Download hybrid_orbitals

Molecular Geometry and Hybrid Orbitals
H-C-H bond angle 90o
+
Molecular Geometry
Why Should I Care About Molecular Geometry?
Molecular geometry (shape) influences...
•Physical properties:
H3C
H
H
H
C
C
C
H
H
H
CH3
CH3
Pentane (bp 36oC)
•Chemical properties: (CH3)3C-Br + HOH3C-Br + HO•Biological properties: Enzyme specificity
H3C
C
CH3
CH3
2,2-Dimethylpropane (bp 10oC)
(CH3)2C=CH2 + BrCH3OH + Br-
Molecular Geometry
How Do We Determine Molecular Shape?
•Empirically (spectroscopy, etc)
•Predict via theory
H
Example:
H
C
H
O
H
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
What Controls Molecular Geometry?
Most common geometry = geometry with lowest energy
Lowest energy results from...
...minimizing electron repulsion (nonbonded interactions; van der Waals repulsions)
...resonance, 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)
Molecular Geometry Survey
Best geometry = largest distance between electron groups
Composition Geometries
AB
Linear
AB2
Linear
Example
A
H
B
B
A
B
O
C
O
180o
Bond angle
Bent
H
A
C
B
B
O
O
180o
AB3
T-shape
B
A
B
H
B
B
H
B
Trigonal planar
B
A
90o
H
H
B
H
B
120o
H
Molecular Geometry Survey
Best geometry = largest distance between electron groups
Composition Geometries
Example
B
AB4
Square planar
B
A
H
B
H
B
Receding back
B
Tetrahedral
A
B
B
B
A
B
B
C
H
H
90o
H
C
B
B
H
180o
109.5o
H
H
Projecting out
Cl
AB5 Trigonal bipyramid
Cl
Cl
P
Cl
Cl
Explore and verify molecular geometry examples with molecular models
Bond Angles
Best geometry = largest distance between electron groups
Are bond angles always equal?
•Bond angles controlled by magnitude of electron group repulsions
H
H
H
C
H
H
C
H
Electron groups equal size
All H-C-H 109.5o
Electron group repulsions equal
Bond angles equal
N
H
H
H
C
H
H
H < LP
H-N-H 107o
C
H
C
H
H < CH3
H-C-H < 109.5o
Electron group repulsions unequal
Bond angles unequal
Which electron groups have greatest repulsion? (Which electron groups are largest?)
Approximate size ranking: H, F < lone pair, Cl, Br, I < all groups of atoms
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)
Not a consideration in Chem 30A
}
...aromaticity
...conjugation
Four attachments
Example:
N
H
H
Trigonal planar due to resonance
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
Molecular Geometry and Orbitals
What orbitals are used for organic molecules?
H (1s) + C (2s, 2px, 2py, 2pz)  C-H bonds
Example: Methane (CH4)
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
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
H
C
H
109.5o
H
H
sp3 carbon
Molecular Geometry and Orbitals
H
Hybridization scheme for atom with three electron groups
Example:
•Carbon = trigonal planar; need three bonds and three hybrid orbitals
H
C
C
H
H
•Three hybrid orbitals come from three atomic orbitals:
s + px + py + pz  sp2 + sp2 + sp2 + pz
Mixed
Top view without pz
120o
Not used
Top view with pz
Side view
pz
sp2
sp2
pz
90o
Molecular Geometry and Orbitals
H
Combine orbitals to build
H
C
H
Side view
C-H s bond
C
H
C sp2 + H 1s
C sp2 + C sp2
C pz + C pz
C-H s bond
C-C s bond
C-C p bond
Top view
p bond not shown
C-C p bond
C-C s bond
Explore and verify with molecular models
p bond included
Molecular Geometry and Orbitals
Hybridization scheme for atom with two electron groups
Example:
H
C
C
H
•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
sp
py
pz
Side view without py
Side view with py
Molecular Geometry and Orbitals
Combine orbitals to build
H
C
Side view
py-py p bond not shown
C-H s bond
C
H
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
Top view
pz-pz p bond not shown
C-C p bond
C-C s bond
Explore and verify with molecular models
Molecular Geometry and Orbitals
A few final notes...
•Other double bonds (C=O, C=N) similar orbital arrangements to C=C
•Other triple bonds (CN) similar orbital arrangements to CC
•C, O, N almost always hybridized in organic molecules
Four electron groups
sp3
H
H
C
H
Three electron groups
sp2
Two electron groups
sp
H
H
O
H
C
H
O
H
C
H
C
N