Molecular Geometry and Hybrid Orbitals

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Molecular Geometry and Hybrid Orbitals
H-C-H bond angle 90o
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Molecular Geometry
Why Should I Care About Molecular Geometry?

• Molecular geometry (shape) influences...
• Physical properties

Pentane (bp 36oC)
2,2-Dimethylpropane (bp 10oC)
(CH3)2CCH2 Br-

• Chemical properties

(CH3)3C-Br HO-
H3C-Br HO-
CH3OH Br-

• Biological properties Enzyme specificity

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Molecular Geometry
How Do We Determine Molecular Shape?

• Empirically (spectroscopy, etc)
• Predict via theory

Example
CH3OH Lewis structure implies planarity But other
geometries can be conceived So what is the real
structure of this molecule?

• Bonds vibrating, bending, etc.
• By geometry we usually mean most common
  geometry
• Most common geometry geometry with lowest energy

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What Controls Molecular Geometry?Most common
geometry geometry with lowest energy
Lowest energy results from...
...minimizing electron repulsion (nonbonded
interactions van der Waals repulsions) ...resonan
ce, conjugation, and aromaticity

• How does the molecule minimize electron
  repulsion?
• Moving electron clouds (electron groups) away
  from each other
• Electron groups Atoms
• Groups of atoms
• Lone pairs
• Valence Shell Electron Pair Repulsion theory
  (VSEPR)

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Molecular Geometry SurveyBest geometry largest
distance between electron groups
Composition Geometries
Example AB
Linear
AB2 Linear
180o
Bond angle
Bent
180o
AB3 T-shape
90o
Trigonal planar
120o
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Molecular Geometry SurveyBest geometry largest
distance between electron groups
Composition Geometries
Example
180o
AB4 Square planar
90o
Receding back
109.5o
Tetrahedral
Projecting out
AB5 Trigonal bipyramid
Explore and verify molecular geometry examples
with molecular models
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Bond AnglesBest geometry largest distance
between electron groups
Are bond angles always equal?

• Bond angles controlled by magnitude of electron
  group repulsions

Electron groups equal size All H-C-H 109.5o
H lt LP H-N-H 107o
H lt CH3 H-C-H lt 109.5o
Electron group repulsions equal
Electron group repulsions unequal
Bond angles equal
Bond angles unequal
Which electron groups have greatest repulsion?
(Which electron groups are largest?)
Approximate size ranking H, F lt lone pair, Cl,
Br, I lt all groups of atoms
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Bond Angles
In general...

• Two electron groups linear
  
• Three electron groups trigonal planar or
  distorted trigonal planar
• Four electron groups tetrahedron or distorted
  tetrahedron
• Warning! Atomic geometry may also be
  influenced by...
• ...resonance (covered in Chem 30A)
• ...conjugation
• ...aromaticity

Not a consideration in Chem 30A
Four attachments
Trigonal planar due to resonance
Example
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Molecular Geometry and Orbitals
Covalent bonds formed by overlap of orbitals

• Example Two H 1s orbitals (spheres) overlap to
  form H-H sigma bond (s bond)

Hydrogen atoms
Hydrogen molecule
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Molecular Geometry and Orbitals
What orbitals are used for organic molecules?
Example Methane (CH4)
H (1s) C (2s, 2px, 2py, 2pz) ? C-H bonds
orthogonal
H-C-H bond angle 90o
H 1s C 2px C 2pz ? wrong H-C-H bond angle!
Therefore this orbital combination incorrect
Verify with your model kit
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Molecular Geometry and Orbitals
Solution (Linus Pauling, 1931)

• Combine s, px, py, and pz orbitals to form
  correct of bonds with correct geometry
• Combinations hybrid orbitals
• Hybridization scheme for atom with four electron
  groups
• CH4 geometry tetrahedral need four bonds and
  four hybrid orbitals
• Orbital conservation four hybrid orbitals come
  from four atomic orbitals

• s px py pz ? sp3 sp3 sp3 sp3

• C sp3 H 1s ? C-H s bond

109.5o
sp3 carbon
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Molecular Geometry and Orbitals
Hybridization scheme for atom with three electron
groups Example

• Carbon trigonal planar need three bonds and
  three hybrid orbitals
• Three hybrid orbitals come from three atomic
  orbitals

s px py pz
? sp2 sp2 sp2 pz
Mixed
Not used
Top view without pz Top view with pz
Side view
pz
sp2
120o
sp2
pz
90o
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Molecular Geometry and Orbitals
C sp2 H 1s C sp2 C sp2 C pz C pz
C-H s bond C-C s bond C-C p bond
Combine orbitals to build
Side view
Top view
C-H s bond
p bond not shown
C-C p bond
p bond included
C-C s bond
Explore and verify with molecular models
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Molecular Geometry and Orbitals
Hybridization scheme for atom with two electron
groups Example

• Carbon linear need two bonds and two hybrid
  orbitals
• Two hybrid orbitals come from two atomic orbitals

s px py pz
? sp sp py pz
Mixed
Not used
Top view without pz Top view with pz
Side view without py Side view with py
sp py pz
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Molecular Geometry and Orbitals
C sp H 1s C sp C sp C py C py C pz C pz
C-H s bond C-C s bond C-C p bond C-C p bond
Combine orbitals to build
Side view py-py p bond not shown
Top view pz-pz p bond not shown
C-H s bond
C-C p bond
C-C s bond
Explore and verify with molecular models
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Molecular Geometry and Orbitals
A few final notes...

• Other double bonds (CO, CN) similar orbital
  arrangements to CC
• Other triple bonds (C?N) similar orbital
  arrangements to C?C
• C, O, N almost always hybridized in organic
  molecules

Four electron groups sp3
Three electron groups sp2
Two electron groups sp