Organic Pedagogical Electronic
Network
Molecular Orbitals
Mariana Neubarth Coelho
Edited by Margaret Hilton
Honors Organic Chemistry
University of Utah
Molecular Orbitals
Molecular Orbital Theory
1929 - John Lennard-Jones describes “Atomic states” and “Molecular states”
Representation of Molecular Orbials
π bond
σ bond
Bonding:
in phase
(matching colors)
Antibonding:
out of phase
(non matching colors)
Lennard-Jones, J.E. (1929) Trans.Faraday Soc. 25, 668. Link
H
H
H
H
H
H
H
H
Bonding:
in phase
(matching colors)
Antibonding:
out of phase
(non matching colors)
Building Energy Diagrams
• Molecular orbitals result of
combination of atomic orbitals.
Example Energy Diagram of CH3Br
the
• Orbitals cannot be created nor
destroyed. The number of molecular
orbitals equals the number of atomic
orbitals involved.
antibonding
MO
• More electronegative atoms
lower-energy atomic orbitals.
energy
• Orbitals need to be in phase in order
to overlap. Thus, bonding orbitals are
in phase and antibonding orbitals are
out of phase.
Br
(AO)
bonding
MO
have
• Lower-energy molecular orbials are
filled first (Aufbau principle).
sp3 C
(AO)
2 atomic orbitals  2 molecular orbitals (MO)
(AO)
1 bonding and 1 antibonding
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
Frontier Molecular Orbitals
Frontier orbitals are where reactions take place.
HOMO: highest-energy occupied molecular orbital – acts as a nucleophile
LUMO: lowest-energy unoccupied molecular orbital – acts as an electrophile
Frontier Orbitals of CH3Br
•
•
•
Because bromine is more electronegative than carbon, the
σ-bond is polarized with bromine bearing more of the
electron density.
H3C-Br (σ bond)
The HOMO contains the electrons of this bond. It has more
contribution from bromine because they are closer in
energy.
The LUMO is empty and has more contribution from
carbon.
If this molecule was attacked by a nucleophile in an SN2
reaction, the antibonding orbital would receive electrons,
forming a new bond between carbon and the nucleophile.
Bromine, the leaving group, would take the electrons from
the C-Br bond.
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
LUMO
energy
•
sp3 C
(AO)
Br
(AO)
HOMO
Why are antibonding orbitals important?
•
The transfer of electrons from one reactant to another requires the overlap of a filled
orbital (HOMO) and an empty orbital (LUMO).
•
Orbitals need to be in phase in order to overlap.
•
If the LUMO is inaccessible, the reaction may not occur.
•
Filled orbitals cannot receive electrons (Pauli exclusion principle).
SN2 reactions
Primary or
secondary
alkyl halides
Tertiary
alkyl
halides
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
This reaction does not occur
because the antibonding
orbital is too hindered.
Why are antibonding orbitals important?
•
The transfer of electrons from one reactant to another requires the overlap of a filled
orbital (HOMO) and an empty orbital (LUMO).
•
Orbitals need to be in phase in order to overlap.
•
If the LUMO is inaccessible, the reaction may not occur.
•
Filled orbitals cannot receive electrons (Pauli exclusion principle).
E2 reactions
Antiperiplanar: the bonding orbital of the
C-H bond and the antibonding orbital of the
C-Br bond must rehybridize in order to
form the π bond (note: they are in phase).
Synperiplanar: the bonding orbital of the
C-H bond and the antibonding orbital of the
C-Br bond are out of phase, therefore
they cannot overlap to form the π bond.
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
Occupied and Unoccupied Orbitals
Occupied
orbitals
can be
occupied non-bonding orbitals (lone pairs)
bonding orbital of a σ bond
bonding orbital of a π bond
Unoccupied
orbitals
can be
unoccupied non-bonding orbitals (cations)
antibonding orbital of a σ bond
antibonding orbital of a π bond
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
Molecular Orbitals: π Systems
Conjugation: p orbitals on adjacent carbons can overlap allowing the π electrons to
delocalize, thus lowering the energy of the molecule. In order for there to be overlap,
orbitals need to be in phase.
1,3 diene
3 nodes
energy
antibonding
2 nodes
LUMO
•
Each carbon is sp2 hybridized.
•
There are 4 electrons in the conjugated
π system of this molecule.
•
The nodes (dashed lines) represent a
switch in phase, meaning that orbitals
can not overlap. No electrons can be
found where there is a node.
•
One node is added (based on
symmetry) for each increase in energy
level.
1 node
HOMO
bonding
0 nodes
4 atomic orbitals

4 moleclar orbitals
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
Molecular Orbitals: π Systems
energy
2 nodes
1 node
0 nodes
Antibonding
•
Each carbon is sp2 hybridized.
•
Conjugation still exists even if there
are only 2 electrons. The positive
charge and the π electrons are
delocalized over the 3 carbons.
•
One node is added (based on
symmetry) for each increase in energy
level.
Non-bonding
Bonding
3 atomic orbitals  3 molecular orbitals
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
Molecular Orbitals: Predicting Interactions
•
The HOMO on one reactant donates electrons to the LUMO on the other. Both reactants have
a HOMO and LUMO, but the best HOMO-LUMO combination will be have the best orbital
overlap, or in other words, the best match in energy.
Example: acid – base chemistry
•
The low LUMO on H3O+ makes it a good acid, but also prevents it from acting as a base
under normal circumstances, despite the presence of a lone pair on the oxygen. Similarly, the
high HOMO on –OH makes it a good base, but also prevents it from acting as an acid despite
the potentially acidic O-H bond.
–OH
As the formal charge on a
species
becomes
more
positive, the energies of its
frontier orbitals decrease.
LUMO
energy
•
H3O+
LUMO
HOMO
AE vs. AE
HOMO
AE: Activation energy
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
Applications: Diels-Alders Reaction
Diels-Alders Reaction
The HOMO on the diene overlaps
with the LUMO on the dienophile.
This is the best match in energy.
energy
LUMO
LUMO
AE
HOMO of
the diene
HOMO
HOMO
LUMO of
the dienophile
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
Applications: Diels-Alders Reaction
Reverse Diels-Alders
•
The presence of an electron withdrawing
group (EWG) on the dienophile decreases its
energy.
•
The presence of an electron donating group
(EDG) on the diene increases its energy.
LUMO
energy
LUMO
AE
HOMO
HOMO
As a consequence,
the HOMO on the
dienophile interacts
with the LUMO on the
diene.
Lewis, D. Journal of Chemical Education, 1999, 76, 1718.
LUMO of
the diene
HOMO of
the dienophile
Problems