Transcript Chapter 10

Chapter 10
Bonding and Molecular
Structure:
Orbital Hybridization and
Molecular Orbitals
Dr. S. M. Condren
Advanced Theories of Chemical Bonding
Atomic Orbitals
Molecules
Dr. S. M. Condren
Two Theories of Bonding
• MOLECULAR ORBITAL
THEORY —
• Robert Mullikan (1896-1986)
• valence electrons are
delocalized
• valence electrons are in
orbitals (called molecular
orbitals) spread over entire
molecule.
Dr. S. M. Condren
Two Theories of Bonding
• VALENCE BOND
THEORY — Linus
Pauling
• valence electrons are
localized between atoms
(or are lone pairs).
• half-filled atomic orbitals
overlap to form bonds.
Dr. S. M. Condren
Sigma Bond Formation by Orbital Overlap
Two s orbitals
overlap
Dr. S. M. Condren
Sigma Bond Formation
Two s
orbitals
overlap
One s
and one p
orbital
overlap
Two p
orbitals
overlap
Dr. S. M. Condren
Using VB Theory
Bonding in BF3
•• ••
F ••
••••
F
••
B
Boron configuration

•••
F• 1s
••

2s

2p
planar triangle
angle = 120o
Dr. S. M. Condren
Bonding in BF3
• How to account for 3 bonds 120o apart using a
spherical s orbital and p orbitals that are 90o
apart?
• Pauling said to modify VB approach with
ORBITAL HYBRIDIZATION
• — mix available orbitals to form a new set
of orbitals — HYBRID ORBITALS — that
will give the maximum overlap, in the
correct geometry.
Dr. S. M. Condren
Bonding in BF3
2p
2s
hydridize orbs.
rearrange electrons
three sp2
hybrid orbitals
Dr. S. M. Condren
unused p
orbital
Bonding in BF3
• The three hybrid orbitals are made from 1
s orbital and 2 p orbitals  3 sp2 hybrids.
•
Now we have 3, half-filled HYBRID orbitals
that can be used to form B-F sigma bonds.
Dr. S. M. Condren
Bonding in BF3

An orbital from each F overlaps one of the
sp2 hybrids to form a B-F  bond.
F


F
B
F
Dr. S. M. Condren
BF3, Planar Trigonal
Dr. S. M. Condren
Bonding in CH4
How do we account for 4
C—H sigma bonds 109o
apart?
Need to use 4 atomic
orbitals — s, px, py, and
pz — to form 4 new
hybrid orbitals pointing in
the correct direction.
Dr. S. M. Condren
109o
Bonding in CH4
Dr. S. M. Condren
Bonding in a Tetrahedron —
Formation of Hybrid Atomic Orbitals
4 C atomic orbitals
hybridize to form
four equivalent sp3
hybrid atomic
orbitals.
Dr. S. M. Condren
Dr. S. M. Condren
Bonding in Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
••
O H
••
sp
Dr. S. M. Condren
2
3
Orbital Hybridization
BONDS
SHAPE
HYBRID REMAIN
2
linear
sp
2 p’s
3
trigonal
planar
sp2
1p
4
tetrahedral
sp3
none
Dr. S. M. Condren
Multiple Bonds
Consider ethylene, C2H4
H
H
120Þ
C
sp
C
H
H
Dr. S. M. Condren
2
Sigma Bonds in C2H4
H
H
120Þ
C
sp
C
H
H
Dr. S. M. Condren
2
π Bonding in C2H4
The unused p orbital on each
C atom contains an
electron and this p orbital
overlaps the p orbital on
the neighboring atom to
form the π bond.

2s
 
2p

 
3 sp 2
hybrid
orbitals

p
orb.
for š
bond
Dr. S. M. Condren
π Bonding in C2H4
The unused p orbital on each C atom contains an
electron and this p orbital overlaps the p orbital
on the neighboring atom to form the π bond.
Dr. S. M. Condren
Multiple Bonding
in C2H4
Dr. S. M. Condren
 and π Bonding in C2H4
Dr. S. M. Condren
 and π Bonding in CH2O
Dr. S. M. Condren
 and π Bonding in C2H2
Dr. S. M. Condren
 and π Bonding in C2H2
Dr. S. M. Condren
Consequences of Multiple Bonding
There is restricted rotation around C=C bond.
Dr. S. M. Condren
Consequences of Multiple Bonding
Restricted rotation around C=C bond.
Dr. S. M. Condren
Diatomic Molecules
AO
H
MO
H2
1s
AO
H
1s
ENERGY
B => bonding electrons
B - AB
2-0
B.O. = ------------ = -------- = 1
2
2
all electrons are paired, thus diamagnetic
Dr. S. M. Condren
AB => antibonding electrons
Diatomic Molecules
AO
He
MO
He2
1s
AO
He
1s
ENERGY
B - AB
2-2
B.O. = ------------ = -------- = 0
2
2
Dr. S. M. Condren
Zero bond order means
it does not exist
Diatomic Molecules
AO
He
MO
He2+
AO
He+
1s
1s
ENERGY
B - AB
2-1
B.O. = ------------ = -------- = 1/2
2
2
One unpaired electron, thus, paramagnetic
Dr. S. M. Condren
AO
B
MO
B2
AO
B
2p
2p
2s
1s
2s
nonbonding
1s
Dr. S. M. Condren
Molecular Orbital Diagram
antibonding MO
end-to-end
bonding MO
antibonding MO
side-by-side
bonding MO
Dr. S. M. Condren
AO
C
MO
C2
AO
C
2p
2p
2s
1s
2s
nonbonding
1s
Dr. S. M. Condren
AO
O
MO
O2
AO
O
2p
2p
2s
1s
2s
nonbonding
1s
Dr. S. M. Condren
Energy
Dr. S. M. Condren