Download - Molecular Geometry and Bonding Theories
Molecular Geometry and Bonding Theories
Two Simple Theories of Covalent Bonding Valence Shell Electron Pair Repulsion
TheoryVSEPRR. J. Gillespie - 1950’s
Valence Bond TheoryHybridized orbitalsL. Pauling - 1930’s & 40’s
Stereochemistry
Study of the 3 dimensional shapes of molecules TWO MODELS VSEPR Theory Valence Bond Theory
Some questions to examine:Why are we interested in shapes?
What role does molecular shape play in life?
How do we determine molecular shapes?
How do we predict molecular shapes?
Determining Molecular Structure
Draw the Lewis dot structureidentify central atom
Count # of regions of high electron density on central atom
VSEPR tells the geometry around central atom
Determining Molecular Structure Identify lone pair effect on ideal
molecular geometry Repeat procedure for more than one
central atom Determine polarity from entire molecular
geometryelectronegativity differences
VSEPR Theory
regions of high electron density around the central atom go as far apart as possible to minimize repulsions
five basic shapesbased on # of regions of high electron density
several modifications of these five basic shapes will also be examined
VSEPR Theory
Two regions of high electron density
VSEPR Theory
Three regions of high electron density
VSEPR TheoryFour regions of high electron density
VSEPR Theory
Five regions of high electron density
VSEPR Theory
Six regions of high electron density
VSEPR Theory
electronic geometry (electron domain)determined by the locations of regions of high
electron density around the central atom(s) molecular geometry
determined by the arrangement of atoms around the central atom(s)
electron pairs are not used in the molecular geometry determination
VSEPR Theory CH
4 - methane
electronic geometrytetrahedral
molecular geometry tetrahedral
bond angles = 109.5o
VSEPR Theory
H2O - water
electronic geometry tetrahedral
molecular geometrybent or angular
bond angle = 104.50
VSEPR Theory
lone pairs of electrons (unshared pairs) require more volume than shared pairs
there is an ordering of repulsions of electrons around central atom
VSEPR Theory
1 lone pair to lone pair repulsion is strongest2 lone pair to bonding pair repulsion is
intermediate3 bonding pair to bonding pair repulsion is
weakest mnemonic for repulsion strengths
lp/lp > lp/bp > bp/bp lone pair to lone pair repulsion is why bond
angles in water are less than 109.50
Valence Bond Theory covalent bonds are formed by overlap of
atomic orbitals atomic orbitals on the central atom can mix
and exchange their character - hybridization
common hybridspink flowers, mules, corn, grass
Valence Bond Theory hybridized orbitals describe same shapes
as VSEPR Name of orbital Shape of orbital
sp3 tetrahedral
sp2 trigonal planar
sp linear
sp3d trigonal bipyramidal
sp3d2 octahedral
Hybrid Orbitals
Electronic Structures Lewis Formulas
1s 2s 2p
Be 3s 3p
Cl [Ne]
Be ··
····
··
.Cl
Hybrid Orbitals
Dot Formula Electronic Geometry
180o - linear
BeCl Cl··
····
····
··BeCl Cl
····
····
····
····
Hybrid Orbitals
VSEPR Polarity
bondspolar very
3.5 51. 3.5 ativitiesElectroneg
Cl - Be- Cl
0.22.0
180o-linear
BeCl Cl····
Hybrid Orbitals
VSEPR Polarity
moleculenonpolar
symmetric are dipoles bond
Cl---Be---Cl
180o-linear
BeCl Cl····
Hybrid Orbitals
Molecular Geometry
same as electronic geometrysymmetrical & nonpolar
Hybrid Orbitals
Valence Bond Theory (Hybridization)
1s 2s 2p 1s sp hyb 2p
Be 3s 3p
Cl [Ne]
Hybrid Orbitals Linear
Hybrid Orbitals
examples BF
3, BCl
3
all are trigonal planar, nonpolar molecules
Hybrid Orbitals
Electronic Structures Lewis Formulas
1s 2s 2p
B 3s 3p
Cl [Ne]
B·· .
Cl··
····
.
Hybrid Orbitals
Dot Formula Electronic Geometry··
B
Cl
Cl Cl··
····
··
···· ··
····
·· ··
B··
··
··
120-trigonal planar
Hybrid Orbitals
VSEPR Polarity
120-trigonal planar
BClCl
Clbondspolar ery v
3.0 1.5 ativitiesElectroneg
Cl - B
1.5
Hybrid Orbitals
Molecular Geometry
Hybrid Orbitals
Valence Bond Theory (Hybridization)
1s 2s 2p 1s sp2 hybrid
B 5s 5p
Cl [Ne]
Hybrid Orbitals Trigonal Planar
Hybrid Orbitals
examples CH
4, CF
4, CCl
4,
SiH
4,
SiF
4
all are tetrahedral, nonpolar moleculesas long as they have the same 4 substituents
Hybrid Orbitals
Electronic Structures Lewis Formulas
2s 2p
C [He] C..
. .
Hybrid Orbitals
Electronic Structures Lewis Formulas
2s 2p
C [He] 1s
H
C..
. .
H .
Hybrid Orbitals
Dot Formula Electronic Geometry
C
H
H
H H......
..C
....
..
..
tetrahedral109.5o bond angles
Hybrid Orbitals
VSEPR Polarity
CH H
H
H
tetrahedralbondspolar slightly
2.1 2.5 ativitiesElectroneg
H- C
0.4
Hybrid Orbitals
Molecular Geometry
Hybrid Orbitals
Valence Bond 2s 2p four sp3 hybrid orbitals
C [He] C [He] 1s H
Hybrid Orbitals Tetrahedron
Hybrid Orbitals
Examples PF5, AsF5, PCl5, etc.
All are trigonal bipyramidal, nonpolar molecules.
Hybrid Orbitals
Electronic Structures Lewis Formulas
4s 4p
As [Ar] 3d10 2s 2p
F [He]
As··
...
F···· .··
Hybrid Orbitals
Dot Formula Electronic Geometry
··
As
F
F
FF
F
··
··
··
····
··
····
····
····
··
··
··
··
··
·· ··trigonal bipyramidal
As
··
··
······
Hybrid Orbitals
VSEPR Polarity
trigonal bipyramid
AsF
F
F
F
F
··
··
··
··
·· ··
··
··
····
··
····
····
bondspolar ry ve
4.0 2.1 ativitiesElectroneg
F- As
1.9
Hybrid Orbitals
VSEPR Polarity
trigonal bipyramid
AsF
F
F
F
F
··
··
··
··
·· ··
··
··
····
··
····
····
symmetric dipoles cancel nonpolar molecule
AsF
F
F
F
F
··
··
··
··
·· ··
··
··
····
··
····
····
Hybrid Orbitals
Molecular Geometry
Hybrid Orbitals
Valence Bond (Hybridization) 4s 4p 4d
As [Ar] 3d10
five sp3 d hybrids
Hybrid Orbitals Trigonal Bipyramidal Molecules
Valence Bond (Hybridization)
Hybrid Orbitals Trigonal Bipyramid Molecules
Valence Bond (Hybridization)
Variations of Trigonal Bipyramidal Shape If lone pairs are incorporated into the
trigonal bipyramidal structure, there are three possible new shapes.
One lone pair - seesaw shape Two lone pairs - T-shape Three lone pairs - linear
Hybrid Orbitals
Example SF6, SeF6, SCl6, etc.
These are octahedral and nonpolar molecules. if all 6 substituents are the same
Hybrid Orbitals
Electronic Structures Lewis Formulas 4s 4p
Se [Ar] 3d10 2s 2p
F [He] ··F·· .··
·· Se·· ..
Hybrid Orbitals
Dot Formula Electronic Geometry
Se
F
F
F
F
F
F
····
··
······
··
····
··
··
··
··
··
··
··
··
····
··
······
··
octahedral
Se
··
··
····
·· ··
Hybrid Orbitals
VSEPR Polarity
octahedral
SeF
F
F
F
F
F
bondspolar ery v
4.0 2.4 ativitiesElectroneg
F- Se
1.6
Hybrid Orbitals
VSEPR Polarity
octahedral
SeF
F
F
F
F
F
symmetric dipoles cancel nonpolar molecule
SeF
F
F
F
F
F
Hybrid Orbitals
Molecular Geometry
Hybrid Orbitals Octahedral Molecules
Valence Bond (Hybridization)
4s 4p 4d
Se [Ar] 3d10
six sp3 d2 hybrids
Hybrid Orbitals Octahedral Molecules Valence Bond (Hybridization)
AB6- No Lone Pairs - Octahedral Molecules
Variations of Octahedral Shape
If lone pairs are incorporated into the octahedral structure, there are two possible new shapes.
One lone pair - square pyramidal Two lone pairs - square planar
Compounds Containing Double Bonds Ethene or ethylene is the simplest double
bond containing organic compound. Lewis Dot Formula for ethene C2H4
N = 2(8) + 4(2) = 24 A = 2(4) + 4(1) = 12 S = 12
Compounds Containing Double Bonds Lewis Dot Formula for ethene C2H4
CC
H
HH
H
C CH
H
H
H····
·· ·· ··
··or
Compounds Containing Double Bonds VSEPR
suggests C atoms at center of two equilateral triangles
C C
H
H
H
H
Compounds Containing Double Bonds Valence Bond Theory
sp2 hybrids for the C atoms
1 electron remains in an unhybridized p orbital
2s 2p three sp2 hybrids 2p
C
Compounds Containing Double Bonds
C atoms are sp2 hybridized with single electrons in each of the lobes
Top view
Compounds Containing Double Bonds
The unhybridized 2p orbital on the C atom lies perpendicular to the planar sp2 lobes.Side view
Compounds Containing Double Bonds
C=C double bond results from head-on overlap of two sp2 hybrid orbitals
Compounds Containing Double Bonds
The portion of the double bond formed from the head-on overlap of the sp2 hybrids is designated as a bond.
Compounds Containing Double Bonds The other portion of the double bond, resulting
from the side-on overlap of the p orbitals, is designated as a bond.
Compounds Containing Double Bonds The final result is a bond that looks like this.
Compounds Containing Triple Bonds Ethyne or acetylene is the simplest triple
bond containing organic compound. Lewis Dot Formula for ethyne C2H2
N = 2(8) + 2(2) = 20 A = 2(4) + 2(1) =10 S = 10
Compounds Containing Triple Bonds Lewis Dot Formula for ethyne C2H2
C C HHCH HC·· ·· ···· ·· or
Compounds Containing Triple Bonds VSEPR
suggests C and H atoms are 180o apart.
H C C H
Compounds Containing Triple Bonds Valence Bond Theory
sp hybrids for the C atoms
2 electrons remain in unhybridized p orbitals
Compounds Containing Triple Bonds Valence Bond Theory
sp hybrids for the C atoms
2 electrons remain in unhybridized p orbitals
2s 2p two sp hybrids 2p
C
Compounds Containing Triple Bonds
triple bond results from head-on overlap of two sp hybrid orbitals
C C
Compounds Containing Triple Bonds Note thatthis gives us one bond and two
bonds.
Compounds Containing Triple Bonds The final result is a bond that looks like this.
Summary of Electronic & Molecular Geometries
Synthesis Question 1 The basic shapes that we have discussed are present in
essentially all molecules. Shown below is the chemical structure of vitamin B6 phosphate. What is the shape and hybridization of each of the indicated atoms in vitamin B6 phosphate?
N+
H
CH3
OH
COH
CH2
OP
O
O
O
1
2
4
5
3
Synthesis Question 1trigonal planar sp2
bent or angular sp3
tetrahedral sp3
trigonal planar sp2
trigonal planar sp2
N+
H
CH3
OH
COH
CH2
OP
O
O
O
1
2
4
5
3
Group Question 1 Shown below is the structure of penicillin-G.
What is the shape and hybridization of each of the indicated atoms in penicillin-G?
CH
C N
CH S
CH
C
CH3
CH3
OH
OO
NHO
CH2CCC
CC C
HH
H
HH
1
23 4
5 6
7
8910
Synthesis Question 2 As we all know, in the wintertime we are more
likely to get shocked when we walk across carpet and touch the door knob. Here is another wintertime experiment to perform. Turn on a water faucet until you have a continuous but small stream of water coming from the faucet. Brush your hair vigorously then hold the brush near the stream of water. You will notice that the stream bends towards the brush. Why does the water bend?
Synthesis Question 2
Since water is a highly polar molecule, it is attracted by the electromagnetic field generated by the hair brush. This causes the stream to bend.
Group Question 2 On a recent “infomercial” it was claimed
that placing a small horseshoe magnet over the fuel intake line to your car’s carburetor would increase fuel mileage by 50%. The reason given for the mileage increase was that “the magnet aligned the molecules causing them to burn more efficiently.” Will this work? Should you buy this product?