Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp)...
Transcript of Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp)...
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Hybridisation I• We want model that describes 'localised' bonds• i.e. a model that matches Lewis structures (line diagrams)• Use hybrid AO (HAO)• Combine AO of each atom BEFORE combining atoms!
H C C H
Ethyne• Combine carbon AO first• Only combine (hybridise) AO we need• Each carbon attached to 2 groups so hybridise 2 AO• Remember conservation of orbitals
2s
2px 2py 2pz
2(sp) 2(sp)
2py 2pz
carbon AO carbon hybrid orbitals 2 x sp (HAO) + 2 x p (AO)
combine
ener
gy
2
=– =
+ =
2s 2px
Hybridisation II
• We form 2 new hybrid AO called sp orbital• sp has 1/2s character & 1/
2p character• Due to direction of
combined AO new HAO are at 180˚
2s 2px 2py 2pz
+ + +
2py 2pz2 x sp AO
same plane 180˚
Carbon Atomic Orbitals
Carbon with 2 Hybrid Orbitals
2 x sp2 x AO
+
=Combination of 2 x carbon AO
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π (2py + 2py)
CH HCσ (1s + sp)
σ (sp + sp)
C H+H
1s2py
2 x sp
Ethyne: hybrid orbitals
• Now combine HAO• Remember each HAO has 1 electron• So each MO has 2 electrons• 2π MO are perpendicular to rest of molecule• REMEMBER each p & π orbital has 2 lobes but is only 1 orbital!• REMEMBER antibonding orbitals also formed (not shown!)• Molecule is linear (straight)• Just one of many models (just very useful!)
H C C Hσ π
π
π (2pz + 2pz)2pz
C
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Hybridisation: sp2 orbitals
Ethene• Each carbon attached to 3 groups so hybridise 3 AO• Produces 3 HAO - the sp2 orbitals• sp2 orbitals have 1/3s character & 2/3p character• Due to direction of combined AO new HAO are at 120˚ & all in the
same plane
C CH
H H
H
2s 2px 2py 2pz
+ + +
3 x sp2
all in the plane
2pz
120˚
Carbon Atomic Orbitals
Carbon with 3 Hybrid Orbitals
3 x sp2
1 x AO
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C CH
HH
H
117.8˚
σ (1s + sp2) σ (sp2 + sp2)
π (2pz + 2pz)12 AO 12 MO6 bonding (shown) +
6 antibonding (not shown)
H
H
+ C
H
H90˚2pz
C
sp2
Ethene
• Simply combine HAO to give σ MO• Combine AO to give π orbital at 90˚ to rest of
molecule• Remember it is one orbital with 2 phases
C CH
H H
Hσ
π
σ
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MO theory for ethene
Bonding
Antibonding
HOMOhighest occupied molecular orbital
π orbital
LUMOlowest unoccupied molecular orbital
π* orbital
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Hybridisation: sp3 orbitals
Methane• Each carbon attached to 4 groups so hybridise all AO• Produces 4 HAO - the sp3 orbitals• sp3 orbitals have 1/4s character & 2/3p character• Due to direction of combined AO new HAO are at 109.5˚ in a
tetrahedral arrangement
HC
HHH
C
2s 2px 2py 2pz
+ + +
4 x sp3
tetrahedral
109.5˚
Carbon Atomic Orbitals
Carbon with 4 Hybrid Orbitals
4 x sp3
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+ C
H
HH
C
H
HH
σ (1s + sp3)
σ (sp3 + sp3)
C
H
HHH
4 identical MO in tetrahedron with σ symmetry (bonds)
σ (1s + sp3)
+ 4 H
s AO of hydrogen
Alkanes
4 sp3 orbitals form tetrahedron
C
HC C
HH
HH
H
C
H
H H
C
H
HH
HC
HHH
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Hybridisation works for all atoms
1s1s
2s
2px 2py 2pz
2(sp3) 2(sp3) 2(sp3) 2(sp3)
nitrogen AO nitrogen hybrid orbitals
combine
ener
gy
NH
HH N
HHH
Ammonia• Nitrogen attached to 4 groups (we count
lone pair)• So it will be sp3 hybridised• Combine with 3 x 1s of H• Gives 3 x s bonds & lone pair in sp3 orbital
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Hybrid orbitalsBorane BH3
• Boron attached to 3 groups so hybridise 3 AO• So boron sp2 and trigonal planar• Has empty p orbital (AO)
H
CH
HH
H
BH
HH
H
NH
HH
• Isoelectronic (same number of electrons) so have the same MO!• Only difference is the energy and hence size
H BH
H BHH
HB = sp2
empty 2p empty 2p
σ (1s + 2(sp2))
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MO works for all molecules
• 34 AO give 17 new bonding MOs (shown) & 17 antibonding MOs• It is only a model - but a very useful one!
O
CC
OCC
H
H H H H
H
C sp2
σ (1s + 2sp2)
σ (C2sp2 + C2sp2)
π (2p + 2p)
O sp3lone pairsp3
C sp3 σ (1s + 2sp3)
σ (1s + 2sp2)
π (O2p + C2p)
lone pairsp2
σ (C2sp2 + O2sp3)
σ (C2sp2 + O2sp2)
O sp2
OH
O
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Reactions & Lewis structures• Organic chemistry & reactions is about making & breaking bonds• Or moving electrons• Below is the reaction of an alkene with bromine
+ Br Br + Br Br
+ Br BrBr
Br
colourless red colourless red
colourless red colourless
• Could draw out all the orbitals each time to explain reaction• Too much like hard work!• Use the diagrams above - but what are they actually showing?
H H = = H H H Hx=
σ (1s+1s)containing 2electrons
H H
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≡ H FFH
CHH
HH H C
H
H
H
≡C + 4H
H
Lewis structures: electron bookkeepingHow do you draw a Lewis structure?
• Draw a dot for each valence electron on each atom (old group number)• Share electrons (form bonds) to get duplet (H) or octet (the rest)• Third row can have >8 electrons• Each bond (line) is TWO electrons
H
Be B C N O F
HeNeLi
lithium = 1 valence electron
carbon = 4 valence electrons
oxygen = 6 valence electrons
1st row
2nd row
group 1 2 3 4 5 6 7 8
C C
valence electrons
HF
Examples
CH4
+ F
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C CHH
HH
≡ C CH
HH
HC + 4HC +
CH
HH O H ≡ H C
H
O
H
HC + + 4HO
Examples of Lewis structures
• Remember: Draw lone-pairs (unshared electrons) - they are important• More than one covalent bond may be required!• Atoms can have less than an octet (very reactive!)
C + + 6HOH3C
CCH3
O ≡O
3
F ≡B + 3 FB FF F
BF
FBF3
acetone
CH2CH2
CH3OH