Organic Chemistry, 5th Edition
L. G. Wade, Jr.
Chapter 15
Conjugated Systems,
Orbital Symmetry, and
Ultraviolet Spectroscopy
Jo Blackburn
Richland College, Dallas, TX
Dallas County Community College District
2003, Prentice Hall
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
2
Resonance Energy
• Heat of hydrogenation for trans-1,3pentadiene 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
3
Relative Stabilities
twice 1-pentene
more substituted
=>
Chaper 15
4
Structure of 1,3-Butadiene
• Most stable conformation is planar.
• Single bond is shorter than 1.54 Å.
• Electrons are delocalized over molecule.
Chaper 15
5
=>
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
6
1 MO for 1,3-Butadiene
• Lowest energy.
• All bonding
interactions.
• Electrons are
delocalized over
four nuclei.
=>
Chaper 15
7
2 MO for 1,3-Butadiene
• 2 bonding
interactions
• 1 antibonding
interaction
• A bonding MO
=>
Chaper 15
8
3* MO for 1,3-Butadiene
• Antibonding MO
• Empty at ground
state
• Two nodes
=>
Chaper 15
9
4* MO for 1,3-Butadiene
• All antibonding
interactions.
• Highest energy.
• Vacant at ground
state.
=>
Chaper 15
10
MO Energy Diagram
The average
energy of
electrons is
lower in the
conjugated
compound.
=>
Chaper 15
11
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
H
H
H
s-cis H
s-trans
Chaper 15
H
H
=>
12
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
H
+
H2C C CH2
+
H2C C CH2
=>
Chaper 15
13
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
14
Addition of HBr
_
Br
_
Br
H H
H H
H3C C C CH2
H3C C C CH2
Br
1,2-addition product
Br
1,4-addition product
=>
Chaper 15
15
Kinetic vs.
Thermodynamic Control
Major product
at 40C
Major product
at -80C
Chaper 15
16
=>
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
17
Allylic Bromination
h
Br2
H
2 Br
H
Br
+ HBr
H
H
H
H
H
H
H
H
Br Br
Br Br
H
Br
H
H
H
H
Br
Chaper 15
+ Br 
=>
H
18
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.
O
N
O
Br + HBr
N H
O
O
Chaper 15
+ Br 2
=>
19
MO’s for the Allylic System
=>
Chaper 15
20
SN2 Reactions of Allylic
Halides and Tosylates
=>
Chaper 15
21
Diels-Alder Reaction
• Otto Diels, Kurt Alder; Nobel prize, 1950
• Produces cyclohexene ring
• Diene + alkene or alkyne with electronwithdrawing group (dienophile)
H
W
C

C
H
H
C W
C H
H
H
Chaper 15
=>
22
Examples of
Diels-Alder Reactions
N
H3C
C
+
H3C
H
diene
C
C
H
H3C
H
H3C
dienophile
O
C
O
C H
H
Diels-Alder adduct
O
C OCH
3
C
C
C
C C N
OCH3
C
+
H
C
OCH3
Chaper 15
O
C OCH
3
=>
23
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
24
Concerted Mechanism
=>
Chaper 15
25
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
26
=>
Regiospecificity
The 6-membered ring product of the
Diels-Alder reaction will have electrondonating and electron-withdrawing
groups 1,2 or 1,4 but not 1,3.
H
D
H
C
D
C
H
W
W
H
not
H
D
C
C
H
D
W
W
W
D
Chaper 15
=>
27
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
=>
28
“Forbidden” Cycloaddition
[2 + 2] cycloaddition of
two ethylenes to form
cyclobutene has antibonding overlap of
HOMO and LUMO
=>
Chaper 15
29
Photochemical Induction
Absorption of correct energy photon will
promote an electron to an energy level
that was previously unoccupied.
=>
Chaper 15
30
[2 + 2] Cycloaddition
Photochemically
allowed, but
thermally
forbidden.
=>
Chaper 15
31
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
32
  * for
ethylene
and
butadiene
=>
Chaper 15
33
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
I
I
s
r
• Graph is absorbance vs. wavelength.
=>
Chaper 15
34
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
35
UV Spectrum of Isoprene
Chaper 15
36
=>
Sample UV Absorptions
=>
Chaper 15
37
Woodward-Fieser Rules
=>
Chaper 15
38
End of Chapter 15
Chaper 15
39