VSEPR Review and Valence Bond Theory

Report
UNIT 6
Theories of Covalent Bonding and
Intro to Organic Chemistry
VSEPR Theory, Molecular Shapes,
and Valence Bond Theory
Electron Domains
Electron domains are the regions in the molecules
where it is most likely to find electrons.
• For a bond (single, double, or triple), the
electron domain is between the two atoms in the
bond and consists of all the electrons involved
in the bond.
• For nonbonding pairs of electrons, the domain
is the nonbonding pair and is centered on a
single atom.
Valence Electrons
• Valence electrons are the electrons in the
outermost unfilled shell of the atom or ion.
These are usually s and p electrons, but can be
d electrons.
• For the main group elements, the number
of valence electrons is the last digit of the
group number.
• Knowing the number of valence electrons
allows us to draw Lewis symbols for the
elements and Lewis structures for compounds.
Lewis Structures
• Lewis structures are very helpful in
studying bonding because they show only
the valence electrons of the atoms or ions.
The Octet Rule
• Atoms tend to gain, lose, or share electrons
until they are surrounded by eight valence
electrons (the s and p subshells are full). We
use this rule to draw Lewis structures for
compounds.
Lewis Structures of Covalent Compounds
Follow these steps in order.
1. Decide which atoms are bonded.
2. Count all valence electrons.
3. Put two electrons in each bond.
4. Complete the octets of the atoms attached to the central atom
except H, which takes a duet.
5. Put any remaining electrons on the central atom.
6. If the central atom has less than an octet, form double or triple
bonds.
Putting Formal Charges on Lewis
Structures
The formal charge of any atom in a compound or ion
may be calculated using the following:
FC = # of valence electrons – number of bonds –
number of nonbonding electrons
FC of O = 6-2-4 = 0
FC of O = 6-1-6 = -1
FC of O = 6-3-2 = +1
Resonance Structures
Three completely equivalent Lewis structures can
be drawn for the nitrate ion, NO3-.
Reality is a blend of the three. There are no double
bonds in the nitrate ion, but each bond is more stable
than just a single bond. All three structures are
resonance structures.
From Lewis Structure to Electron
Domain Geometry via VSEPR
• The Lewis structure shows the covalent
bonds (solid lines) and the nonbonding
electrons (dots) that are present in a
compound.
• This allows the identification of the
electron domains of the molecule. Domains
are the regions in the molecules where it is
most likely to find electrons.
• Electron domain geometry gives bond
angle and hybridization.
VSEPR Theory
Valence Shell Electron Pair
Repulsion – qualitative
explanation of molecular
shapes
• Electrons in a domain are
subject to electrostatic
repulsion from the electrons in
the other domains. The
domains will orient themselves
so as to minimize this
repulsion.
• The orientation of these
domains is a function of the
number of domains around the
central atom and is one of
several simple geometric
figures.
Bond Angles when a Nonbonding
Electron Pair or Multiple Bond is Present
• Nonbonding electron pairs
take up more space than
bonding pairs and have the
effect of squeezing
(decreasing) the bond angles
among the atoms.
• Multiple bonds exert more
repulsion than single bonds,
and have the same effect on
bond angle as the nonbonding
electron pair.
Bond Angles for Atoms in Organic Molecules:
Effect of Nonbonding Electron Pairs
Example: C in an alkane
4 bonding domains
tetrahedral geometry
Bond angles 109.5°
Example: O in an ether
4 electron domains
tetrahedral geometry
Example: N in an amine
4 electron domains
tetrahedral geometry
Bond angles on N are
<109.5° because the epair takes up extra
space.
Bond angles on O are
<109.5° because the two
e- pairs take up extra
space.
Some Electron Domain Geometries for C
in Organic Molecules
carbocations
H-C≡C-H
Valence Bond Theory
Lewis Structures – Explain bonding as a sharing
of electron pairs and geometry through VSEPR.
VB Theory - A more quantitative approach to
explaining bonding. Here bonds are explained by
the overlap of orbitals on the two atoms in the
bond.
ENERGY
Orbital Energy Diagram
4s
4p
3s
3p
2s
1s
2p
3d
The periodic table may also
be used to determine the
electron configuration of the
elements.
Valence Bond Theory
Bonds occur from the overlap of atomic orbitals.
Cl: 1s22s22p63s23p5
H: 1s1
The bond in H-H is formed
by the overlap of the two H
1s orbitals.
The bond in H-Cl is formed by the overlap
of the H 1s orbital with the Cl 3p orbital.
How would you describe the Cl-Cl bond?
Bonds formed by end-to-end overlap are called sigma (σ)bonds.
Valence Bond Theory – Hybrid Orbitals
Describing bonds as the overlap of s and p orbitals
explains some geometries, but certainly not the
tetrahedral geometries (e.g. H2O).
What orbitals are overlapping here?
Valence Bond Theory – Hybrid Orbitals
Since the orbitals (s, p, d, f, etc.) are all mathematical
solutions to the Schrödinger wave equation, it is true
that linear combinations of these orbitals are also
solutions to the Schrödinger wave equation.
In other words, we may mix s, p, and d orbitals to
make new, hybrid orbitals that are also valid.
sp Hybrid Orbitals
Orbital energy
Consider HC≡CH. VSEPR says the molecule is linear.
How can hybrid orbitals explain the geometry?
2p
2s
C ground state (the orbitals of C
are what determine the geometry
of HC≡CH.)
1s
Two e- are available for bonding,
but the geometry is wrong.
sp Hybrid Orbitals
Orbital energy
Consider HC≡CH. VSEPR says the molecule is linear.
How can hybrid orbitals explain the geometry?
2p
Energy is used to promote
one 2s e- to a 2p orbital.
2s
A 2s orbital and a 2p orbital
mix to make two new
orbitals. The other two 2p
orbitals are unchanged.
1s
sp Hybrid Orbitals
Orbital energy
Consider HC≡CH. VSEPR says the molecule is linear.
How can hybrid orbitals explain the geometry?
sp
2p
The 2s orbital and one 2p orbital mix to
form two sp hybrid orbitals. The energy
of mixing is more than paid back when the
C-H and C≡C bonds are formed.
1s
sp Hybrid Orbitals
C
C
C
C
C
Hybrid orbitals have a small lobe and a large one. The large lobe
allows more overlap and, therefore, the formation stronger bonds. The
energy needed to make the hybrid orbital is paid back, with interest, in
the formation of stronger bonds.
sp2 Hybrid Orbitals
Consider H2C=CH2. VSEPR says the molecule is linear.
How can hybrid orbitals explain the geometry?
One 2p orbital remains
unhybridized.
Orbital energy
2p
sp2
Mixing the three orbitals gives three sp2
hybrid orbitals with the same energy.
1s
What orbitals overlap to form each of the C-H bonds?
sp2 Hybrid Orbitals
sp3 Hybrid Orbitals
Consider CH4. How can hybrid orbitals explain this
tetrahedral geometry?
Orbital energy
sp3
Mixing the four orbitals gives four sp3 hybrid
orbitals with the same energy.
1s
The VSEPR geometry tells you the hybridization:
Tetrahedral VSEPR geometry  sp3 hybrid orbitals.
sp3 Hybrid
Orbitals
What orbitals
overlap to form
each of the C-H
bonds in
methane?
sp3d Hybrid Orbitals
PCl5 has the shape of a trigonal bipyramid.
P ground state:
3s
3p
3d
Energy is used to
promote 3s e- to 3d:
Hybridize:
sp3d
3d
Trigonal bipyramid VSEPR geometry  sp3d hybrid
orbitals.
sp3d2 Hybrid Orbitals
SF6 has the shape of an octahedron.
S ground state:
3s
3p
3d
Energy is used to promote
3s and a 3p e- to 3d:
Hybridize:
sp3d2
3d
Octahedral VSEPR geometry  sp3d2 hybrid orbitals.
Use Lewis structures to get the electron
domain geometry, and that will lead to the
bond angles and hybridization.
# of edomains
2
3
4
5
6
e- domain
geometry
linear
trigonal
planar
tetrahedral
trigonal
bipyramidal
octahedral
bond angle
hybridization
180°
120°
sp
sp2
109.5°
90°,120°,180°
sp3
sp3d
90°,180°
sp3d2
Hybridization and Bond Angles in Larger
Molecules
Just identify the geometry/hybridization around
each atom in succession.
sp2, trigonal planar geometry,
bond angle is >120° (due to
double bond)
H :O:
..
|
||
H—C—C—O—H
acetic acid
|
¨
3, bent geometry, bond angle is
sp
H
<109.5°
sp3, tetrahedral geometry,
bond angle is 109.5°
What orbitals overlap
to form each of the
bonds in acetic acid?
Hybridization and Bond Angles in Larger
Molecules
H
|
H—C—C≡C—H
propyne
|
sp, linear geometry, 180°
H
sp3, tetrahedral geometry,
109.5°
Valence Bond Descriptions
H
|
H—C—C≡C—H
|
H
What orbitals overlap
to form each of the
bonds in propyne?
• The three C-H bonds shown at the left are each formed by the
overlap of a H 1s orbital with one of the four C sp3 hybrid orbitals.
• The C-C bond is formed by the overlap of the remaining sp3
orbital on the first C with one of the sp orbitals on the second C.
• The C≡C triple bond is formed by: 1) the overlap of C sp
orbitals (sigma bond), 2) the overlap of C 2py orbitals (pi bond),
and 3) the overlap of C 2pz orbitals (pi bond).
• The final C-H bond is formed by the overlap of the C sp orbital
with a H 1s orbital.

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