Molecular Orbital Theory
(MO Theory)
1
z
MO theory are derived from a linear
combination of atomic orbitals of nearby
atoms to form the molecular orbitals.
– There are two possible combinations
• Adding two atomic orbitals forms a bonding
MO.
• Subtracting two atomic orbitals forms an
antibonding MO.
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Forming of H2
σ*1s
σ1s
H
H2
H
3
MO from atomic p orbital
End-to-end
combination give a
pair of σ MOs
Side-to-side
combination give a
pair of π MOs
Energy order :
σ 2 p < π 2 p < π 2* p < σ 2* p
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Electronic configurations of some
molecules
ethylene
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allyl system
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3
1,3-butadiene
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Orbital symmetry and the chemical reaction
”
”
A chemical reaction involves the crossing of an
energy barrier.
The reacting molecules seek the easiest path: low
path to avoid climbing any higher than is necessary;
and a broad path, to avoid undue restrictions on the
arrangement of atoms.
” Bonding is a stabilizing factor; the stronger bonding,
the more stable the system.
” If a reaction is to follow the easiest path, it must take
place in the way that maintains maximum bonding
during the reaction process.
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Orbital symmetry and the chemical reaction
”
”
Now bonding as we visualize it, results from overlap
of orbitals.
The theory lay in the mathematics, and
R.B.Woodward, Roald Hoffmann and K.FuKui made
predictions, which have since been borne out by
experiment. (1965 Nobel Prizes)
” Orbital symmetry effects are observed in
concerted reaction where several bonds are
being made or broken simultaneously.
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Orbital symmetry and the chemical reaction
”
”
Woodward and Hoffmann formulated “rules”, and
described certain reaction paths as symmetryallowed and others as symmetry-forbidden.
All of this applies only to concerted reactions, and
refers to the relative ease with which they take place.
” A symmetry-forbidden reaction which the concerted
mechanism is very difficult, it will probably do in a
different way: by a different concerted path (symmetryallowed) or by a stepwise, non-concerted mechanism.
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Pericyclic reaction
Three reaction types
™ Electrocyclic reaction (Intramolecular cyclization)
™ Cycloaddition reaction (Intermolecular Diel-Alder rxn.)
™ Sigmatropic reaction (Intramolecular rearrangement)
Bond breaking and forming : single atom or molecule with
conjugated double bond
Reagent : Thermal / Photochemical
Mechanism : concerted reaction (bond breaking and
forming occur in the same time)
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Electrocyclic reaction
™
Under the influence of heat or light, a conjugated
polyene can undergo isomerization to form a cyclic
compound with a single bond between the terminal
carbons.
1,3,5-hexatriene
1,3-cyclohexadiene
™ one double bond disappears and the remaining double
bonds shift their positions.
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Electrocyclic reaction
™
The reverse process can also take place : a single
bond is broken and a cyclic compound yields an
open-chain polyene.
1,3-butadiene
cyclobutene
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The stereochemistry of electrocyclic reaction
™
Interconversion of 2,4-hexadiene and
3,4-dimethylcyclobutene
H3C
H3 C
H
H
trans,cis-2,4-hexadiene
CH3
CH3
H
H
cis-3,4-dimethylcyclobutene
hν
H3C
H
H
CH3
trans,trans-2,4-hexadiene
CH3
H
H
CH3
trans-3,4-dimethylcyclobutene
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1, 3 butadiene
ψ1
ψ2
ψ3
ψ4
=
=
=
=
0.3717φ1 + 0.6015φ2 + 0.6015φ3 + 0.3717φ4
0.6015φ1 + 0.3717φ2 - 0.3717φ3 – 0.6015φ4
0.6015φ1 – 0.3717φ2 - 0.3717φ3 + 0.6015φ4
0.3717φ1 - 0.6015φ2 + 0.6015φ3 – 0.3717φ4
0 node
Sketch butadiene MO
Node คือชวงทีม่ ีการเปลี่ยนเครื่องหมาย
1 node
Coefficient เปนตัวกําหนด
ขนาดของ AO. อยางคราวๆ
ของ coefficient
2 node
3 node
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Thermal cyclization of a disubstituted
butadiene
RHC
ψ4
ψ3
C HC CHR
H
The highest occupied molecular orbital
of a conjugated diene is ψ2.
It is the electrons in this orbital that will
form the bond that closes the ring.
ψ2
ψ1
Bond formation requires overlap of
terminal lopes, in this case overlap
of lopes on C-1 and C-4 of diene.
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ψ2
HOMO of ground state
conjugated diene
Conrotatory
bonding
Disrotatory
antibonding
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Thermal cyclization of substituted butadiene
H3C
H3C
H
H
conrotatory
CH3
CH3
H
H
cis-3,4-dimethylcyclobutene
trans,cis-2,4-hexadiene
CH3 H
H3C
H
H
CH3
conrotatory
trans,trans-2,4-hexadiene
H
CH3
trans-3,4-dimethylcyclobutene
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Photochemical cyclization of substituted
butadiene
On absorption of light, butadiene is converted
into the excited state.
ψ3
Photochemical condition
ψ2
Thermal condition
hν
H3C
H
H
CH3
trans,trans-2,4-hexadiene
disrotatory
bonding
CH3
CH3
H
H
cis-3,4-dimethylcyclobutene
Give the photochemical cyclization pdt. of cis,cis-2,4-hexadiene
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