Pericyclic Reactions
• Pericyclic reactions are concerted processes in which bond orbitals overlap in a continuous
cycle in the transition state
• A concerted process is when bond breaking and bond making occurs simultaneously
M O
o
-
all reacting orbitals overlap in a
&
Nuc
continuous cycle, hence it is a
LG
pericyclic reaction
concerted reaction but not pericyclic
because the orbitals do not form a ring
Molecular Orbitals in Conjugates Systems
1. Approach to determine the phases of orbitals: 2. Approach to determine orbitals coefficients:
• For an alkene with n p-orbitals, draw n • Draw a line with n+2 equally spaced dotes for
horizontal lines stacked on top of each other an alkene with n p-orbitals
(MO's) • Superimpose sine waves onto the lines so
• On each line, draw n p-orbitals with an that they start and end at the terminal dots -
increasing number of nodes (starting from zero the number of times the sine wave crosses
nodes) - each MO must be symmetric or the lines increases incrementally starting at
antisymmetric zero, indicating the nodes
• Invert the phase of the orbitals at each node • Invert the phase of the orbitals at each node
• Fill orbitals with electrons, two per MO • The height of the p-orbitals on each dot
equals the size of the corresponding orbital
coefficient
agg • The size of the orbital coefficient is a
reflection of the electron density on the
corresponding orbital
6 p Orbitals (n =
b) be
,
G8 88
,
, , -
-8888 -
e
888-un
- -
- # HOMS n
888 8881
⑳
-
, #
# HOMS
00000 8
-
000000
onodes #
6 dots
. Onodes #
,3. Orbital coefficients for substituted polyalkenes:
• Replace an electron withdrawing substituent with a CH₂⁺ group, and an electron-donating
substituent with a CH₂⁻ group
• Determine the orbital coefficients as shown above
• The HOMO and LUMO coefficients for a substituted alkene are assumed to be an average
between the unsubstituted compound and the alkene fragment of the compound with the CH₂⁺/CH₂⁻
groups
one
G
H ge
2p orbitals 3 orbitals
(n 2) , ze
=
(n =
3) 4e
,
#average I
mo- -
umo
Homo LUMO alkene
-
-
fragment
g
M A cation +
g
II
e ge
porpitas an
·-
porpitas
-
-
# ·
~
00
, Orbital Correlation Diagrams
• Orbital correlation diagrams are used to predict whether pericyclic reactions are allowed or
forbidden based on orbital symmetry
• The Woodward-Hoffman rule dictates that the symmetry of frontier molecular orbitals (FMOs)
must be maintained during a reaction
• For these symmetry allowed reactions, MOs interacts leading to smooth bond formation
1-
#
I -
-
-
Ih &
G
O O -
-
:,
Tt3
87
·
-
I
i
88
-
-
-
It ,
O
Relative Energy of MO's
• A Frost circle is a mnemonic to
determine the relative energy
levels of π molecular orbitals in
·
cyclic, conjugated systems
• The longer the conjugation length,
the smaller the HOMO-LUMO
energy gap
• EWGs causes the HOMO and
other orbitals to increase in energy
• EDGs causes the LUMO to
decrease in energy
ethylene butadiene
• Pericyclic reactions are concerted processes in which bond orbitals overlap in a continuous
cycle in the transition state
• A concerted process is when bond breaking and bond making occurs simultaneously
M O
o
-
all reacting orbitals overlap in a
&
Nuc
continuous cycle, hence it is a
LG
pericyclic reaction
concerted reaction but not pericyclic
because the orbitals do not form a ring
Molecular Orbitals in Conjugates Systems
1. Approach to determine the phases of orbitals: 2. Approach to determine orbitals coefficients:
• For an alkene with n p-orbitals, draw n • Draw a line with n+2 equally spaced dotes for
horizontal lines stacked on top of each other an alkene with n p-orbitals
(MO's) • Superimpose sine waves onto the lines so
• On each line, draw n p-orbitals with an that they start and end at the terminal dots -
increasing number of nodes (starting from zero the number of times the sine wave crosses
nodes) - each MO must be symmetric or the lines increases incrementally starting at
antisymmetric zero, indicating the nodes
• Invert the phase of the orbitals at each node • Invert the phase of the orbitals at each node
• Fill orbitals with electrons, two per MO • The height of the p-orbitals on each dot
equals the size of the corresponding orbital
coefficient
agg • The size of the orbital coefficient is a
reflection of the electron density on the
corresponding orbital
6 p Orbitals (n =
b) be
,
G8 88
,
, , -
-8888 -
e
888-un
- -
- # HOMS n
888 8881
⑳
-
, #
# HOMS
00000 8
-
000000
onodes #
6 dots
. Onodes #
,3. Orbital coefficients for substituted polyalkenes:
• Replace an electron withdrawing substituent with a CH₂⁺ group, and an electron-donating
substituent with a CH₂⁻ group
• Determine the orbital coefficients as shown above
• The HOMO and LUMO coefficients for a substituted alkene are assumed to be an average
between the unsubstituted compound and the alkene fragment of the compound with the CH₂⁺/CH₂⁻
groups
one
G
H ge
2p orbitals 3 orbitals
(n 2) , ze
=
(n =
3) 4e
,
#average I
mo- -
umo
Homo LUMO alkene
-
-
fragment
g
M A cation +
g
II
e ge
porpitas an
·-
porpitas
-
-
# ·
~
00
, Orbital Correlation Diagrams
• Orbital correlation diagrams are used to predict whether pericyclic reactions are allowed or
forbidden based on orbital symmetry
• The Woodward-Hoffman rule dictates that the symmetry of frontier molecular orbitals (FMOs)
must be maintained during a reaction
• For these symmetry allowed reactions, MOs interacts leading to smooth bond formation
1-
#
I -
-
-
Ih &
G
O O -
-
:,
Tt3
87
·
-
I
i
88
-
-
-
It ,
O
Relative Energy of MO's
• A Frost circle is a mnemonic to
determine the relative energy
levels of π molecular orbitals in
·
cyclic, conjugated systems
• The longer the conjugation length,
the smaller the HOMO-LUMO
energy gap
• EWGs causes the HOMO and
other orbitals to increase in energy
• EDGs causes the LUMO to
decrease in energy
ethylene butadiene
