1.15 Bonding in Methane and Orbital Hybridization

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1.15Bonding in Methane andOrbital Hybridization
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Structure of Methane

• tetrahedral
• bond angles 109.5
• bond distances 110 pm
• but structure seems inconsistent withelectron
  configuration of carbon

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Electron configuration of carbon

• only two unpaired electrons
• should form s bonds to only two hydrogen atoms
• bonds should be at right angles to one another

2p
2s
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sp3 Orbital Hybridization
2p

• Promote an electron from the 2s to the 2p
  orbital

2s
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sp3 Orbital Hybridization
2p
2p
2s
2s
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sp3 Orbital Hybridization
2p

• Mix together (hybridize) the 2s orbital and the
  three 2p orbitals

2s
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sp3 Orbital Hybridization
2p
2 sp3

• 4 equivalent half-filled orbitals are consistent
  with four bonds and tetrahedral geometry

2s
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Shapes of orbitals
p
s
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Nodal properties of orbitals
p



s
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Shape of sp3 hybrid orbitals
p

• take the s orbital and place it on top of the p
  orbital

s
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Shape of sp3 hybrid orbitals
s p

• reinforcement of electron wave in regions where
  sign is the same
• destructive interference in regions of opposite
  sign

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Shape of sp3 hybrid orbitals
sp hybrid

• orbital shown is sp hybrid
• analogous procedure using three s orbitals and
  one p orbital gives sp3 hybrid
• shape of sp3 hybrid is similar

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Shape of sp3 hybrid orbitals
sp hybrid

• hybrid orbital is not symmetrical
• higher probability of finding an electron on one
  side of the nucleus than the other
• leads to stronger bonds

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The CH s Bond in Methane
In-phase overlap of a half-filled 1s orbital of
hydrogen with a half-filled sp3 hybrid orbital of
carbon

sp3
s
H
C
gives a s bond.

HC s
C
H
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Justification for Orbital Hybridization

• consistent with structure of methane
• allows for formation of 4 bonds rather than 2
• bonds involving sp3 hybrid orbitals are stronger
  than those involving s-s overlap or p-p overlap

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1.16sp3 Hybridization and Bonding in Ethane
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Structure of Ethane
C2H6
CH3CH3

• tetrahedral geometry at each carbon
• CH bond distance 110 pm
• CC bond distance 153 pm

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The CC s Bond in Ethane

• In-phase overlap of half-filled sp3
  hybridorbital of one carbon with half-filled
  sp3hybrid orbital of another.
• Overlap is along internuclear axis to give a s
  bond.

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The CC s Bond in Ethane

• In-phase overlap of half-filled sp3
  hybridorbital of one carbon with half-filled
  sp3hybrid orbital of another.
• Overlap is along internuclear axis to give a s
  bond.

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1.17sp2 Hybridization and Bonding in Ethylene
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Structure of Ethylene
C2H4 H2CCH2

• planar
• bond angles close to 120
• bond distances CH 110 pm CC 134 pm

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sp2 Orbital Hybridization
2p

• Promote an electron from the 2s to the 2p
  orbital

2s
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sp2 Orbital Hybridization
2p
2p
2s
2s
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sp2 Orbital Hybridization
2p

• Mix together (hybridize) the 2s orbital and two
  of the three 2p orbitals

2s
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sp2 Orbital Hybridization
2p
2 sp2

• 3 equivalent half-filled sp2 hybrid orbitals plus
  1 p orbital left unhybridized

2s
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sp2 Orbital Hybridization
p
2 of the 3 sp2 orbitalsare involved in s
bondsto hydrogens the otheris involved in a s
bondto carbon
2 sp2
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sp2 Orbital Hybridization
s
p
s
2 sp2
s
s
s
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p Bonding in Ethylene
the unhybridized p orbital of carbon is involved
in p bondingto the other carbon
p
2 sp2
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p Bonding in Ethylene
p
2 sp2

• each carbon has an unhybridized 2p orbital axis
  of orbital is perpendicular to the plane of the s
  bonds

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p Bonding in Ethylene
p
2 sp2

• side-by-side overlap of half-filledp orbitals
  gives a p bond
• double bond in ethylene has a s component and a
  p component

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1.18sp Hybridization and Bonding in Acetylene
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Structure of Acetylene
C2H2

• linear
• bond angles 180
• bond distances CH 106 pm CC 120 pm

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sp Orbital Hybridization
2p

• Promote an electron from the 2s to the 2p
  orbital

2s
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sp Orbital Hybridization
2p
2p
2s
2s
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sp Orbital Hybridization
2p

• Mix together (hybridize) the 2s orbital and one
  of the three 2p orbitals

2s
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sp Orbital Hybridization
2p
2 p
2 sp2

• 2 equivalent half-filled sp hybrid orbitals plus
  2 p orbitals left unhybridized

2s
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sp Orbital Hybridization
2 p
1 of the 2 sp orbitalsis involved in a s bondto
hydrogen the otheris involved in a s bondto
carbon
2 sp2
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sp Orbital Hybridization
s
2 p
s
2 sp2
s
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p Bonding in Acetylene
the unhybridized p orbitals of carbon are
involved in separate p bonds to the other carbon

2 p
2 sp2
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p Bonding in Acetylene
2 p
2 sp2

• one p bond involves one of the p orbitals on each
  carbon
• there is a second p bond perpendicular to this one

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p Bonding in Acetylene
2 p
2 sp2
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p Bonding in Acetylene
2 p
2 sp2
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1.19Which Theory ofChemical Bonding is Best?
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Three Models

• Lewis
• most familiareasiest to apply
• Valence-Bond (Orbital Hybridization)
• provides more insight than Lewis model
• ability to connect structure and reactivity to
  hybridization develops with practice
• Molecular Orbital
• potentially the most powerful method
• but is the most abstract
• requires the most experience to use effectively