Chapter 15
Conjugated Systems,
Orbital Symmetry,and
Ultraviolet Spectroscopy
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Chaper 15 2
Definitions
Conjugated double bonds are separated by
one single bond,Example,1,3-pentadiene.
Isolated double bonds are separated by
two or more single bonds,1,4-pentadiene.
Cumulated double bonds are on adjacent
carbons,Example,1,2-pentadiene,
=>
Chaper 15 3
Resonance Energy
Heat of hydrogenation for trans-1,3-
pentadiene is less than expected.
H for 1-pentene is 30.0 kcal/mol and for
trans-2-pentene is 27.4 kcal/mol,so expect
57.4 kcal for trans-1,3-pentadiene.
Actual?H is 53.7 kcal,so the conjugated
diene is more stable.
Difference,(57.4 – 53.7) 3.7 kcal/mol,is the
resonance energy,=>
Chaper 15 4
Relative Stabilities
twice 1-pentene
more substituted
=>
Chaper 15 5
Structure of 1,3-Butadiene
Most stable conformation is planar.
Single bond is shorter than 1.54?.
Electrons are delocalized over molecule.
=>
Chaper 15 6
Constructing
Molecular Orbitals
Pi molecular orbitals are the sideways
overlap of p orbitals.
p orbitals have 2 lobes,Plus (+) and minus
(-) indicate the opposite phases of the wave
function,not electrical charge.
When lobes overlap constructively,(+ and
+,or - and -) a bonding MO is formed.
When + and - lobes overlap,waves cancel
out and a node forms; antibonding MO,=>
Chaper 15 7
1 MO for 1,3-Butadiene
Lowest energy.
All bonding
interactions.
Electrons are
delocalized over
four nuclei,
=>
Chaper 15 8
2 MO for 1,3-Butadiene
2 bonding
interactions
1 antibonding
interaction
A bonding MO
=>
Chaper 15 9
3* MO for 1,3-Butadiene
Antibonding MO
Empty at ground
state
Two nodes =>
Chaper 15 10
4* MO for 1,3-Butadiene
All antibonding
interactions.
Highest energy.
Vacant at ground
state,
=>
Chaper 15 11
MO Energy Diagram
The average
energy of
electrons is
lower in the
conjugated
compound.
=>
Chaper 15 12
Conformations of
1,3-Butadiene
s-trans conformer is more stable than
the s-cis by 2.3 kcal.
Easily interconvert at room temperature.
H
H
H
H
H
H
s - t r a n s s - c i s
H
H
H
H
H
H
=>
Chaper 15 13
Allylic Cations
Carbon adjacent to C=C is allylic.
Allylic cation is stabilized by resonance.
Stability of 1? allylic? 2? carbocation.
Stability of 2? allylic? 3? carbocation.
H 2 C C
H
C H 2
+
H 2 C C
H
C H 2
+
=>
Chaper 15 14
1,2- and 1,4-Addition
to Conjugated Dienes
Electrophilic addition to the double bond
produces the most stable intermediate.
For conjugated dienes,the intermediate
is a resonance stabilized allylic cation.
Nucleophile adds to either carbon 2 or 4,
both of which have the delocalized
positive charge,
=>
Chaper 15 15
Addition of HBr
B r
_
B r
_
H 3 C C
H
C
H
C H 2
B r
H 3 C C
H
C
H
C H 2
B r
1,2 - a d d it i o n p r o d u c t 1,4 - a d d it i o n p r o d u c t
=>
Chaper 15 16
Kinetic vs,
Thermodynamic Control
Major product
at 40?C
Major product
at -80?C
=>
Chaper 15 17
Allylic Radicals
Stabilized by resonance.
Radical stabilities,1? < 2? < 3? < 1? allylic.
Substitution at the allylic position competes
with addition to double bond.
To encourage substitution,use a low
concentration of reagent with light,heat,or
peroxides to initiate free radical formation,
=>
Chaper 15 18
Allylic Bromination
B r
H
H
HH
H
H
H
H
H
H
+ HBrB r B r B r B r
H
H
B rH
H
H
H
B r
+ Br?
=>
B r2h?B r 2
Chaper 15 19
Bromination Using NBS
N-Bromosuccinimide (NBS) provides a
low,constant concentration of Br2.
NBS reacts with the HBr by-product to
produce Br2 and prevent HBr addition.
=>
N B r
O
O
+ H Br N H
O
O
+ B r2
Chaper 15 20
MO’s for the Allylic System
=>
Chaper 15 21
SN2 Reactions of Allylic
Halides and Tosylates
=>
Chaper 15 22
Diels-Alder Reaction
Otto Diels,Kurt Alder; Nobel prize,1950
Produces cyclohexene ring
Diene + alkene or alkyne with electron-
withdrawing group (dienophile)
C
C
H H
H W
C
C
H
H
W
H
=>
Chaper 15 23
Examples of
Diels-Alder Reactions
=>
+
O
C
O C H 3
C
C
C
O O C H 3
C
C
O
O C H 3
C
O C H 3
O
C
H 3 C
H 3 C
N
C
C
H
C
HH
+
H 3 C
H 3 C
C
C
H
C N
H
H
diene dienophile Diels-Alder adduct
Chaper 15 24
Stereochemical Requirements
Diene must be in s-cis conformation.
Diene’s C1 and C4 p orbitals must
overlap with dienophile’s p orbitals to
form new sigma bonds.
Both sigma bonds are on same face of
the diene,syn stereochemistry,
=>
Chaper 15 25
Concerted Mechanism
=>
Chaper 15 26
Endo Rule
The p orbitals of the electron-withdrawing
groups on the dienophile have a secondary
overlap with the p orbitals of C2 and C3 in
the diene.
=>
Chaper 15 27
Regiospecificity
The 6-membered ring product of the
Diels-Alder reaction will have electron-
donating and electron-withdrawing
groups 1,2 or 1,4 but not 1,3.
D
C
C
H W
H H
W
D
D C
C
H W
H H D
W
WD
n o t
=>
Chaper 15 28
Symmetry-Allowed Reaction
Diene contributes
electrons from its
highest energy
occupied orbital
(HOMO).
Dienophile receives
electrons in its
lowest energy
unoccupied orbital
(LUMO),=>
Chaper 15 29
“Forbidden” Cycloaddition
[2 + 2] cycloaddition of
two ethylenes to form
cyclobutene has anti-
bonding overlap of
HOMO and LUMO
=>
Chaper 15 30
Photochemical Induction
Absorption of correct energy photon will
promote an electron to an energy level
that was previously unoccupied.
=>
Chaper 15 31
[2 + 2] Cycloaddition
Photochemically
allowed,but
thermally
forbidden.
=>
Chaper 15 32
Ultraviolet Spectroscopy
200-400 nm photons excite electrons
from a? bonding orbital to a?*
antibonding orbital.
Conjugated dienes have MO’s that are
closer in energy.
A compound that has a longer chain of
conjugated double bonds absorbs light
at a longer wavelength,=>
Chaper 15 33
=>
* for
ethylene
and
butadiene
Chaper 15 34
Obtaining a UV Spectrum
The spectrometer measures the intensity
of a reference beam through solvent only
(Ir) and the intensity of a beam through a
solution of the sample (Is).
Absorbance is the log of the ratio
Graph is absorbance vs,wavelength.
=>
I
I
s
r
Chaper 15 35
The UV Spectrum
Usually shows broad peaks.
Read?max from the graph.
Absorbance,A,follows Beer’s Law:
A =?cl
where? is the molar absorptivity,c is
the sample concentration in moles per
liter,and l is the length of the light path
in centimeters.
Chaper 15 36
UV Spectrum of Isoprene
=>
Chaper 15 37
Sample UV Absorptions
=>
Chaper 15 38
Woodward-Fieser Rules
=>
Chaper 15 39
End of Chapter 15
Conjugated Systems,
Orbital Symmetry,and
Ultraviolet Spectroscopy
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Chaper 15 2
Definitions
Conjugated double bonds are separated by
one single bond,Example,1,3-pentadiene.
Isolated double bonds are separated by
two or more single bonds,1,4-pentadiene.
Cumulated double bonds are on adjacent
carbons,Example,1,2-pentadiene,
=>
Chaper 15 3
Resonance Energy
Heat of hydrogenation for trans-1,3-
pentadiene is less than expected.
H for 1-pentene is 30.0 kcal/mol and for
trans-2-pentene is 27.4 kcal/mol,so expect
57.4 kcal for trans-1,3-pentadiene.
Actual?H is 53.7 kcal,so the conjugated
diene is more stable.
Difference,(57.4 – 53.7) 3.7 kcal/mol,is the
resonance energy,=>
Chaper 15 4
Relative Stabilities
twice 1-pentene
more substituted
=>
Chaper 15 5
Structure of 1,3-Butadiene
Most stable conformation is planar.
Single bond is shorter than 1.54?.
Electrons are delocalized over molecule.
=>
Chaper 15 6
Constructing
Molecular Orbitals
Pi molecular orbitals are the sideways
overlap of p orbitals.
p orbitals have 2 lobes,Plus (+) and minus
(-) indicate the opposite phases of the wave
function,not electrical charge.
When lobes overlap constructively,(+ and
+,or - and -) a bonding MO is formed.
When + and - lobes overlap,waves cancel
out and a node forms; antibonding MO,=>
Chaper 15 7
1 MO for 1,3-Butadiene
Lowest energy.
All bonding
interactions.
Electrons are
delocalized over
four nuclei,
=>
Chaper 15 8
2 MO for 1,3-Butadiene
2 bonding
interactions
1 antibonding
interaction
A bonding MO
=>
Chaper 15 9
3* MO for 1,3-Butadiene
Antibonding MO
Empty at ground
state
Two nodes =>
Chaper 15 10
4* MO for 1,3-Butadiene
All antibonding
interactions.
Highest energy.
Vacant at ground
state,
=>
Chaper 15 11
MO Energy Diagram
The average
energy of
electrons is
lower in the
conjugated
compound.
=>
Chaper 15 12
Conformations of
1,3-Butadiene
s-trans conformer is more stable than
the s-cis by 2.3 kcal.
Easily interconvert at room temperature.
H
H
H
H
H
H
s - t r a n s s - c i s
H
H
H
H
H
H
=>
Chaper 15 13
Allylic Cations
Carbon adjacent to C=C is allylic.
Allylic cation is stabilized by resonance.
Stability of 1? allylic? 2? carbocation.
Stability of 2? allylic? 3? carbocation.
H 2 C C
H
C H 2
+
H 2 C C
H
C H 2
+
=>
Chaper 15 14
1,2- and 1,4-Addition
to Conjugated Dienes
Electrophilic addition to the double bond
produces the most stable intermediate.
For conjugated dienes,the intermediate
is a resonance stabilized allylic cation.
Nucleophile adds to either carbon 2 or 4,
both of which have the delocalized
positive charge,
=>
Chaper 15 15
Addition of HBr
B r
_
B r
_
H 3 C C
H
C
H
C H 2
B r
H 3 C C
H
C
H
C H 2
B r
1,2 - a d d it i o n p r o d u c t 1,4 - a d d it i o n p r o d u c t
=>
Chaper 15 16
Kinetic vs,
Thermodynamic Control
Major product
at 40?C
Major product
at -80?C
=>
Chaper 15 17
Allylic Radicals
Stabilized by resonance.
Radical stabilities,1? < 2? < 3? < 1? allylic.
Substitution at the allylic position competes
with addition to double bond.
To encourage substitution,use a low
concentration of reagent with light,heat,or
peroxides to initiate free radical formation,
=>
Chaper 15 18
Allylic Bromination
B r
H
H
HH
H
H
H
H
H
H
+ HBrB r B r B r B r
H
H
B rH
H
H
H
B r
+ Br?
=>
B r2h?B r 2
Chaper 15 19
Bromination Using NBS
N-Bromosuccinimide (NBS) provides a
low,constant concentration of Br2.
NBS reacts with the HBr by-product to
produce Br2 and prevent HBr addition.
=>
N B r
O
O
+ H Br N H
O
O
+ B r2
Chaper 15 20
MO’s for the Allylic System
=>
Chaper 15 21
SN2 Reactions of Allylic
Halides and Tosylates
=>
Chaper 15 22
Diels-Alder Reaction
Otto Diels,Kurt Alder; Nobel prize,1950
Produces cyclohexene ring
Diene + alkene or alkyne with electron-
withdrawing group (dienophile)
C
C
H H
H W
C
C
H
H
W
H
=>
Chaper 15 23
Examples of
Diels-Alder Reactions
=>
+
O
C
O C H 3
C
C
C
O O C H 3
C
C
O
O C H 3
C
O C H 3
O
C
H 3 C
H 3 C
N
C
C
H
C
HH
+
H 3 C
H 3 C
C
C
H
C N
H
H
diene dienophile Diels-Alder adduct
Chaper 15 24
Stereochemical Requirements
Diene must be in s-cis conformation.
Diene’s C1 and C4 p orbitals must
overlap with dienophile’s p orbitals to
form new sigma bonds.
Both sigma bonds are on same face of
the diene,syn stereochemistry,
=>
Chaper 15 25
Concerted Mechanism
=>
Chaper 15 26
Endo Rule
The p orbitals of the electron-withdrawing
groups on the dienophile have a secondary
overlap with the p orbitals of C2 and C3 in
the diene.
=>
Chaper 15 27
Regiospecificity
The 6-membered ring product of the
Diels-Alder reaction will have electron-
donating and electron-withdrawing
groups 1,2 or 1,4 but not 1,3.
D
C
C
H W
H H
W
D
D C
C
H W
H H D
W
WD
n o t
=>
Chaper 15 28
Symmetry-Allowed Reaction
Diene contributes
electrons from its
highest energy
occupied orbital
(HOMO).
Dienophile receives
electrons in its
lowest energy
unoccupied orbital
(LUMO),=>
Chaper 15 29
“Forbidden” Cycloaddition
[2 + 2] cycloaddition of
two ethylenes to form
cyclobutene has anti-
bonding overlap of
HOMO and LUMO
=>
Chaper 15 30
Photochemical Induction
Absorption of correct energy photon will
promote an electron to an energy level
that was previously unoccupied.
=>
Chaper 15 31
[2 + 2] Cycloaddition
Photochemically
allowed,but
thermally
forbidden.
=>
Chaper 15 32
Ultraviolet Spectroscopy
200-400 nm photons excite electrons
from a? bonding orbital to a?*
antibonding orbital.
Conjugated dienes have MO’s that are
closer in energy.
A compound that has a longer chain of
conjugated double bonds absorbs light
at a longer wavelength,=>
Chaper 15 33
=>
* for
ethylene
and
butadiene
Chaper 15 34
Obtaining a UV Spectrum
The spectrometer measures the intensity
of a reference beam through solvent only
(Ir) and the intensity of a beam through a
solution of the sample (Is).
Absorbance is the log of the ratio
Graph is absorbance vs,wavelength.
=>
I
I
s
r
Chaper 15 35
The UV Spectrum
Usually shows broad peaks.
Read?max from the graph.
Absorbance,A,follows Beer’s Law:
A =?cl
where? is the molar absorptivity,c is
the sample concentration in moles per
liter,and l is the length of the light path
in centimeters.
Chaper 15 36
UV Spectrum of Isoprene
=>
Chaper 15 37
Sample UV Absorptions
=>
Chaper 15 38
Woodward-Fieser Rules
=>
Chaper 15 39
End of Chapter 15
