CHM102
CarbonCompoundsandChemicalBonding:
Valencebondtheory,shapesoforbitals;
Hybridiza8on/LCAO,Molecularorbitaltheory;
Hyperconjuga8on;Resonance;Tautomerismetc,
(1hr)
Whatdeterminestheshapeofmoecules?
Lewisstructure
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ShapesallowmaximumdistancesbetweenatomsinABn
molecules
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Canwepredicttheseshapesfromchemicalstructure?
WhydomanyABnmoleculeshavesomanyshapes
Valence-shellelectron-pairrepulsion(VSEPR)model
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Electronsinmoleculesbehaveliketheseballoons
Op8mumgeometryexistsforeachnumberofballoons.
Abondingpairofelectronsdefinesaregioninwhichtheelectronsaremost
likelytobefound-theregionisanelectrondomain
Nonbondingpair(orlonepair)ofelectrons
-definesanelectrondomainthatislocatedprincipallyononeatom
Eachnonbondingpair,singlebond,ormul4plebond
producesasingleelectrondomainaroundthecentralatom
inamolecule.
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VSEPRmodel:Electrondomainsarenega8vely
chargedandthereforerepeloneanother.
Thebestarrangementofagivennumberofelectrondomains
istheonethatminimizestherepulsionsamongthem
Twoelectrondomainsorientlinearly,threedomainsorientina
trigonal-planarfashion,andfourorienttetrahedrally.
Twoelectrondomainsorientina
linearelectron-domaingeometry
Neitherdomainisanonbondingpairofelectrons,the
moleculargeometryisalsolinear
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Thesegeometriesareimportantbecause
theyincludealltheshapesusuallyseenin
moleculesorionsthatobeytheoctetrule
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EffectofNonbondingElectronsandMul8pleBondsonBond
Angles
Allthreehaveatetrahedralelectron-domaingeometry,buttheirbond
anglesdifferslightly.Why?
Bondanglesdecreaseasthenumberofnonbondingelectronpairs
increases.
•  bondingpairofelectronsisa^ractedbybothnucleiofthebondedatoms,
•  nonbondingpairisa<ractedprimarilybyonlyonenucleus
Nonbonding-lessnucleara^rac8on,itselectrondomainisspreadoutmorein
spacethanistheelectrondomainforabondingpair
Electrondomainsfornonbondingelectronpairsexertgreaterrepulsiveforceson
adjacentelectrondomainsandtendtocompressbondangles.
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Rela8vevolumesoccupiedbybondingandnonbonding
electrondomains
Mul8plebondscontainahigherelectronic-chargedensitythansinglebonds,
Mul8plebondsalsorepresentenlargedelectrondomains
3electrondomainssurroundthecentral
atom,wemightexpectatrigonalplanar
geometrywithbondangles120o
Thedoublebond,however,seemstoact
muchlikeanonbondingpairofelectrons,
reducingthebondangleto111.4°:
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MoleculeswithExpandedValenceShells
Atomsfromperiod3andbeyondmaybesurroundedbymorethanfourelectron
pairs.
Moleculeswith5or6electrondomainsaroundthecentralatomhavemolecular
geometriesbasedoneitheratrigonal-bipyramidal(fivedomains)oroctahedral
(sixdomains)electron-domaingeometry
5electrondomainsisthetrigonal
bipyramid(twotrigonalpyramids
sharingabase).
Eachaxialdomainmakesanangle
withanyequatorialdomain.Each
equatorialdomainmakesa120°
anglewitheitheroftheothertwo
equatorialdomainsanda90°angle
witheitheraxialdomain
Inatrigonal-bipyramidalgeometry,therearetwotypesofposi8ons
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fortheouteratoms.
Non-bondingdomain
preferesequitorial
ratherthanaxial?Why
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Non-bondingdomain
preferesaxialratherthan
axial?Why
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ShapesofLargerMolecules
VSEPRmodelcanbeextendedtomorecomplexmolecules
BondanglesaboutCand
theOatomsdeviatefrom
idealvaluesof120°and
109.5°-spa8aldemands
ofmul8plebondsand
nonbondingelectronpairs
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COVALENTBONDINGANDORBITALOVERLAP
VSEPRmodel-meansforpredic8ngmoleculargeometriesbutdoesnot
explainwhybondsexistbetweenatoms
Useatomicorbitalstoexplainbondingandtoaccountformoleculargeometries?
Valence-bondtheoryofchemicalbonding:Lewis’sno8onofelectron-pairbonds+
theideaofatomicorbitals
•  Bondingelectronpairsareconcentratedintheregionsbetweenatoms
•  Nonbondingelectronpairslieindirectedregionsofspace.
Waysinwhichatomicorbitalscanmixwithoneanother
Lewistheory:Covalentbondingbetweenatomssharingelectrons,concentrates
electrondensitybetweenthenuclei
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Valence-bondtheory:visualizethebuildupofelectrondensity
betweentwonuclei-whenavalenceatomicorbitalofoneatom
sharesspace,oroverlaps,withavalenceatomicorbitalofanother
atom
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HYBRIDORBITALS
TheVSEPRmodel-goodjobatpredic8ngmolecularshape,thoughhas
noobviousrela8onshiptothefillingandshapesofatomicorbitals.
Basedontheshapesandorienta8onsofthe2sand2porbitalona
carbonatom,itisnotobviouswhyaCH4moleculeshouldhavea
tetrahedralgeometry.
Atomicorbitalsaremathema8calfunc8onsthatcomefromthe
quantummechanicalmodelforatomicstructure
Atomicorbitalsonanatom(usuallythecentralatom)mixtoformnew
orbitalscalledhybridorbitals.Theshapeofanyhybridorbitalis
differentfromtheshapesoftheoriginalatomicorbitals.
Numberofhybridorbitalsonanatomequalsthenumberofatomic
orbitalsthataremixed.
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BeF2
Accordingtothevalence-bondmodel,alineararrangementofelectrondomains
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impliessphybridiza4on
Forma8onofsp2hybridorbitals
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Forma8onofsp3hybridorbitals
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Hybridiza8onwithnonbondingpairsofelectrons
..
H-O-H
..
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Carbon Hybridizations
Unhybridized
↑↓
↑
↑
2p
2s
sp hybridized
↑
↑
↑
2sp
↑
2p
sp2 hybridized
↑
↑ ↑
2sp2
sp3 hybridized
↑
↑
↑
↑
2p
↑
3
2sp
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MULTIPLEBONDS
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FormaGonofandbondsinformaldehyde
H2CO
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Multiple Bonds
In triple bonds, as in
acetylene, two sp orbitals
form a σ bond between
the carbons, and two
pairs of p orbitals overlap
in π fashion to form the
two π bonds.
ResonanceStructures,Delocaliza8on,andπ Bonding
Bondingelectronsarelocalized-electronsareassociatedtotallywiththetwoatomsthat
formthebond.
Moleculesinvolvingmoreπ bonds?
Eachcarbonissurroundedbythreeatomsat
120oangles,theappropriatehybridsetissp2
SixlocalizedC-Cσbonds
SixlocalizedC-Hσbondsfromthe
sp2hybridorbitals
Eachunhybridized2porbitalis
occupiedbyoneelectron,leaving
sixelectronstobeaccountedfor
bypbonding
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Theπbondsformedfrom
overlapoftheunhybridized2p
orbitalsonthesixcarbonatoms.
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Delocalized πbondsinbenzene
Delocaliza8onoftheelectronsinitsπbondsgivesbenzeneaspecialstability.
Delocaliza8onofbondsisalsoresponsibleforthecolorofmanyorganic
molecules.
Op8maloverlapoftheunhybridizedporbitals-alltheatomsinvolvedina
delocalizedπbondingnetworkshouldlieinthesameplane.
Restric8onimpartsacertainrigiditytothemoleculethatisabsentinmolecules
containingonlyσbonds
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Electron-domaingeometryatnitrogenis
trigonalplanar:sp2hybridiza8onatN
ConstructthethreeN¬Oσ bondspresent
ineachresonancestructure.
Unhybridized2porbitalontheNatom
canbeusedtomakeπ bonds.
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Rota8onabouttheC=Cdoublebondinethylenebreaksthe π bond
Rodsaresensi8vetodimlight-usedinnightvision.
Conesaresensi8vetocolors.
Thetopsoftherodsandconescontainamolecule
calledrhodopsin:protein,opsin,bondedtoareddish
purplepigmentcalledre4nal.
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MOLECULARORBITALS
Valence-bondtheoryandHybridatomicorbitals:fromLewis
structurestora8onalizingtheobservedgeometriesofmoleculesin
termsofatomicorbitals.
Valence-bondmodel-doesnotexplainallaspectsofbonding
forex,describingtheexcitedstatesofmolecules,
Howmoleculesabsorblight,givingthemcolor?
Moresophis8catedmodelcalledmolecularorbitaltheory
Electronsinatomscanbedescribedbywavefunc8ons,whichwe
callatomicorbitals.
Molecularorbitaltheorydescribestheelectronsinmoleculesby
usingspecificwavefunc8onscalledmolecularorbitals(MO)
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Molecular Orbital Theory
•  In MO theory, we apply Schrödinger’s wave
equation to the molecule to calculate a set of
molecular orbitals.
•  In this treatment, the electrons belong to the
whole molecule—so the orbitals belong to the
whole molecule.
–  delocalization
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LCAO
•  The simplest assumption starts with the
atomic orbitals of the atoms adding together
to make molecular orbitals. This is called
the linear combination of atomic orbitals
method.
•  Because the orbitals are wave functions,
the waves can combine either
constructively or destructively.
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Molecular Orbitals
•  When the wave functions combine constructively,
the resulting molecular orbital has less energy
than the original atomic orbitals and is called a
bonding molecular orbital.
–  σ, π
–  most of the electron density between the nuclei
•  When the wave functions combine destructively,
the resulting molecular orbital has more energy
than the original atomic orbitals and is called an
antibonding molecular orbital.
–  σ*, π*
–  most of the electron density outside the nuclei
–  nodes between nuclei
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Theoverlapof2atomicorbitals,leadsto2molecular
orbitals
Destruc4vecombina4on.
Electrondensity
iszero
an8bondingmolecular
orbital
Electrondensitytobe
moreorlesscanceledin
thecentralregionwhere
thetwooverlap.
Greatestelectrondensity
onoppositesidesofthe
twonuclei.
Electroninanan8bonding
MOrepelledfromthe
bondingregion,higher
energy)lessstable
Construc4vecombina4on:the
bondingmolecularorbital.
Energyoftheresul8ngMOis Lowerenergy,morestablethan
lowerthantheenergyofthe
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twoatomicorbitals
the1satomicorbitalofan
isolatedhydrogenatom
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H2
σ*An8bondingMO
LUMO
σbondingMO
HOMO
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Molecularorbitalscanexplainstabilityofmolecules
ElectrondensityinboththebondingMOandthean8bondingMOofH2iscentered
abouttheinternuclearaxis-sigmamolecularorbitals(byanalogytoσbonds).
Energy-leveldiagram
EnergydecreaserealizedingoingfromHe
•  MOislowerinenergythanthe1s
atomicorbitalstotheHebondingMOisoffset
atomicorbitals
bytheenergyincreaserealizedingoingfrom
•  H2moleculeismorestablethanthe
theatomicorbitalstotheHean8bondingMO.
twoseparateHatoms.
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He2isanunstablemolecule
Bondorder
Bondorderof1representsasinglebond,
Bondorderof2representsadoublebond,
Bondorderof3representsatriplebond.
MOtheoryalsotreatsmoleculeswithoddnumberofelectrons,
bondordersof1/2,,3/2or5/2arepossible.
H2=1;He2=0
He2+?
Forma8onofHe2+inthegasphase
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1.ThenumberofMOsformedequalsthenumberofatomic
orbitalscombined.
2.Atomicorbitalscombinemosteffec8velywithotheratomic
orbitalsofsimilarenergy.
3.Theeffec8venesswithwhichtwoatomicorbitalscombineis
propor8onaltotheiroverlap.Astheoverlapincreases,the
energyofthebondingMOisloweredandtheenergyofthe
an4bondingMOisraised.
4.EachMOcanaccommodate,atmost,twoelectrons,with
theirspinspaired(Pauliexclusionprinciple).
5.WhenMOsofthesameenergyarepopulated,oneelectron
enterseachorbital(withthesamespin)beforespinpairing
occurs(Hund’srule)
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MolecularOrbitalsfrom2pAtomicOrbitals
Theelectrondensitylies
alongtheinternuclearaxis,
sotheyaremolecular
orbitals:σ2pand:σ*2p
2porbitalsoverlapsideways
andthusconcentrateelectron
densityaboveandbelowthe
internuclearaxis.
MOsofthistypearecalledpi
(Π)molecularorbitals
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Energy-leveldiagramforMOsofperiod2homonuclear
diatomicmolecules.
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Effectofinterac8onof2sand2porbitals
•  The smaller p-block elements in the second period have a
sizable interaction between the s and p orbitals.
•  This flips the order of the σ and π molecular orbitals in these
elements.
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LUMO
Thehighestoccupiedmolecular
orbital(HOMO)istheMOof
highestenergythathaselectrons
init.Thelowestunoccupied
molecularorbital(LUMO)isthe
MOoflowestenergythatdoes
nothaveelectronsinit.
HOMO
HOMO–LUMOgap—isrelated
totheminimumenergy
neededtoexciteanelectronin
themolecule.
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Hyperconjuga8on
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Tautomerism
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