicalbonding,istheelectron.
Electronsare'said'tooccupyorbitalsinanatom. Thetermorbitalissomewhat'erroneous'aselectronsdonot'orbit'aroundanucleus. Theseorbitals,also,donotreallyexist. Theyaremathematicalconstructswhichmakeuseofawavefunction,ψ,thatdescribesthemotionofanelectron.
Anorbitalis,morecorrectly,amathematicalfunction,4πr2ψ2,whichdescribesaregionofhighprobabilityin3Dspace,aboutanucleus,whereanelectronmaybefound. Inorganicchemistryoneneedstobemostfamiliarwiththes-andp-typeorbitals. Theseorbitalscanbedescribedbyusingquantumnumbers. Theorbitalsfortheelectronsincarbon,andtheassociatedquantumnumbersaregivenbelow. Thenucleusofthecarbonatomwouldresideatthecentreofthex,y,zcoordinate.
Youshouldbefamiliarwiththemeaningofquantumnumbers,howtonameanorbitalusingquantumnumbersorwhatorbitalasetofquantumnumbersdefines.Trysomequestions. Ifyourstrugglingyou'llneedtoreviewthemeaningofquantumnumbers.
Theelectronconfigurationofanatomdescribesthenumberofelectronsthatatompossessesandtheorbitalsinwhichthoseelectronsaretobefound.
e.g. electronconfigurationsofcarbon
simple
1s22s22p2
shortform
[He]2s22p2
orbitalenergydiagram
Youshouldbefamiliarwithhowtodetermineanelectronconfigurationforanatomandidentifythevalenceelectrons. Youshouldbeabletoidentifybothgroundandexcitedstateelectronconfigurations. Trysomequestions. Ifyouarehavingtroubleyoushouldreviewhowtodetermineelectronconfigurations.
Bondtypes:
Whyisitimportanttounderstandthestructuralarrangementofelectronsinanatom?
Ineverydaylife,chemistryarisesoutoftheinteractionofmoleculeswithmolecules...butwhatisamolecule?
Itisacollectionofatoms. Whentheatomscometogethertheirelectroncloudsinteractwithoneanotherandifthisinteractionis'favourable'thenbondingoccurs.
Oneofthedrivingforcesbehindbondingisanatomsdesiretoobtainastablevalenceelectronconfiguration. Fortheatomsthataremostimportantinorganicchemistry,(N,C,O,Cl,Br,andI), thisisanoctetofelectrons,whichissimiliartothevalenceelectronconfigurationofthenearestnoblegas(ns2np6).
Ionicbondsresultwhenelectronsaretranferedbetweenatoms.
Covalentbondsresultwhenvalenceelectronsaresharedbetweenatoms.
Youshouldbeabletoidentifywhetheranionicorcovalentbondisbeingformedbetweenatoms. Trysomequestions. Ifyouarehavingdifficultythenyouneedtoreviewbonding.
Mostbondswithinorganicmoleculeswillbecovalent. Theexeceptionswillbecompoundswhichpossessmetalatoms(wherethemetalatomsshouldbetreatedasions). Itisimportanttonotethattheelectrondistributionwithinacovalentbondisnotalwayssymmetrical. Ifitisnotthenthebondissaidtobepolarasopposedtonon-polar.
Ifabondiscovalentyoushouldbeabletoidentifywhetheritispolarornon-polar. Trysomequestions. Ifyouarehavingdifficultythenyouneedtoreviewbondpolarity.
LewisDiagrams:
LewisDiagramsgivetheconnectivitybetweenatomsinamolecule,theytellyounothingaboutthearrangementoftheatomsofamoleculewithin3Dspace(thatislefttoVSEPRorHybridisation).
YoushouldbeabletodrawaLewisdiagramforbothorganicandinorganicmoleculesasthelatterareoftenusedasreagentsinorganicsyntheses.Examples. IfyouarenotyetcomfortablewithdeteminingaLewisdiagram,takealookatanoutlineofhowtodetermineaLewisdiagram.
Inreality,electronsinamoleculearebeingdelocalizedwithinthatmolecule. Consequently,morethanoneLewisdiagramcanbeusedtodescribemoleculeswhichpossessmultiplebonds. CollectivelytheseLewisdiagramsarethenknownasresonancestructuresandtheirsumisoftenrepresentedastheresonancehybrid(notearesonancehybridisnotaLewisdiagram!
).
Youshouldbeabletodrawallreasonableresonancestructuresforagivenorganicmolecule.Examples. Ifyouarenotyetcomfortablewithexaminingtheresonanceinamoleculeyoushouldreviewtheconceptofresonance.
ShapesofMolecules:
Inordertoviewsomeofthestructuresforthissection,youwillneedtheweb"plug-in","CHIME".Ifyoudonothaveitalreadyinstalled,youshouldbeabletodownloaditforfreefromthelinkedWebsite.CHIMEisabrowserthatallowsyoutovisualiseandmanipulatemoleculesas3Dstructures. Checkhereifyoudon'tknowhowtouseChime.
Whendeterminingtheshapeofamoleculethepremiseisthatgroupsofelectronssurroundinganatomarerepelledasfaraspossiblefromeachothertominimizetheoverallenergyofthemolecule. Groupsofelectronscanrefertoelectronsthatparticipateinbonding(single,double,ortriple)ortothosethatarenon-bonding(e.g.lonepairelectrons). ThenumberandtypesofgroupsaredeterminedfromaLewisdiagram.
Notonlyisitimportantforachemisttobeabletovisualizeastructure,theymustalsobeabletoeffectively(clearly!
)transferthisknowledgeontoapieceofpaper!
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Amoleculeisvisualizedasifitwerecenteredintheplaneofthepaperand,generally,sothatthemostnumberofatomspossibleareinthatplane. Unbrokenlinesareusedtorepresentbondsintheplaneofthepaper,hashedwedgesrepresentbondsgoingbackoutoftheplaneofpaperandsolidwedgesrepresentbondscomingoutoftheplaneofthepaper.
Theremaybeseveraldifferentwaysthatyoucanrepresentamoleculeonapieceofpaper. Youshouldbeabletodiscriminatewhichofthosedrawingsmostclearlyrepresentthestructureofamolecule.
Youshouldbeabletodeterminetheshapeofamoleculebylookingattheelectronicandmoleculargeometryaboutallnon-terminalatoms. Indoingsoyoumustbeabletopresentcleardrawingsofyourconclusions.Trysomequestions. IfyouarehavingproblemsdoingthisyouwillneedtoreviewVSEPR.
Oncetheshapeofamoleculeisknown,byreferringtothesumofthebondpolarities,themolecularpolaritycanbedetermined. Thisisimportantwhenexaminingthereactivityofamolecule.
Youshouldbeabletoindicateamolecule'spolarity.
Hybridisation:
Covalentbondsareformedwhenatomicorbitalsoverlap. Therearetwotypesoforbitaloverlapthatanorganicchemistneedstobefamiliarwith. Sigma,σ,overlapoccurswhenthereisonebondinginteractionthatresultsfromtheoverlapoftwoorbitals. Pi,π,overlapoccurswhentwobondinginteractionsresultfromtheoverlapoforbitals.
Theorganicchemistalsoneedstorealisehowtheseorbitaloverlapsrelatetothetypeofbondingthatisoccuringbetweenatoms:
singlebond σoverlap
doublebond σandπoverlaps
triplebond σandtwoπoverlaps
Ifonetriestocorrelatetheoverlapofatomicorbitalstotheshapeofamolecule,however,theexpectedgeometrydoesnotcorrespondtoamaximumorbitaloverlap. Takealookatmethane,CH4.VSEPRpredictsatetrahedralgeometryaboutthecarbonatombutthisisnotachievedwhenoneconsidersamaximumorbitaloverlapbetweenfour1sorbitalsofHandthe2s,2px,2pyand2pzorbitalsofcarbon.
Hybridisationisasolutiontothisproblem. Itistheimaginarymixingofthe2s,2px,2pyand2pzatomicorbitalsofcarbontoformanewsetof'hybrid'orbitalsthatorientthemselvesinthedesiredVSEPRgeometry. Thehybridorbitalsareequivalenttooneanothermakingallorbitaloverlapsequivalent,therefore,allC-Hbondinginteractionsequivalent.
Hybridorbitalsarenamedbyconsideringthetypeandnumberofatomicorbitalsfromwhichtheyarose. ForCH4thenthehybridisationforthecarbonissp3. Oneseesthatthehybridisationofanatomcanbedeterminedveryquicklybyconsideringthenumberofelectrongroupsaboutanatom. Hybridorbitalsar
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