Chapter1ChemicalBondingandChemicalStructure

Organic Chemistry, 5th ed. Marc Loudon

Eric J. Kantorowski California Polytechnic State University San Luis Obispo, CA

Chapter1Overview

•  1.1Introduc,on•  1.2ClassicalTheoriesofChemicalBonding•  1.3StructureofCovalentCompounds

•  1.4ResonanceStructures•  1.5WaveNatureoftheElectron

•  1.6ElectronicStructureoftheHydrogenAtom

•  1.7ElectronicStructureofMoreComplexAtoms•  1.8AnotherLookattheCovalentBond:MolecularOrbitals

•  1.9HybridOrbitals

2

Organicchemistry

•  Thebranchofsciencethatdealswithcompoundsthatcontaincarbon

•  Vitalism:Onlybiologicalsystems(e.g.,plants,animals)couldproduceorganiccompounds

•  Wohler’ssynthesisofurea(1828),begantounderminevitalism

31.1Introduc6on

WhyStudyOrganicChemistry?

•  Organicchemistryliesattheheartofthemodernchemicalindustry

•  Centraltomedicineandpharmacy

•  Interfaceofphysicalandbiologicalsciences•  Everydayapplica,ons:Plas,cs,tex,les,communica,ons,transporta,on,food,clothing,cosme,cs,etc.

41.1Introduc6on

ElectronsinAtoms

•  Valenceshell:Theoutermostshellofelectronssurroundinganatom

•  Atomsgainorlosevalenceelectronstoachievenoblegasconfigura,on

•  Octetrule:Elementswanteightvalenceelectrons

•  Chemistryhappensbecauseofthebehaviorofvalenceelectrons!

51.2ClassicalTheoriesofChemicalBonding

IonicCompounds

•  Chemicalcompoundswithionspresent•  Ionicbondsareomni‐direc6onal

•  Candissociateintofreeions

61.2ClassicalTheoriesofChemicalBonding

CovalentBonds

•  Whentwoatomsshareelectrons•  Covalentbondsaredirec6onal•  LewisStructuresuse“:”and“–”torepresentbonds

71.2ClassicalTheoriesofChemicalBonding

LewisStructures

•  Nota,onforelectron‐pairbonds•  Sharedelectrons(bonding)•  Unsharedelectrons(non‐bonding=lonepairs)•  Strivetosa,sfytheoctetrule(and2forH)

81.2ClassicalTheoriesofChemicalBonding

FormalCharge

•  Thechargeassignedtoanatom•  Findvalence#(=Group#ofatom)

•  Subtract#ofunsharedelectrons•  Subtracthalfofthebondingelectrons

91.2ClassicalTheoriesofChemicalBonding

ThePolarCovalentBond

•  Unevensharingofelectronsinabond•  ElectronegaDvitydeterminesdirec,on

101.2ClassicalTheoriesofChemicalBonding

ElectrostaDcPotenDalMaps

•  Agraphicaldepic,onofelectrondistribu,on

111.2ClassicalTheoriesofChemicalBonding

DipoleMoment(µ)

•  Dependsonchargesepara,onanddistance•  µ=qr(avectorquan,ty)

121.2ClassicalTheoriesofChemicalBonding

BondPolarityandFormalCharge

•  Formalchargeisonlyabookkeepingtool•  Formalchargemayormaynotcorrespondtoactualcharge!

131.2ClassicalTheoriesofChemicalBonding

MolecularStructure

•  AtomicconnecDvity:Howatomsinamoleculeareconnected

•  Moleculargeometry:Howfarapartatomsareandhowtheyarearrangedinspace

•  Knowingatomicconnec,vityandmoleculargeometryrevealstheoverallstructure

141.3StructuresofCovalentCompounds

MethodsforDeterminingMolecularGeometry

•  X‐raycrystallography:Atomicarrangementofsolids

•  ElectrondiffracDon:Atomicarrangementofgaseoussubstances

•  Microwavespectroscopy:Structuralinfoofgaseoussubstances

•  Nocomparablemethodsavailableforanalysisofaliquid

151.3StructuresofCovalentCompounds

MolecularGeometry

•  Moleculargeometryinfluenceschemicalreac,vity

•  Bondlength:distancebetweennuclei•  Bondangle:anglebetweeneachpairofbonds

161.3StructuresofCovalentCompounds

BondLength

•  Increaseswithatomsinhigherrows•  Decreaseswithincreasingbondorder•  Decreasestowardhigheratomicnumberalongarow

171.3StructuresofCovalentCompounds

BondAngle

•  Bondanglesdetermineshape•  Valence‐shellelectron‐pairrepulsion(VSEPR)•  Usemolecularmodels!

•  Line‐and‐wedgestructures

181.3StructuresofCovalentCompounds

CommonShapestoKnow

•  Tetrahedron

•  TrigonalPlanar

191.3StructuresofCovalentCompounds

CommonShapestoKnow

•  Linear

•  Trigonalpyramidal

201.3StructuresofCovalentCompounds

DihedralAngle

•  Alsoknownasthetorsionalangle•  Rota,oncanoccuralongsinglebonds

211.3StructuresofCovalentCompounds

ResonanceStructures

•  AsingleLewisstructuremaynotbeenough•  MoreLewisstructures=moreinfoonelectrondistribu,on(thesearenotinequilibrium!)

•  The“true”electrondistribu,onisaweightedaverageoftheindividualcontributors

•  Suchstructuresareresonance‐stabilized

221.4ResonanceStructures

QuantumViewoftheElectron

•  Covalentchemicalbonds:Thesharingofoneormoreelectronpairsbetweentwoatoms

•  Thismodelishighlyusefulbuthasitslimita,ons

•  Quantummechanicsdescribesanalternate,moredetailedview

231.5WaveNatureoftheElectron

HeisenbergUncertaintyPrinciple

•  Impossibletoknowtheposi,onandvelocityofapar,cle

•  Notimportantforlargeobjects(cars,baseballs,andevenmolecules)

•  Highlyimportantforverysmallobjects(electrons)

•  Theloca,onofanelectronis“fuzzy”

241.5WaveNatureoftheElectron

Orbitals,QuantumNumbers,andEnergy

•  Atomicorbital:Describesthewaveproper,esofanelectroninanatom

•  Eachorbitalisdescribedby3quantumnumbers(n,l,ml)

251.6ElectronicStructureoftheHydrogenAtom

SpaDalCharacterisDcsofOrbitals

•  3‐Dorbitalpicture=90%probabilitylevel•  Node:Adefinedregionofnoelectrondensity•  1sand2sorbitals:

261.6ElectronicStructureoftheHydrogenAtom

SpaDalCharacterisDcsofOrbitals

•  Threedegenerateporbitals(px,py,pz)•  Orthogonaltooneanother•  2porbital:

271.6ElectronicStructureoftheHydrogenAtom

ElectronConfiguraDon

•  AuXauprinciple:Buildupprinciple•  Electronspin(ms):+1/2(up)or‐1/2(down)

281.7ElectronicStructuresofMoreComplexAtoms

ElectronConfiguraDon

•  Hund’srule:placeelectronsintodegenerateorbitalsbeforepairingthem

•  Pauliexclusionprinciple:Notwoelectronscanhavethesamefourquantumnumbers

291.7ElectronicStructuresofMoreComplexAtoms

MolecularOrbitalTheory

•  Combiningatomicorbitals(AOs)generatesmolecularorbitals(MOs)

•  nAOs→nMOs

301.8AnotherLookattheCovalentBond:MolecularOrbitals

MolecularOrbitalTheory

•  AddingAOsresultsinconstruc6veinterac,on•  Remember:Orbitalsaredescribedbymathema6calfunc6ons‐theycanbeaddedandsubtracted

311.8AnotherLookattheCovalentBond:MolecularOrbitals

MolecularOrbitalTheory

•  Subtrac,ngAOsresultsindestruc6veinterac,on

•  Subtrac,nganAOisthesameasaddinganAOofoppositephase

321.8AnotherLookattheCovalentBond:MolecularOrbitals

OrbitalInteracDonDiagram

331.8AnotherLookattheCovalentBond:MolecularOrbitals

BondinginMethane

•  TheMOpictureofmethanedistributeselectronsthroughouttheen,remolecule

•  Totalelectrondensityinmethane:

341.9HybridOrbitals

ConstrucDngMethane

•  Amoreuseful(butlessaccurate)pictureofbondinginmethanerequireshybridorbitals

•  ThemixingprocessappliedtoAOstomakeMOsiscalledhybridizaDon

351.9HybridOrbitals

BondinginMethane

•  Mixings+px+py+pz→Foursp3MOs

361.9HybridOrbitals

BondinginMethane

•  Eachsp3MOcanthencombinewithaHatom

371.9HybridOrbitals

BondinginAmmonia

•  Thehybridorbitalpictureaccommodatesmoleculeswithlonepairstoo(e.g.,:NH3)

381.9HybridOrbitals

BondinginAmmonia

•  Thehybridorbitaldescrip,onofammonia•  Note,thesmallrearlobesofeachsp3orbitalarecommonlyomioedforclarity

391.9HybridOrbitals