3. Hybridization of Atomic Orbitals.pdf

Hybridization

1. Hybridization
of
Atomic Orbitals
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2. HybridizationofAtomicOrbitals
The process of mixing of atomic orbitals of different types
(s, p, d, etc) of the same atom to give a set of new
equivalent is called hybridization.
•Theseneworbitalsarecalledhybridorbitals.
•Thenumberofhybridorbitalsisequaltothenumberof
atomicorbitalscombined.
•Thehybridorbitalsariseonlyduringtheprocessof
bondinganddonotexistinthefreeatom.
•Orbitalhybridizationoccursintheexcitedstate.

3. •Manyelements,forexample,Be,BandC,formcompoundswhose
formationcannotbeexplainedonthebasisoftheconfigurationof
theelementsinthegroundstate.
•However,asatisfactorydescriptionofcompoundsofthese
elementscanbeobtainedthroughanorbitalhybridization
approach.
•Theatomicorbitalsofcarboncanbehybridizedinthreedifferent
waysdependinguponthennumberofatomsbondedtothecarbon
atom.

4. •Theelectronicconfigurationofcarboninitsgroundoratomic
stateis
•Ithasonlytwounpairedelectrons(partiallyfilledorbitals)which
canformonlytwocovalentbonds,butcarbonformsfourbondsin
themajorityofitscompounds.
•Itstetravalencycanbeexplainedbyassumingthatanelectron
fromthe2sorbitalispromotedtoanempty2p
z
orbitalgivingthe
excitedstateaconfiguration
•Initsexcitedstate,carboncanformfourbondsbutthesewould
notbeequivalent.
•Threep-orbitalswouldformthebondsalongthethreemutually
perpendicularaxes,whereasthefourthbondarisingfrom2s
orbitalwouldbenon-directionalbecausesorbitalsarespherically
symmetrical.

5. •ThisshowsthattwodifferenttypesofC-Hbondsareinvolvedin
theformationofCH
4
molecule.Inactualpractice,allthefour
bondsinCH
4
areequivalentwithabondangleof109.50
o
.
•Thishasbeenexplainedbytheprocessofhybridizationinwhich
2sandthree2porbitals(p
x
,p
y
andp
z
)intheexcitedstateare
mixedtoformasetoffourequivalenthybridorbitals.
•Theprocessofcombiningthesandthethreeporbitalstogetfour
sp
3
orbitalsiscalledsp
3
hybridization.
•Thesymbolsp
3
signifiesthateachsp
3
orbitalismadeupofsand
porbitalsintheratioof1:3.
•Thesp
3
orbitalsaredirectedfromthecenterofaregular
tetrahedrontoitsfourcorners,makinganangleof109.50
o
witheachother.
•Thesp
3
hybridizationistherefore,alsocalledtetrahedral
hybridization.

6. •Thesesp
3
orbitalsarenon-coplanar(donotlieinthesameplane).
•Thesmalllobeisoftenomittedwhendepictinghybridorbitals.
Hybridization of s and three p orbitals to form 4sp
3
hybrid orbitals.

7. BondinginMethane
•Themethanemoleculeisformedbytheoverlapoffoursp
3
orbitalsofcarbonwith1sorbitalsoffourhydrogenatoms.
•Themethanemoleculecontainsfourσbonds,eachduetosp
3
-s
overlapandeachH-C-Hbondangle=109.50
o
(tetrahedral).
•Thesp
3
hybridizationoccurswhencarbonisbondedtofourother
atoms.
•Thepromotionofan-electronfromthe2stoa2porbitalrequires
energy406KJmol
-1
,butbydoingthiscarbonformsfourcovalent
bondsinsteadoftwobondsandreleasesenergy887KJmol
-1
.
•Theformationoftwoadditionalbondsreleasesenergymorethan
compensateforthatrequiredtoexcitedtheelectron.

9. Multiple (Localized) Bond:
Trigonal and Digonal Hybridization
Trigonalor sp
2
Hybridization
•Whencarbonisbondedtoonlythreeotheratomsasin
ethene,C
2
H
2
(onecarbonandtwoHatoms),the2sand
twoofthethree2porbitalsofeachcarbonatomare
hybridizedtogivethreeequivalentsp
2
hybridorbitals
ofeachcarbon,leavingtheremaining2porbital
unhybridized.
•Eachsp
2
orbitaliscomposedofsandporbitalsinaratioof1:2.
•Thesethreesp
2
orbitalslieinthesameplaneandmakeanangle
120
o
witheachother,thehybridized2porbitalisperpendicularto
thisplane.
•Thesp
2
hybridizationis,therefore,alsocalledtrigonal
hybridization.

12. •Inethene,twosp
2
hybridizedcarbonatomsformaσbond
betweenthembytheoverlapofonesp
2
orbitalfromeachcarbon,
theremainingtwosp
2
orbitalsofeachcarbonformσbondsto
fourhydrogenthroughoverlapwiththe1sorbitalsofthe
hydrogenatoms.
•Thetwounhybridizedporbitalsoneoneachcarbonare
perpendiculartotheaxisjoiningthetwocarbonnucleiandtheir
axesareparallel.
•Thesetwoporbitalsareinaperfectpositiontooverlapelaterally
(side-to-side)togeneratetwonewmolecularorbitals,i.e.,a
bondingπorbitalandanantibondingπ*orbital.
•Theelectronsfillthelowenergybondingπorbitalleavingthe
highenergyantibondingπ*orbitalempty.
•Thissidewiseoverlapofporbitalsresultsapi(π)bond.
•Thuscarbon-carbondoublebondconsistsoftwodifferentkinds
ofbonds,aσbondandaπbond.
•Theπbondismorediffusedandisweakerthanthatofσbond.

14. Linear or Diagonal or spHybridization
•Wheneachcarbonatom(asinacetylene)isbondedtoonlytwo
otheratoms,the2sorbitalandone2porbitalofcarbonare
hybridizedtoform2sphybridorbitals,leavingtwo2porbitalsas
unhybridized.
•Theaxesofthetwosporbitalsformastraightline.
•Theanglebetweenthetwosporbitalsisthus180
o
.
•Thesphybridizationis,therefore,calledlinearordiagonal
hybridization.
•The two unhybridized2p orbitals are perpendicular to the axis
that passes through the centreof the 2sp orbitals.

16. •Inacetylene,H-C≡C-Htwosphybridorbitals(i.e.,onefrom
eachcarbon)overlapstoformaσbondbetweenthem.
•Theremainingtwosporbitalsateachcarbonoverlapwith1s
orbitalsfromhydrogenatomstoproducetwosigma(σ)C-H
bonds.
•Theresidualtwo2porbitalsoneachcarbonalsooverlapside-to-
sidetoformtwoπbonds.
•Thusinacetylenethereisoneσbondandtwoπbondsbetween
thetwocarbonatoms.

18. Hybridizations of Nitrogen
•Nitrogenhasfiveelectronsinitsvalenceshell
likecarbon,thenitrogenatomalsoundergoessp
3
,sp
2
andsphybridization.
•Whenevernitrogenisattachedtothreeotheratomsorgroups(as
inammoniaandmethylamine,)then2sandthree2porbitals(p
x
,
p
y
andp
z
)mixedtoformfourequivalenthybridorbitals.
•Thefoursp
3
orbitalsofnitrogenaredirectedtowardsthecorners
ofaregulartetrahedron,justasthesp
3
orbitalsofcarbondo.
•Considertheformationofmethylamine.
•Inmethylaminebothnitrogenandcarbonaresp
3
hybridized.
•Theformationofadoublebondbetweencarbonandoxygenand
atriplebondbetweencarbonandnitrogencanbeexplainedina
similarmanner.

20. •Oneofthesp
3
orbitalsofnitrogeniscompletelyfilledand
cannottakepartinbondformation.
•TheC-NσbondinCH
3
NH
2
isformedbyoverlapofansp
3
orbitalofnitrogenandsp
3
orbitalofcarbon.
•EachN-Hσbondisformedbytheoverlapofansp
3
orbitalofN
andsorbitalofhydrogen.
•EachC-Hσbondisformedbyoverlapofansp
3
orbitalof
carbonandsorbitalofhydrogen.
•Allbondanglesareapproximatelytetrahedral.

21. •TheH-N-Hbondangleinmethylamineishowever,105.9
o
whichisslightlylessthanthenormaltetrahedralangleof109
o
28'.
•Thisisbecausethesp
3
orbitalcontainingtheunsharedpairof
electronsissomewhatlargerthanthosecontainingbondpairs.
•Thustheanglesbetweentheσbondsontheothersideof
moleculearecompressedslightly.
•InCH
3
NH
2
theN-Hbondlengthis1.011A
o
andtheC-Nbond
lengthisabout1.47A
o
indifferentamines.
•Also,nitrogencanformcompoundsusingsp
2
andsphybrid
orbitals.
•Whenevernitrogenisbondedtotwootheratomsorgroups(as
iniminesorazocompounds),itusestwosp
2
hybridorbitals
plustheunhybridizedp
z
orbitalstoformitsbonds.

23. •Whenevernitrogenisbondedtoonlyoneotheratomorgroupof
atoms(asinH-CN,methylcyanide,ornitrogen),itusesansp
hybridorbitalplusthetwounhybridized2porbitals(p
y
andp
z
)to
formitsbonds.
Bondinginhydrogencyanide,H-C≡N

24. HybridizationsofOxygen
•Oxygenhassixelectronsinitsvalenceshell;
•Likecarbonornitrogen,theoxygenundergoessp
3
andsp
2
hybridizations.
•Wheneveroxygenisbondedtotwootheratomsorgroups(asin
water,ethers,oralcoholsorphenols),then2sandthree2porbitals
(p
x
,p
y
andp
z
)mixtoform4sp
3
equivalenthybridorbitals.
•Thefoursp
3
hybridorbitalsofoxygenaredirectedtowardsthe
cornersofaregulartetrahedron,justasthesp
3
orbitalsofcarbon
ornitrogen.

26. •Inmethanolbothcarbonandoxygenaresp
3
hybridized.
•Twoofthesp
3
orbitals,arecompletelyfilledandcannottake
partinbondformation.
•TheC-Obondinmethanolisformedbyoverlapofansp
3
orbitalofcarbonandansp
3
orbitalofoxygen.
•TheO-Hbondisformedbyoverlapofansp
3
orbitalof
oxygenandsorbitalofhydrogen.
•TheC-O-Hbondangleis105
o
,whichislessthanthenormal
tetrahedronangle.
•Thisisbecausethetwocompletelyfilledsp
3
orbitalsof
oxygenrepeleachother,whichresultsinreductionofbond
angle.
Structure of methanol

27. •Oxygencanalsoformcompoundsusingsp
2
hybridorbitals.
•Wheneveroxygenisbondedtooneotheratomorgroup(asin
aldehydesandketones)itusessp
2
orbitalsplustheunhybridizedp
x
orbitaltoformitsbonds.
•Inaldehydesandketones,thecarbonylgroup,liketheC-Cdouble
bondofalkanes,iscomposedofoneσbondandoneπbond.
•Boththecarbonandoxygenaresp
2
hybridized.
•Theσbondisformedbytheoverlapofansp
2
orbitalofcarbonand
ansp
2
orbitalofoxygen.
•Theπbondisformedbytheoverlapofunhybridizedporbitalsof
thetwoatoms.Thetwounsharedelectronspairsofoxygenoccupy
thesp
2
hybridorbitalsofoxygen.
•Becausethecarbonylcarbonissp
2
hybridized,thethreeatoms
attachedtoitlieinthesameplane.
•Thebondanglebetweentheattachedatomsareapproximately
120
o
.
•Oxygendoesnotundergosphybridization.

28. Bonding in aldehydes and ketones.

29. Hybridization of Sulphur
•Atomicnumberofsulphurissixteen.Theelectronic
configurationofsulphuris
•Thesulphurundergoessp
3
,sp
2
andsp
3
d
2
hybridizations.
•Insp
3
hybridization,3s,3p
x
,3p
y
,3p
z
atomicorbitalsofthe
valenceshellofsulphurcombinetoformfoursp
3
hybridized
orbitals.
•Twoofthesehybridizedorbitalscontaintwoelectronseach(the
lone-pairelectrons),whiletheothertwoaresinglyoccupied.

30. •IntheformationofthiolmoleculeRSH,onesinglyoccupiedsp
3
orbitalofS-atomoverlapswiththesp
3
orbitalofRgroupto
formaσ(sigma)bond.
•Thesecondsinglyoccupiedsp
3
orbitalofSoverlapswith1s
orbitalofHatomtoformanotherσbond.
•Sinceinthiolssulphurissp
3
hybridized,the<R-S-Hshouldbe
normally109.5
o
.
•Buttheactual<RSHisshrinktoamuchlessvalue,99.4
o
incase
ofCH
3
SH.
•Thisisattributedtothefactthatthelonepair-lonepair
interactionsovershadowtosomeextentthebondpair-bondpair
repulsionsofS-RandS-Hbonds.
•Asaresult<RSHisreducedconsiderably.

31. •Indimethylsulphide,(CH
3
)
2
S,sulphurisalsosp
3
hybridized,
twosp
3
orbitalsareoccupiedbybondingpairofelectronsand
twobylonepairs.
Bonding in thiol.

32. p
π
-d
π
Bonding (π
d-p
Bond)
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33. p
π
-d
π
Bonding (π
d-p
Bond)
•Ingeneral,atomsofthesecondrowoftheperiodictabledo
notformstablebondsofthetypediscussedinethaneand
acetylene(πbondsformedbyoverlapofparallelporbitals).
•However,thereisanothertypeofdoublebondthatis
particularlycommonforthesecondrowatoms,sulfurand
phosphorus.
•Forexample,suchadoublebondisfoundindimethyl
sulfoxide,(CH
3
)
2
S=O,anddimethylsulphone,
inwhichsulphurissp
3
hybridized.
•In(CH
3
)
2
S=O,thetwomethylgroups(Rgroups)arebonded
tosulphurbyσbondsusingtwosp
3
orbitals;theOatomis
bondedtoSbyaσbondsusingthirdsp
3
orbitalandaπ
d-p
bond(oneofthesulphurelectronispromotedto3dorbital);a
lonepairoccupiesthefourthsp
3
orbitalofsulphur.

34. •Insulphones,therearefourσbondstofoursp
3
orbitalsofsulphur
andtwoπ
d-p
bonds,onetoeachoxygenatom.
•WhensulphurisbondedtosixotheratomsasinSF
6
,thenSatom
usesallthesixvalenceshellelectronsinformingsixS-Fσbonds.
p orbital
dorbital
p
π
-d
π
Bonding (π
d-p
Bond)

35. •Ones(3s),threep(3p
x
,3p
y
and3p
z
)andtwod(3d)orbitalsof
sulphurundergosp
3
d
2
hybridizationtoformsixequalenergy
hybridorbitalswhicharedirectedtowardthecornersofregular
octahedron.
•Eachhybridorbitalcontainoneelectron.
•Eachsp
3
d
2
hybridorbitalisoverlappedbyahalf-filled2porbital
fromfluorinetoformatotalofsixcovalentbonds.
•ExaminationoftheoctahedralmoleculeshowsthatF-S-Fbond
anglesare90and180andthebondsarequitepolar.
•Becausethemoleculeissymmetrical,thelargebonddipoles
cancel,andthemoleculeisnon-polar.
Octahedral shape of SF
6
molecule

36. •Wehavedescribedthelocalizedchemicalbondsthatare
constitutedfromoverlapoftwoatomicorbitalsandarecentered
aroundtwonuclei.
•Bondsinwhichthebondingelectronsaresharedbyonlytwo
nucleiinamoleculearecalledlocalizedbonds.
•Asinglecovalentbondiscalledsigma(σ)bond.
•Inmultiplelocalizedbondsboththeσandπelectronsare
confinedbetweenonlytwoatoms(i.e.,nuclei).
•Inotherwordstheelectrondensityisconcentratedbetweentwo
atoms.
•Compoundswhichhavelocalizedbondscanbedescribedbya
singleLewisstructure.
•Alsothepredictedbondlengths,bondenergiesanddipole
momentsagreefairlywiththeexperimentallyobservedvalues.
•Theirchemicalreactivityandspectralbehaviourisalso
predictable.

37. Difference between an AO, a Hybrid AO,
a MO and a Localized MO
•Anatomicorbital(AO)isaregionofspaceinanatomin
whichanelectronmayexist.
•Ahybridatomicorbitalismathematicallyfabricated(made
up)fromsomenumberofatomicorbitalstoexplain
equivalencyofbonds.
•Amolecularorbitalisaregionofspaceabouttheentire
moleculecapableofaccommodatingelectrons.
•Alocalizedmolecularorbitalisaregionofspacebetweena
pairofbondedatomsinwhichthebondingelectronsare
assumedtobepresent.