Transcript Frontier Molecular Orbitals

Third Year
Organic Chemistry Course
CHM3A2
Frontier Molecular Orbitals
and
Pericyclic Reactions
- Prof Jon A Preece School of Chemistry
University of Birmingham
Prof Preece’s Powerpoint Lecture Presentations
and answers to questions can be found at…
www.nanochem.bham.ac.uk
Teaching Resources
Username: Undergradchem
Password: Preece57nano
Queries on course after reading around the subject
to [email protected].
Be Specific with the problem(s) in your email.
Give me three times when you are free to see me.
I will email you back with a time to see me.
Course Synopsis
Part
Contents
1
Pericyclic Reactions
These lectures will begin with a definition of Pericyclic reactions, and will be
exemplified by considering examples of cycloaddation, sigmatropic, and electrocyclic reactions. It will be
highlighted how it is possible to use FMO theory (and other theories) to predict the constitution and
stereochemical outcome of the products. Attention will be drawn to the cyclic transition state and the number of
electrons involved (Huckel or Mobius), highlighting that when 4n+2 electrons are involved the reaction proceeds
readily under thermal conditions, and the reversibility of such reactions. The concept of Linear Combination of
Atomic Orbitals to form a bond(s) (and antibond(s)) will be revised, and extended to the linear combination of
frontier molecular orbitals. The p-molecular orbitals of ethene, butadiene and 1,3,5-hexatriene will be considered
and the identities of the HOMO and LUMO will be established, as well as the FMOs of a C–H bond.
2i
Electrocyclic Reactions
This lecture will extend the predicative nature of FMO theory regarding the
stereochemical outcomes to electrocyclic reactions for 4 and 6 p-electron transition states (by defining the
disrotatory or conrotatory movement of the termini of the HOMO in the Transition State).
2ii
Cycloaddition Reactions
These lectures will introduce cycloaddition reactions and the concepts of (i) phase
relationships of the FMOs, (ii) geometry of approach of the FMOs (suprafacial and antarafacial will be
defined), and (iii) minimum energy differences between the HOMO and LUMO. These concepts will be
exemplified by several Diels-Alder and related reactions. Attention will be drawn to the nature (chemical and
stereochemistry) of substituents and their stereochemistry in the product.
3
Photochemically Induced Pericyclic reactions
These lecture will extend the predicative nature of FMO
theory regarding the outcomes of electrocyclic reactions and cycloaddition reactions by considering how they can
be induced photochemically, to give alternative stereochemical outcomes and allow reactions that did not go
thermally.
Part 1.
Frontier Molecular Orbitals
Constructing molecular orbitals and identifying the frontier
molecular orbitals
Part 2.
Thermal Pericyclic Reactions
(i) Electrocyclic Reactions using FMO Theory
(ii) Cycloaddition Reactions using FMO Theory
Part 3.
Photochemical Pericyclic Reactions
(i) Electrocyclic Reactions using FMO Theory
(ii) Cycloaddition Reactions using FMO Theory
Second Year Organic Chemistry Course
CHM3A2
Recommended Reading
I Fleming
Frontier Orbitals and Organic Chemical Reactions, John Wiley and Sons, 1996.
Part 1:
Ch 1 and Ch 2
Part 2 and 3:
Ch 4
Second Year Organic Chemistry Course
CHM3A2
Frontier Molecular Orbitals and
Pericyclic Reactions
Part 1(i):
The Questions FMO Analysis Can
Answer

100%
0%
Ionic And Radical Reactions
To date you have seen two broad categories of reaction:
(i)
Ionic reactions
Here pairs of electrons move in one direction
e.g. SN2, SN1, E2 and E1 mechnisms
B
H
R1R4
(ii)
R2 3
R
BH
Cl
R1
R2
R4
R3
Cl
Radical reactions
Here single electrons move in a correlated manner
e.g. chlorination of alkanes
CH3
Cl
Cl
H3C
Cl
Cl
Pericyclic Reactions
Pericyclic reactions are the third distinct class.
They involve cyclic transition states
In which all bond breaking and bond making steps take place in
commensurate manner
And there is no sense of the flow of electrons.
Pericyclic Reactions: Electrocyclic Reactions
Stereospecific Reaction

Clockwise
Anti-Clockwise
100%
0%
There is no real senses of flow for
the electrons in pericyclic reactions
Pericyclic Reactions: Cycloaddition Reactions
Kinetic
Product
Stereospecific Reaction

CO2Me
CO2Me
Thermodynamic
Product
CO2Me
CO2Me
CO2Me
CO2Me
100%
0%
Regiospecific Reaction
MeO
CHO

MeO
CHO
MeO
CHO
0%
100%
Revision: 1,3–Syndiaxial Interactions
Br
1,3-syndiaxial interactions
Br
H
H
H
H
Br
Br
H
H
H
H
H
1
H
2
3
H
axial
H
Br
H
Br
equitorial
H
H
H
H
H
H
Br
H
H
H
Thermodynamic and Kinetic Control
Thermodynamic
Product
Kinetic
Product
Formed in
Cycloaddition MeO C
2
Reaction
CO2Me
Not Formed in
Cycloaddition
MeO2C
Reaction
CO2Me
H
H
H
CO2Me
CO2Me
CO2Me
H
CO2Me
MeO2C
H
MeO2C
MeO2C
H
MeO2C
H
H
H
Pericyclic Reactions: Sigmatropic Reactions
D
Stereospecific
Reaction
H
D
Regiospecific
Reaction
Me
Me
Me
H
100%
D
0%
Pericyclic Reactions: Why are they so specific?
Pericyclic reactions show high degrees of
(i)
Stereoselectivity
(ii)
Regioselectivity, and
(iii)
Diastereoselectivity
Thus, an obvious question to ask ourselves at this point is why are
pericyclic reactions so selective?
To help begin to answer this question we shall briefly need to revise the
SN2 reaction mechanism where YOU WILL remember that this reaction
type was highly stereoselective leading to inversion of chiral centres.
Revision: SN2 Reaction Mechanism
Bimolecular
Process
R1
Nucleophile attacks from behind
the C-Cl s-bond.
Cl
Nu
R
3
R2
Rate = k[R-Hal][Nu]
sp3
This is where the s*-antibonding
orbital of the C-Cl bond is
situated.
Rate
Determinig
Step
R1
1
–
2
Nu
Cl
R3
Bond
Forming
1
–
2
R2
Bond
Breaking
Transition State –
Energy Maxima
sp2
R1
Nu
R3
R2
Inversion
of Configuration
Cl
The concerted flow of both pairs of electrons in the SN2 reaction
mechanism leads to the transition state which allows the
stereochemical information to be retained,
i.e. a stereoselective reaction.
This SN2 reaction mechanism should be contrasted to the SN1 reaction
mechanism where the arrow-pushing is the same but the two pairs
electrons do not flow in a concerted fashion. Instead, the haloalkane
C-Cl bond heterolytically cleaves to give the planar sp2 hybridised
carbocation reactive intermediate. Now the nucleophile can attack
from either side of the carbocation leading to racemisation,
i.e. a non-stereoselective reaction.
Revision: Transition States
Discussion of reaction mechanisms frequently include discussions of the
nature of the transition state for each step in a reaction sequence – or at least
for the slowest or rate limiting step.
A transition state is the point of highest energy in a reaction or in each step of
a reaction involving more than one step.
The nature of the transition state will determine whether the reaction is a
difficult one, requiring a high activation enthalpy (G‡), or an easy one.
Transition states are always energy maxima, I.e. at the top of the energy hill,
and therefore, can never be isolated: there are no barriers to prevent them
from immediately “rolling” downhill to form the reaction products or
intermediates (or even reform the starting materials).
A transition states structure is difficult to identify accurately. It involves partial
bond cleavage and partial bond formation. However, it is nigh on impossible to
estimate whether the transition state is an early one (looks more like the
starting materials) or a late one (looks more like the products)
Revision: Transition States
Transition
State
Energy
Maxima
E
n
e
r
g
y
G‡
Starting
Material
A+B
Go
Reaction Coordinate
Product
C+D
Pericyclic Reactions: Transition States
Thus, now we can start to understand why pericyclic reactions are so
highly stereo-, regio-, and diasteroselective.
Pericyclic reactions involve concerted flow of pairs of electrons going
through transition states which retains stereochemical information that
was present in the starting material.
Pericyclic Reactions Involve Cyclic Transition States
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Cyclic
Transition
State
CO2Me
CO2Me
Pericyclic reactions involve ene and polyene units.
Thus, the transition states involve the overlap of pmolecular orbitals in the case of electrocyclic and
cycloaddition reactions, and a p-molecular orbital
and s-molecular orbital in the case of sigmatropic
reactions.
CO2Me
CO2Me
How do the orbitals overlap?
CO2Me
CO2Me
Frontier Molecular Orbitals
In order to understand the selectivity of pericyclic reactions, we need to
understand these molecular orbitals and how they overlap.
In particular, we need to know how the Frontier Molecular Orbitals
(FMOs) interact in the starting material(s) which lead to the cyclic
transition states.
We will first revise some simple molecular orbitals of a C-H s-bond and
a C=C p-bond and then extend this analysis to highly conjugated linear
polyenes and related structures/
Second Year Organic Chemistry Course
CHM2C3B
Frontier Molecular Orbitals and
Pericyclic Reactions
Part 1(ii):
Frontier Molecular Orbitals
Electronic
Ground State
LUMO
HOMO
Therm al
Reactions
4
3 nodes
3
2 nodes
2
1 node
1
0 nodes
CHM2C3B
– Introduction to FMOs –
– Learning
Objectives Part 1 –
Frontier Molecular Orbitals
After completing PART 1 of this course you should have an understanding of, and be able to demonstrate, the following terms, ideas and
methods.
(i)
Given a set of n p-orbitals you should be able to construct a molecular orbital energy level
diagram
results from their combination.
(ii)
In this diagram you should be able to identify for each MO
nodes
the symmetric (S) or antisymmetric (A) nature of the MO towards a C2
axis or mirror plane
the bonding, nonbonding or antibonding nature of it
(iii)
For a set of n molecular orbitals you should be able to identify the frontier molecular orbitals.
the highest occupied molecular orbital (HOMO )
the lowest unoccupied molecular orbital (LUMO)
(iv)
The HOMO (thermal reaction) interactions are important when evaluating the probability of an
unimolecular reaction occurring and the stereochemical outcome – see electrocyclic reactions.
The HOMO/LUMO (thermal reaction) interactions of the reacting species are important when
evaluating the probability of (i) a bimolecular reaction occurring and the stereochemical
outcome– see cycloaddition reactions, and (ii) a unimolecular reaction occurring and the
stereochemical outcome – see sigmatropic reactions.
The geometry, phase relationship and energy of interacting HOMOs and LUMOS is important for
evaluating the probability of a reaction occurring and the stereochemical outcome.
which
s-Bond
Two s Atomic Orbitals
Molecular Orbitals
ANTI-BONDING
MOLECULAR
ORBITAL
E
N
E
R
G
Y
Nodal Plane
s*
+
s ATOMIC ORBITAL
BONDING
MOLECULAR
ORBITAL
s
s-Bond
One s Atomic Orbital and One sp3 Atomic Orbital
Molecular Orbitals
ANTI-BONDING
MOLECULAR
ORBITAL
E
N
E
R
G
Y
s*
sp3 ATOMIC ORBITAL
+
BONDING
MOLECULAR
ORBITAL
s
p-Bond:
Two p Atomic Orbitals
Molecular Orbitals
Nodal Plane
ANTI-BONDING
MOLECULAR
ORBITAL
E
N
E
R
G
Y
p*
+
p-atomic orbitals
BONDING
MOLECULAR
ORBITAL
p
The linear combination of
n atomic orbitals
leads to the formation of
n molecular orbitals
A SIMPLE Mathematical Description of a MO
The combination of two (or more) p-atomic orbitals (or any orbitals) to
afford 2 p-molecular orbitals can be described by the following simple
mathematical relationship
p* = ccf1 + cdf2
p = caf1 + cbf2
fm =
Cn =
Electronic distribution in the atomic orbitals
Coeffecient: a measure of the contribution which the
atomic orbital is making to the molecular orbital
The probability of finding an electron in an occupied molecular orbital is 1.
The probability of finding an electron in an occupied molecular orbital is the Sc2
Thus, for the ethene p-molecular orbitals…
p* = ccf1 + cdf2
Sc2 = cc2 + cd2 = 1
1
2
Cc = 1/√2
p*
Cd = -1/√2
Negative
Sc2 = ca2 + cb2 = 1
p = caf1 + cbf2
1
2
p
Ca = 1/√2
Cb = 1/√2
So what about the combination of 3 or 4
or 5 or 6 p-atomic orbitals.
That
is
to
systems…
consider
conjugated
The Allyl Cation, Radical and Anion – 3p AOs to give 3p MOs
Cl
H
Cl
Polar
Solvent
B
Cl
Cl
BH
Allyl Cation
Allyl Radical
Allyl Anion
Thus, allyl systems result from the combination of 3 conjugated p-orbitals.
Therefore, this will result in 3 p-molecular orbitals.
When we constructed the p-molecular orbitals of ethene, each
contributing AO was the same size, i.e. the coeffecient c were 1/√2 or 1/√2.
When there are three or more p-atomic orbitals combining the size of each
contributing p-atomic orbital will not be equal (but they will be
symmetrical about the centre).
Finally, we refer to the p-MOs and p*-MOs as 1, 2, 3 (…n)
The Allyl p-Molecular Orbitals
We can consider the molecular orbital (the electron density) being described
by a SINE WAVE starting and finishing one bond length beyond the
molecule…
3 = 2 Nodes
Nodal
position
4/3 = 1.33
+
+
3
+
1.33
Nodes
_
+
_
+
2 = 1 Nodes
1 = 0 Nodes
Nodal
position
4/2 = 2
Nodal
position
4/1 = 4
+
+
2
+
2
+
1
+
2
3
4
1
+
4
For our analysis of molecular orbitals we do not have to
concern ourselves with the coefficients.
We can draw the p-AOs that make up the p-MOs all the
same size.
However, we have to always remember they are not the
same size.
But it is of the utmost importance that we know how to
calculate where the nodes are placed
Bonding, Non-Bonding, and Anti-bonding Levels
Energy
Anti-bonding
Anti-bonding Level
3
2
2
Non-bonding Level
Non-bonding
Bonding Level
Bonding
1
1
We can consider the molecular orbital (the electron density) being described
by a sine wave starting and finishing one bond length beyond the molecule…
LUMOs and HOMOs
LUMO = Lowest Unoccupied Molecular Orbital
HOMO = Highest Occupied Molecular Orbital
Allyl
Cation
(2e)
3
2 nodes
2
1 node
LUMO
1
0 nodes
HOMO
Allyl
Radical
(3e)
Allyl
Anion
(4e)
LUMO
LUMO
HOMO
HOMO
Question 1: 4 p-Molecular Orbital System – Butadiene
Construct the p-molecular orbitals of butadiene.
Identify the number of nodes, nodal positions, HOMO and LUMO.
n
Number of
Nodes
Nodal
Position
Answer 1: 4 p-Molecular Orbital System – Butadiene
Construct the p-molecular orbitals of butadiene.
Identify the number of nodes, nodal positions, HOMO and LUMO.
n
4
Number of
Nodes
Nodal
Position
3
+
+
+
+
+
+
+
+
LUMO
+
+
+
+
HOMO
+
+
+
+
5/4 = 1.25
3
2
5/3 = 1.66
2
1
5/2 = 2.5
1
0
5/1 = 5
1
2
3
4
5
A Reminder: Sinusodal Wave Function
SIMPLE
MORE COMPLEX
4
3 nodes
5/4
1.25
3
2 nodes
5/3
1.66
2
1 node
5/2
2.5
1
1
2
3
4
5
0 nodes
5/1
5
Coefficients, cn
Each molecular orbital is described by an equation…
n= caf1 + cbf2 + ccf3 + cnfn
Where c is referred to as the coefficient
Such that the…
Sc2 = 1
That is to say the probability of finding an electron in a molecular orbital is 1
3= caf1 + cbf2 + ccf3 + cdf4
3
1.66
We Keep FMO Analysis Simple!!
For the purpose of this course and the third year course (Applied Frontier
Molecular Orbitals and Stereoelectronic Effects) you are expected
(i)
to be able to place the nodal planes in the correct place
(ii)
but not to be able to assign the coefficients to the molecular
orbitals.
That is to say you can draw the p-orbitals that make up each
molecular orbital as the same size, whilst remembering that
in reality they are not and for high level FMO analysis this
needs to be taken into account.
Question 2: 5 p-Molecular Orbital System – Pentadienyl
Construct the p-molecular orbitals of the cyclopentenyl system.
Identify the number of nodes and nodal positions.
n
Number of
Nodes
Nodal
Position
Molecular
Orbitals
Answer 2: 5 p-Molecular Orbital System – Pentadienyl
Construct the p-molecular orbitals of the cyclopentenyl system.
Identify the number of nodes and nodal positions.
n
Number of
Nodes
Molecular
Orbitals
Nodal
Position
5
4
6/5 = 1.2
+
+ + +
+
4
3
6/4 = 1.5
+
+ + +
+
3
2
6/3 = 2
+
+ + +
+
2
1
6/2 = 3
+
+ + +
+
1
0
6/1 = 6
+
+ + +
+
1
2
3
4
5
6
Question 3: Pentadienyl Cation, Radical & Anion
Introduce the electrons and identify the HOMOs and LUMOs
Pentenyl
cation
5
4 nodes
4
3 nodes
3
2 nodes
2
1 node
1
0 nodes
Pentenyl
radical
Pentenyl
anion
Answer 3: Pentadienyl Cation, Radical & Anion
Introduce the electrons and identify the HOMOs and LUMOs
Pentenyl
cation (4e)
Pentenyl
radical (5e)
5
4 nodes
4
3 nodes
3
2 nodes
LUMO
2
1 node
HOMO
1
0 nodes
Pentenyl
anion (6e)
LUMO
LUMO
HOMO
HOMO
Question 4: Pentadienyl Cation & Anion
Generate the cation and anion and draw the resonance structures of the above species
Cl
H
Answer 4: Pentadienyl Cation, Radical & Anion
Generate the cation and anion and draw the resonance structures of the above species
Cl
H
B:
6 p-Molecular Orbital System – 1, 3, 5-Hexatriene
6
5 nodes
5
4 nodes
4
3 nodes
LUMO
3
2 nodes
HOMO
2
1 node
1
0 nodes
7 p-Molecular Orbital System
cation (6e) radical (7e) anion (8e)
Nodal Plane
Position
7
6 nodes
8/7 = 1.14
6
5 nodes
8/6 = 1.33
5
4 nodes
8/5 = 1.6
4
3 nodes
8/4 = 2
3
2 nodes
8/3 = 2.67
2
1 node
8/2 = 4
1
0 nodes
8/1 = 8
LUMO
HOMO
LUMO
LUMO
HOMO
HOMO
Question 5: 6p MO System
Electr ons
By shading the p atomic orbitals,
generate the molecular orbitals for
hexa-1,3,5-triene .
Identify
the
number
of
6
nodes
molecular
5
With reference to both a mirror
4
characterising
each
orbital.
plane (m) and a two-fold axis,
designate the orbitals as symmetric
(S) or antisymmetric (A).
3
Using arrows to represent electrons,
associate the six p-electrons with
the appropriate molecular orbitals
of hexa-1,3,5-triene in its ground
2
state.
Finally,
LUMO.
identify
the
HOMO
and
1
m or C2
Nodes
m
C2
Answer 5: 6p MO System
m or C2
By shading the p atomic orbitals,
generate the molecular orbitals for
hexa-1,3,5-triene .
Identify
the
number
of
6
Nodes
m
C2
5
A
S
4
S
A
nodes
molecular
5
With reference to both a mirror
4
LUMO
3
A
S
3
HOMO
2
S
A
2
1
A
S
1
0
S
A
characterising
each
orbital.
plane (m) and a two-fold axis,
designate the orbitals as symmetric
(S) or antisymmetric (A).
Using arrows to represent electrons,
associate the six p-electrons with
the appropriate molecular orbitals
of hexa-1,3,5-triene in its ground
state.
Finally,
LUMO.
identify
the
HOMO
and
Question 6: MO System
Protonation of A affords B. Draw the three resonance
structures of B in which the positive charge has
H
O
formally been shifted from the oxygen atom onto
O
H3O
H2O
three of the five carbon atoms
.
B
A
H
Considering only these three resonance
structures, how many
O
(i) carbon atoms are involved in the
hybrid structure,
(ii) carbon p-orbitals are there,
(iii) p-electrons are associated with the
carbon atoms, and
(iv) molecular orbitals are associated
with the combination of these carbon porbitals
In an analogous fashion to how question 1 was set out, draw out the molecular orbitals resulting from the p-orbital
combination on this carbon framework, making sure you identify all of the items listed in question 1.
Answer 6: 5p MO System
H
Protonation of A affords B. Draw the three resonance
O
O
structures of B in which the positive charge has
H3O
formally been shifted from the oxygen atom onto
H2O
three of the five carbon atoms
.
B
A
H
O
Considering only these three resonance
structures, how many
(i) carbon atoms are involved in the
hybrid structure,
(ii) carbon p-orbitals are there,
(iii) p-electrons are associated with the
carbon atoms, and
(iv) molecular orbitals are associated
with the combination of these carbon porbitals
5
5
4
5
O
H
O
H
O
H
In an anologous fashion to how question 5 was set out, draw out the molecular orbitals resulting from the p-orbital
combination on this carbon framework, making sure you identify all of the items listed in question 5.
O
Pentenyl
cation (4e)
Mirror Plane
C2 axis
5
4 nodes
S
A
4
3 nodes
A
S
3
2 nodes
LUMO
S
A
2
1 node
HOMO
A
S
1
0 nodes
S
A
Second Year Organic Chemistry Course
CHM2C3B
Frontier Molecular Orbitals and
Pericyclic Reactions
Part 1(iii):
HOMO and LUMO Combination
What is the Driving Force for Controlling Pericyclic
Reactions?
The driving force which controls the product outcome in
pericyclic reactions is the in phase combination of the
FMOs (the HOMO and LUMO) of the reacting species in
the transition state.
FMO Theory is Extremely Powerful.
Pericyclic Reactions Involve Conjugated Polyene Systems
Pericyclic reactions involve conjugated polyene systems.
Enes and Polyenes are made by the linear combination of p-AOs.
Thus, we first need to construct the molecular orbitals of polyenes.
Then we need to identify the Frontier Molecular Orbitals.
Finally, we will need to construct the correct geometry for orbital
overlap of the FMOs in the transition states of the reactions.
HOMOs and LUMOs
Highest Occupied Molecular Orbitals
Lowest Unoccupied Molecular Orbitals
In bimolecular reactions (like the SN2 and the Diels-Alder reaction), interaction
between the two molecular components is represented by interaction between
suitable molecular orbitals of each.
The extent of the interaction depends upon the geometry of approach of the
components since the relative geometry affects the amount of possible overlap.
It also depends on the phase relationship of the orbitals – and also upon their
energy of separation, a small energy favouring a greater interaction.
Generally, the two reactants will interact, via the highest occupied molecular
orbital (HOMO) of one component and the lowest unoccupied molecular orbital
(LUMO) of the other component, the so-called frontier molecular orbitals (FMOs).
Consider the next five frames to appreciate this paragraph of text. Consider an
SN2 Reaction…
Revision: Transition State Geometries of Nucleophiles
Attacking sp3 Tetrahedral Centres
TET
sp3
Nu
Nu
X

 = 180°
Nu
X
X
Inversion of Configuration
Supports this Attack Angle
s* C–X
NucleophileHOMO
LUMO
s* C–Nu
X
Nu
sC–Nu
s* C–X
LUMO
NucleophileHOMO
The orbital containing the lone pair of electrons on the Nu
is the…
HOMO (Highest Occupied Molecular Orbital)
The s* orbital of the C-X bond is the…
LUMO (Lowest Unoccupied Molecular Orbital)
Any bimolecular reaction can be analysed in this fashion
Frontier Molecular Orbital Theory (FMOs)
This analysis of FMOs (HOMOs and LUMOs) for such a simple reactions may
seem pointless for a simple SN2 reaction.
It is not!
Understand it.
Appreciate that for a bimolecular reaction the HOMO of one component
interacts with the LUMO of the second component.
(Additionally, for
unimolecular reaction the HOMO of the molecular component dictates the
reaction course).
In this course we will examine the use of FMOs to explain and predict the
outcomes of a class of reactions referred to as pericyclic.
The use of FMOs is an extremely powerful tool to the synthetic organic chemist
when analysing and predicting the outcome of pericyclic reactions.
CHM3A2
– Introduction to FMOs –
– Summary
Sheet Part 1 –
Frontier Molecular Orbitals
Molecular orbital theory is a powerful and versatile asset to the practice of organic chemistry. As a theory of bonding it
has almost superseded the valence bond theory.
Molecular orbital theory has
proven amenable to pictorial non-mathematical expression,
given the right answers to some decisive questions in organic chemistry,
proven the theory of most theoretical chemists,
given insight into not only to the theory of bonding, but also to the theory of making and breaking chemical bonds, and
proven a theory which has been able to explain the pattern of reactivity in a class of reactions, known as pericyclic
reactions.
In this course we will concentrate solely on the use of MO theory in predicting the outcome of pericyclic reactions. But it
should not be forgotten that MO theory is applicable to other types of chemical reraction
To understand the importance of MO theory, we shall consider three types of pericyclic reactions and show how frontier
molecular orbitals of the reactants can be used in a predicative nature to work out whether the reaction will proceed and
what the stereo/regiochemical outcome will be.
The three types of pericyclic reactions we will consider are
electrcyclic reactions
cycloaddition reactions
sigmatropic reactions
We will see how it is possible to predict the stereoselectivity, diastereoselectivity, and regioselectivity of pericyclic
reactions by the analysis of the FMOs of the transition states
The precise construction of the p-molecular orbitals by the linear combination of p-atomic orbitals is extremely important if
FMO theory is to yield the correct stereochemical product outcomes,
Key points to note when constructing p-molecular orbitals from the combination of p-AOs are
(i)
(ii)
(iii)
(iv)
(v)
the combination of n Aos always affords n MOs
The lowest p-MOs (1) has no nodal planes
The next highest (2) has one nodal plane, and so on
The nodal planes need to be placed exactly in the Mos as described in the lecture notes
Electrons fill from the lowest MO first with no more than two electrons in each MO.