molecular geometry and hybrid orbitals
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Molecular Geometry and Hybrid Orbitals
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H-C-H bond angle 90o
Molecular Geometry
Why Should I Care About Molecular Geometry?
•Chemical properties:
•Biological properties: Enzyme specificity
H3C C
CH3
CH3
CH3
H3C C
H
H
C
H
H
C
H
H
CH3
Pentane (bp 36oC) 2,2-Dimethylpropane (bp 10oC)
H3C-Br + HO- CH3OH + Br-
(CH3)3C-Br + HO- (CH3)2C=CH2 + Br-
Molecular geometry (shape) influences...
•Physical properties:
Molecular Geometry
How Do We Determine Molecular Shape?•Empirically (spectroscopy, etc)•Predict via theory
Example: H C
H
O H
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...
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)
...minimizing electron repulsion (nonbonded interactions; van der Waals repulsions)
...resonance, conjugation, and aromaticity
Molecular Geometry SurveyBest geometry = largest distance between electron groups
Composition Geometries Example AB Linear
AB2 Linear
A B H H
A BB
AB B
Bond angle
O C O
CO O
AB3 T-shape
Trigonal planar
B A B
B
180o
A
B
B
B
H B H
H
B
H
H
H 120o
180o
90o
Bent
Molecular Geometry SurveyBest geometry = largest distance between electron groups
Composition Geometries Example
AB5 Trigonal bipyramid
Tetrahedral
B A
B
B
B
AB4 Square planar H C
H
H
H 90o
180o
C
H
H H
HA
B
B B
B109.5o
Explore and verify molecular geometry examples with molecular models
A
B
B B
B
Cl
PCl
ClCl
Cl
Receding back
Projecting out
Bond AnglesBest geometry = largest distance between electron groups
Are bond angles always equal?
Electron group repulsions equal Electron group repulsions unequal
Which electron groups have greatest repulsion? (Which electron groups are largest?)
Electron groups equal sizeAll H-C-H 109.5o
C
H
H H
H NH H
H
H < LPH-N-H 107o
•Bond angles controlled by magnitude of electron group repulsions
Approximate size ranking: H, F < lone pair, Cl, Br, I < all groups of atoms
C
C
H H
H
HH
H
CC
H < CH3
H-C-H < 109.5o
Bond angles equal Bond angles unequal
Bond Angles
•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
Example:
Not a consideration in Chem 30A}
N H
H
Four attachments
In general...
Trigonal planar due to resonance
Molecular Geometry and Orbitals
•Example: Two H 1s orbitals (spheres) overlap to form H-H sigma bond ( bond):
Hydrogen atoms
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Hydrogen molecule
Covalent bonds formed by overlap of orbitals
Molecular Geometry and Orbitals
H (1s) + C (2s, 2px, 2py, 2pz) C-H bonds
Therefore this orbital combination incorrect
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orthogonal
What orbitals are used for organic molecules? Example: Methane (CH4)
H-C-H bond angle 90o
H 1s + C 2px + C 2pz wrong H-C-H bond angle!
Verify with your model kit
Molecular Geometry and Orbitals
•s + px + py + pz sp3 + sp3 + sp3 + sp3
•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
Solution (Linus Pauling, 1931)
C
H
H H
H109.5o
sp3 carbon
•C sp3 + H 1s C-H bond
Molecular Geometry and Orbitals
s + px + py + pz
•Carbon = trigonal planar; need three bonds and three hybrid orbitals
•Three hybrid orbitals come from three atomic orbitals:
Hybridization scheme for atom with three electron groups Example:
Mixed Not used
C
H
H
C
H
H
Top view without pz Top view with pz Side view
sp2 pz
sp2 + sp2 + sp2 + pz
120o
90o
pzsp2
Molecular Geometry and Orbitals
C sp2 + H 1s
C sp2 + C sp2
C pz + C pz
Combine orbitals to build C
H
H
C
H
H C-H bondC-C bondC-C bond
Side view Top view
bond not shown
bond included
C-H bond
C-C bond
C-C bond
Explore and verify with molecular models
Molecular Geometry and Orbitals
s + px + py + pz
•Carbon = linear; need two bonds and two hybrid orbitals
•Two hybrid orbitals come from two atomic orbitals:
Hybridization scheme for atom with two electron groups Example:
Mixed Not used
Top view without pz Top view with pz Side view without py Side view with py
sppy
pz
sp + sp + py + pz
H C C H
Molecular Geometry and Orbitals
C sp + H 1s
C sp + C sp
C py + C py
C pz + C pz
Combine orbitals to build
C-H bondC-C bondC-C bondC-C bond
Side view py-py bond not shown
C-H bond
C-C bond
C-C bond
Top view pz-pz bond not shown
Explore and verify with molecular models
H C C H
Molecular Geometry and Orbitals
•Other double bonds (C=O, C=N) similar orbital arrangements to C=C
•Other triple bonds (CN) similar orbital arrangements to CC
•C, O, N almost always hybridized in organic molecules
A few final notes...
Four electron groupssp3
Three electron groupssp2
Two electron groupssp
C O H
H
H
H
C
H
H
O C NC
H
H
H
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