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CHAPTER 8. Molecular Structure & Covalent Bonding Theories. Chapter Goals. A Preview of the Chapter Valence Shell Electron Pair Repulsion (VSEPR) Theory Polar Molecules:The Influence of Molecular Geometry Valence Bond (VB) Theory. Chapter Goals. Molecular Shapes and Bonding. - PowerPoint PPT Presentation
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1
CHAPTER 8
Molecular Structure & Covalent Bonding Theories
2
Chapter Goals
1. A Preview of the Chapter
2. Valence Shell Electron Pair Repulsion (VSEPR) Theory
3. Polar Molecules:The Influence of Molecular Geometry
4. Valence Bond (VB) Theory
3
Chapter Goals
5. Linear Electronic Geometry: AB2 Species
6. Trigonal Planar Electronic Geometry: AB3 Species
7. Tetrahedral Electronic Geometry: AB4 Species
8. Tetrahedral Electronic Geometry: AB3U Species
9. Tetrahedral Electronic Geometry: AB2U2 Species
10. Tetrahedral Electronic Geometry – ABU3 Species11. Trigonal Bipyramidal Geometry12. Octahedral Geometry 13. Compounds Containing Double Bonds14. Compounds Containing Triple Bonds15. A Summary of Electronic and Molecular Geometries
Molecular Shapes and BondingMolecular Shapes and Bonding
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Stereochemistry Stereochemistry is the study of the
three dimensional shapes of molecules. Some questions to examine in this
chapter are:1. Why are we interested in shapes?
2. What role does molecular shape play in life?
3. How do we determine molecular shapes?
4. How do we predict molecular shapes?
5
Molecular Shapes
The shape of a molecule plays an important role in its reactivity.
By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.
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Two Simple Theories of Covalent Bonding
Valence Shell Electron Pair Repulsion Theory
– Commonly designated as VSEPR
– Principal originator
• R. J. Gillespie in the 1950’s
Valence Bond Theory
– Involves the use of hybridized atomic orbitals
– Principal originator
• L. Pauling in the 1930’s & 40’s
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VSEPR Theory In order to attain maximum stability, each
atom in a molecule or ion arranges the
electron pairs in its valence shell in such a
way to minimize the repulsion of their
regions of high electron density:
(a) Lone (unshared or nonbonding) pairs of
electrons
(b) Single bond
(c) Double bond
(d) Triple bond
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VSEPR Theory
These four types of regions of high
electron density (where the electron are)
want to be as far apart as possible. The
electrons repel each other.
There are five basic molecular shapes
based on the number of regions of high
electron density around the central atom.
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VSEPR TheoryThese are the
regions of high
electron density
around the
central atom for
two through six
electron
densities around
a central atom.
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Electron-Density Geometries
All one must do is count the number of electron density in the Lewis structure.
The geometry will be that which corresponds to that number of electron density.
HH
H:
:
Tetrahedral
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VSEPR Theory1.1. Electronic geometryElectronic geometry is determined by the
locations of regions of high electron density around the central atom(s).
2.2. Molecular geometryMolecular geometry determined by the arrangement of atoms around the central atom(s).
Electron pairs are not used in the molecular geometry determination just the positions of the atoms in the molecule are used.
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Molecular Geometries
The electron-density geometry is often not the shape of the molecule, however.
The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.
electron-densityGeometry - tetrahedral
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VSEPR Theory
An example of a molecule that has the same electronic and molecular geometries is methane - CH4.
Electronic and molecular geometries are tetrahedral.
H
C
HHH
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VSEPR Theory An example of a molecule that has
different electronic and molecular geometries is water - H2O.
Electronic geometry is tetrahedral. Molecular geometry is bent or angular.
H
C
HHH
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VSEPR Theory
Lone pairs of electrons (unshared pairs)
require more volume than shared pairs.
– Consequently, there is an ordering of
repulsions of electrons around central atom.
Criteria for the ordering of the repulsions:
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VSEPR Theory1 Lone pair to lone pair is the strongest repulsion.2 Lone pair to bonding pair is intermediate
repulsion.3 Bonding pair to bonding pair is weakest
repulsion. 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 109.5o.
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VSEPR Theory
lp/bp
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Multiple Bonds and Bond Angles
Double and triple bonds place greater electron density on one side of the central atom than do single bonds.
Therefore, they also affect bond angles.bp/bp
repulsion
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Nonbonding Pairs and Bond Angle
Nonbonding pairs are physically larger than bonding pairs.
Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.
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Nonbonding Pairs and Bond Angle
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Polarity
In Chapter 7 we discussed bond dipoles.
But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.
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Polar Molecules: The Influence of Molecular Geometry
Molecular geometry affects molecular polarity.– Due to the effect of the bond dipoles and
how they either cancel or reinforce each other.
A B A
linear molecule nonpolar
A B A
angular molecule
polar
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Polarity
By adding the individual bond dipoles, one
can determine the overall dipole moment for
the molecule.
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Polarity
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Polar Molecules: The Influence of Molecular Geometry
Polar Molecules must meet two requirements:
1. One polar bond or one lone pair of electrons on central atom.
2. Neither bonds nor lone pairs can be symmetrically arranged that their polarities cancel.
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Polarity
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Valence Bond (VB) Theory
Covalent bonds are formed by the overlapoverlap of atomic orbitals.
Atomic orbitals on the central atom can mix and exchange their character with other atoms in a molecule.– Process is called hybridizationhybridization.
Hybrids are common:1. Pink flowers 2. Mules
Hybrid Orbitals have the same shapes as predicted by VSEPR.
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Valence Bond (VB) Theory
Regions of High Electron
Density
Electronic Geometry
Hybridization
2 Linear sp
3 Trigonal planar
sp2
4 Tetrahedral sp3
5 Trigonal bipyramidal
sp3d
6 Octahedral sp3d2
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Molecular Shapes and BondingMolecular Shapes and Bonding
In the next sections we will use the following terminology:A = central atom
B = bonding pairs around central atom
U = lone pairs around central atom For example:
AB3U designates that there are 3 bonding pairs and 1 lone pair around the central atom.
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Linear Electronic Geometry:AB2
Species (No Lone Pairs of Electrons on A)
Some examples of molecules with this geometry are: BeCl
2, BeBr
2, BeI
2, HgCl
2, CdCl
2
All of these examples are linear, nonpolar molecules.
Important exceptions occur when the two substituents are not the same!BeClBr or BeIBr will be linear and polar!
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Linear Electronic Geometry:AB
2 Species (No Lone Pairs of Electrons
on A)Electronic Geometry
H
C
HHH
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Linear Electronic Geometry:AB
2 Species (No Lone Pairs of Electrons
on A)Polarity
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Linear Electronic Geometry:AB
2 Species (No Lone Pairs of Electrons
on A)Valence Bond Theory (Hybridization)
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Linear Electronic Geometry:AB2 Species (No Lone Pairs of
Electrons on A)
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Linear Electronic Geometry:AB2 Species (No Lone Pairs of
Electrons on A)
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Trigonal Planar Electronic Geometry: AB3 Species (No
Lone Pairs of Electrons on A) Some examples of molecules with this
geometry are: BF3, BCl3
All of these examples are trigonal planar, nonpolar molecules.
Important exceptions occur when the three substituents are not the same!BF2Cl or BCI2Br will be trigonal planar and
polar!
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Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons
on A)
Dot Formula Electronic Geometry
H
C
HHH
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Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of
Electrons on A)
Polarity
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Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of
Electrons on A)Valence Bond Theory (Hybridization)
3s 3p
Cl [Ne]
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Trigonal Planar Electronic Geometry:
AB3 Species (No Lone Pairs of
Electrons on A)
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Trigonal Planar Electronic Geometry: AB3 Species (No Lone
Pairs of Electrons on A)
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Tetrahedral Electronic Geometry: AB
4 Species (No Lone Pairs of
Electrons on A) Some examples of molecules with this
geometry are: CH
4, CF
4, CCl
4,
SiH
4,
SiF
4
All of these examples are tetrahedral, nonpolar molecules.
Important exceptions occur when the four substituents are not the same!CF3Cl or CH2CI2 will be tetrahedral and polar!
43
Tetrahedral Electronic Geometry: AB
4 Species (No Lone Pairs of Electrons
on A)
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Tetrahedral Electronic Geometry: AB
4 Species (No Lone Pairs of
Electrons on A)
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Tetrahedral Electronic Geometry: AB
4 Species (No Lone Pairs of
Electrons on A)
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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of
Electrons on A) Some examples of molecules with this geometry
are:
NH3, NF3, PH3, PCl3, AsH3
These molecules are our first examples of central atoms with lone pairs of electrons.Thus, the electronic and molecular geometries
are different.All three substituents are the same but
molecule is polarpolar. NH3 and NF3 are trigonal pyramidal, polar
molecules.
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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Valence Bond Theory
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Tetrahedral Electronic Geometry:
AB3U Species (One Lone Pair of Electrons on A)
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Electronic Geometry
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Molecular Geometry
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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Polarity
57
Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of
Electrons on A) Some examples of molecules with this geometry
are: H2O, OF2, OCl2, H2S
These molecules are our first examples of central atoms with two lone pairs of electrons.Thus, the electronic and molecular geometries are
different.Both substituents are the same but molecule is polarpolar.
Molecules are angular, bent, or V-shaped and polar.
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Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of
Electrons on A)Valence Bond Theory (Hybridization)
2s 2pO [He]
four sp3 hybrids
H
C
HHH
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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs
of Electrons on A) Some examples of molecules with this
geometry are: HF, HCl, HBr, HI, FCl, IBr
These molecules are examples of central atoms with three lone pairs of electrons.Again, the electronic and molecular geometries are
different.
Molecules are linear and polar when the two atoms are different.Cl2, Br2, I2 are nonpolarnonpolar.
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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)Dot Formula
H F··
····
··
Electronic Geometry
F
H :
:
:
tetrahedral
Molecular Geometry
F
H :
:
:
3 lone pairs
linear
PolarityHF is a polar molecule.
H
C
HHH
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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs
of Electrons on A)Valence Bond Theory (Hybridization)
2s 2pF [He]
four sp3 hybrids
F
H :
:
:
tetrahedral
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Some examples of molecules with this geometry
are: PF5, AsF5, PCl5, etc.
These molecules are examples of central atoms
with five bonding pairs of electrons.
The electronic and molecular geometries are the same.
Molecules are trigonal bipyramidal and nonpolar
when all five substituents are the same.
If the five substituents are not the same polar polar molecules
can result, AsF4Cl is an example.
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
Valence Bond Theory
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
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Molecular Geometry
Trigonal Bipyramidal
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new shapes.
1. One lone pair - Seesaw shape
2. Two lone pairs - T-shape
3. Three lone pairs – linear
The lone pairs occupy equatorial positions because they are 120o from two bonding pairs and 90o from the other two bonding pairs.
– Results in decreased repulsions compared to lone pair in axial position.
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
AB4U molecules have:
1. trigonal bipyramid electronic geometry
2. seesaw shaped molecular geometry
3. and are polar
One example of an AB4U molecule is
SF4
Hybridization of S atom is sp3d.
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Molecular Geometry
H
C
HHH
Lewis Dot
Electronic Geometry
seesaw
70
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
AB3U2 molecules have:
1. trigonal bipyramid electronic geometry
2. T-shaped molecular geometry
3. and are polar
One example of an AB3U2 molecule is
IF3
Hybridization of I atom is sp3d.
72
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
Molecular Geometry
H
C
HHH
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
AB2U3 molecules have:
1.trigonal bipyramid electronic geometry
2.linear molecular geometry
3.and are nonpolar
One example of an AB3U2 molecule is
XeF2
Hybridization of Xe atom is sp3d.
74
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
Molecular Geometry
H
C
HHH
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Trigonal Bipyramidal ElectronicGeometry: AB5, AB4U, AB3U2,
and AB2U3
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Some examples of molecules with this geometry are: SF6, SeF6, SCl6, etc.
These molecules are examples of central atoms with six bonding pairs of electrons.
Molecules are octahedraloctahedral and nonpolar nonpolar when all six substituents are the same.If the six substituents are not the same polarpolar
molecules can result, SF5Cl is an example.
77
Nonpolar
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
78
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
If lone pairs are incorporated into the octahedral
structure, there are two possible new shapes.
1. One lone pair - square pyramidal
2. Two lone pairs - square planar
The lone pairs occupy axial positions because
they are 90o from four bonding pairs.
– Results in decreased repulsions compared to
lone pairs in equatorial positions.
81
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
AB5U molecules have:
1.octahedral electronic geometry
2.Square pyramidal molecular geometry
3.and are polar.
One example of an AB4U molecule is
IF5
Hybridization of I atom is sp3d2.
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Molecular Geometry
H
C
HHH
83
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
AB4U2 molecules have:
1.octahedral electronic geometry
2.square planar molecular geometry
3.and are nonpolar.
One example of an AB4U2 molecule is
XeF4
Hybridization of Xe atom is sp3d2.
84
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Molecular Geometry Polarity
H
C
HHH
nonpolar
85
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
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Sigma () Bonds
Sigma bonds are characterized by– Head-to-head overlap.– Cylindrical symmetry of electron density about the
internuclear axis.
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Pi () Bonds
Pi bonds are characterized by– Side-to-side overlap.– Electron density above
and below the internuclear axis.
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Single Bonds
Single bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.
89
Multiple Bonds
In a multiple bond one of the bonds is a bond and the rest are bonds.
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Compounds Containing Double Bonds
Ethene or ethylene, C2H4, is the simplest organic compound containing a double bond.
Lewis dot formula
N = 2(8) + 4(2) = 24
A = 2(4) + 4(1) = 12
S = 12 Compound must have a double bond to obey
octet rule.
CC
H
HH
H
C CH
H
H
H····
·· ·· ··
··or
91
Compounds Containing Double Bonds
Valence Bond Theory (Hybridization)
C atom has four electrons.Three electrons from each C
atom are in sp2 hybrids. One electron in each C atom
remains in an unhybridized p orbital
VSEPR Theory suggests that the C atoms are at center of trigonal planes.
C C
H
HH
H
92
Compounds Containing Double Bonds
An sp2 hybridized C atom has this shape.
Remember there will be one electron
in each of the three lobes.
Top view of an sp2 hybrid
The single 2p orbital is perpendicular tothe trigonal planar sp2 lobes.The fourth electron is in the p orbital.
Side view of sp2 hybrid with p orbital included.
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Compounds Containing Double Bonds
Two sp2 hybridized C atoms plus p orbitals in proper orientation to form C=C double bond.
The head-on overlap of the sp2 hybrids is designated
as a bond.
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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.
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Compounds Containing Double Bonds
Thus a C=C bond looks like this and is made of two parts, one and one bond.
H
C
HHH
H
C
HHH
96
Multiple Bonds In a molecule like
formaldehyde (shown at left) an sp2 orbital on carbon overlaps in fashion with the corresponding orbital on the oxygen.
The unhybridized p orbitals overlap in fashion.
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Compounds Containing Triple Bonds
Ethyne or acetylene, C2H2, is the simplest triple bond containing organic compound.
Lewis Dot FormulaN = 2(8) + 2(2) = 20
A = 2(4) + 2(1) =10
S = 10
Compound must have a triple bond to obey octet rule.
C C HHCH HC·· ·· ···· ·· or
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Compounds Containing Triple Bonds
Valence Bond Theory (Hybridization)Carbon has 4 electrons. Two of the electrons are in sp
hybrids. Two electrons remain in unhybridized p orbitals.
VSEPR Theory suggests regions of high electron
density are 180o apart.
H C C H
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The head-on overlap of the sp2 hybrids is designated
as a bond.
The two 2p orbital are perpendicular to the sp lobes.The third and fourth electrons are in the p orbitals.
An sp hybridized C atom has this shape. Remember there will be one electron in each of the two lobes.
Compounds Containing Triple Bonds
100
Compounds Containing Triple Bonds
A bond results from the head-on overlap of two sp hybrid orbitals.
101
Compounds Containing Triple Bonds The unhybridized p orbitals form two bonds.
Note that a triple bond consists of one and
two bonds.
H
C
HHH
The final result is a bond
that looks like this.
102
Larger MoleculesIn larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.
103
Larger Molecules
This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.
104
N
CN
C
CC
NC
NCH
HH
CH H
H
H
CH
HH
O
O
Here is the structure for most students’ friend: CAFFEINE
1- Assign hybridization on C, N, and O. Beware I did not put
the lone pairs of electrons into the chemical drawing.
2- How many sigma bonds are present?
3- How many pi bonds are present?
4- How many lone pairs of electrons are present? (You have to look for them)
105
N
CN
C
CC
NC
NH
CH H
H
H
CH
HH
O
O
Here is the structure Theobromine, one of the components of TEA
1- Assign hybridization on C, N, and O. Beware I did not put
the lone pairs of electrons into the chemical drawing.
2- How many sigma bonds are present?
3- How many pi bonds are present?
4- How many lone pairs of electrons are present? (You have to look for them)
106
End of Chapter 8
This is a difficult chapter.
Essential to your understanding of chemistry!
107
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