Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp)...

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1 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 2p x 2p y 2p z 2(sp) 2(sp) 2p y 2p z carbon AO carbon hybrid orbitals 2 x sp (HAO) + 2 x p (AO) combine energy

Transcript of Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp)...

Page 1: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 2: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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

Page 3: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 4: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 5: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

π

σ

Page 6: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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MO theory for ethene

Bonding

Antibonding

HOMOhighest occupied molecular orbital

π orbital

LUMOlowest unoccupied molecular orbital

π* orbital

Page 7: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 8: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 9: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 10: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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))

Page 11: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 12: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 13: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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

Page 14: Hybridisation I - Massey Universitygjrowlan/intro/lecture4.pdf3 π (2py + 2py) H C C H σ (1s + sp) σ (sp + sp) H + C H 1s 2py 2 x sp Ethyne: hybrid orbitals • Now combine HAO •

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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