Chapter 13
Conjugated Unsaturated
Systems
Created by
Professor William Tam & Dr. Phillis Chang
Ch. 13 - 1
About The Authors
These PowerPoint Lecture Slides were created and prepared by Professor
William Tam and his wife, Dr. Phillis Chang.
Professor William Tam received his B.Sc. at the University of Hong Kong in
1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an
NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard
University (USA). He joined the Department of Chemistry at the University of
Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and
Associate Chair in the department. Professor Tam has received several awards
in research and teaching, and according to Essential Science Indicators, he is
currently ranked as the Top 1% most cited Chemists worldwide. He has
published four books and over 80 scientific papers in top international journals
such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem.
Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her
M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She
lives in Guelph with her husband, William, and their son, Matthew.
Ch. 13- 2
1. Introduction

A conjugated system involves at least
one atom with a p orbital adjacent to
at least one p bond
● e.g.
conjugated
diene
allylic
radical
allylic
cation
allylic
anion
O
enone
enyne
Ch. 13 - 3
2. Allylic Substitution and the
Allyl Radical
vinylic
carbons
(sp2)
X
X2
low temp
CCl4
X
X2
allylic
carbon
(sp3)
high temp
(and low conc.
of X2)
+ H
X
X
Ch. 13 - 4
2A. Allylic Chlorination
(High Temperature)
+ Cl2
400oC
gas phase
Cl + H
Cl
Ch. 13 - 5

Mechanism
● Chain initiation
Cl
Cl
2 Cl
● Chain propagation
H
+
Cl
+ H
(allylic radical)
Cl
Ch. 13 - 6

Mechanism
● Chain propagation
+ Cl
Cl
Cl
+ Cl
● Chain termination
+
Cl
Cl
Ch. 13 - 7
H
+
H
DHo = 369 kJmol-1
H
+
H
DHo = 465 kJmol-1
Ch. 13 - 8
H
X
+
+
H
Relative stability
of radicals:
X
Eact
(low)
Eact
(high)
o
o
+
HX
+
HX
o
allylic > 3 > 2 > 1 > vinylic
Ch. 13 - 9
Ch. 13 - 10
2B. Allylic Bromination with N-Bromosuccinimide (Low Concentration of Br2)

NBS is a solid and nearly insoluble in CCl4
● Low concentration of Br•
Br
H
+ O
N
O
(NBS)
h or ROOR
heat, CCl4
H
Br + O
N
O
Ch. 13 - 11

Examples
Br
NBS
ROOR, CCl4
heat
NBS
ROOR, CCl4
heat
Br
Ch. 13 - 12
3. The Stability of the Allyl Radical
3A. Molecular Orbital Description of
the Allyl Radical
Ch. 13 - 13
Ch. 13 - 14
3B. Resonance Description of the
Allyl Radical
2
2
1
3
1
3
3
3
2
2
1
1
Ch. 13 - 15
4. The Allyl Cation

Relative order of Carbocation stability
>
>
(3o)
(3o allylic)
>
>
>
(2o)
(allylic)
(1o)
(vinylic)
Ch. 13 - 16
5. Resonance Theory Revisited
5A. Rules for Writing Resonance Structures
 Resonance structures exist only on paper.
Although they have no real existence of
their own, resonance structures are useful
because they allow us to describe
molecules, radicals, and ions for which a
single Lewis structure is inadequate
 We connect these structures by doubleheaded arrows (), and we say that the
hybrid of all of them represents the real
molecule, radical, or ion
Ch. 13 - 17

In writing resonance structures, we are
only allowed to move electrons
resonance structures
H
H
not resonance structures
Ch. 13 - 18

All of the structures must be proper
Lewis structures
:O:
X
O
10 electrons!
not a proper
Lewis structure
Ch. 13 - 19

All resonance structures must have the
same number of unpaired electrons
X
Ch. 13 - 20

All atoms that are part of the
delocalized p-electron system must lie
in a plane or be nearly planar
delocalization
of p-electrons
no
delocalization
of p-electrons
Ch. 13 - 21

The energy of the actual molecule is
lower than the energy that might be
estimated for any contributing
structure

Equivalent resonance structures make
equal contributions to the hybrid, and a
system described by them has a large
resonance stabilization
Ch. 13 - 22

The more stable a structure is (when
taken by itself), the greater is its
contribution to the hybrid
(3o allylic cation)
(2o allylic cation)
greater contribution
Ch. 13 - 23
5B. Estimating the Relative Stability
of Resonance Structures
 The more covalent bonds a structure
has, the more stable it is
(more stable)
O
(more stable)
(less stable)
O
(less stable)
Ch. 13 - 24

Structures in which all of the atoms
have a complete valence shell of
electrons (i.e., the noble gas structure)
are especially stable and make large
contributions to the hybrid
O
this carbon has
6 electrons
O
this carbon has
8 electrons
Ch. 13 - 25

Charge separation decreases stability
OMe
(more stable)
OMe
(less stable)
Ch. 13 - 26
6.

Alkadienes and Polyunsaturated
Hydrocarbons
Alkadienes (“Dienes”)
4
2
1
2
3
1,3-Butadiene
4
2
1
6
1
3
3
5
4
6
5
1,3-Cyclohexadiene
(2E,4E)-2,4-Hexadiene
Ch. 13 - 27

Alkatrienes (“Trienes”)
4
2
1
3
6
5
8
7
(2E,4E,6E)-Octa-2,4,6-triene
Ch. 13 - 28

Alkadiynes (“Diynes”)
1
2
4
3
5
6
2,4-Hexadiynes

6
Alkenynes (“Enynes”)
5
1
4
3
2
5
3
2
6
7
8
4
1
Hex-1-en-5-yne
(2E)-Oct-2-en-6-yne
Ch. 13 - 29

Cumulenes
enantiomers
H
H
C
H
C
H
H
C
C
H
H
C
C
H
(Allene)
(a 1,2-diene)
Ch. 13 - 30

Conjugated dienes

Isolated double bonds
Ch. 13 - 31
7. 1,3-Butadiene: Electron
Delocalization
7A. Bond Lengths of 1,3-Butadiene
1.47 Å
4
2
1
3
1.34 Å
sp3
1.54 Å
sp2 sp3
1.50 Å
sp sp3
1.46 Å
Ch. 13 - 32
7B. Conformations of 1,3-Butadiene
trans
single
bond
single
bond
cis
(s-cis)
(s-trans)
H
H
(less stable)
Ch. 13 - 33
7C. Molecular Orbitals of 1,3-Butadiene
Ch. 13 - 34
8. The Stability of Conjugated
Dienes

Conjugated alkadienes are
thermodynamically more stable than
isomeric isolated alkadienes
H o (kJmol-1)
2
+ 2 H2
2
2 x (-127)=-254
+ 2 H2
=-239
Difference
15
Ch. 13 - 35
Ch. 13 - 36
9. Ultraviolet–Visible
Spectroscopy

The absorption of UV–Vis radiation is
caused by transfer of energy from the
radiation beam to electrons that can be
excited to higher energy orbitals
Ch. 13 - 37
9A. The Electromagnetic Spectrum
Ch. 13 - 38
9B. UV–Vis Spectrophotometers
Ch. 13 - 39
Ch. 13 - 40

Beer’s law
A = excxℓ
or
e =
A
cxℓ
A =
e =
c =
ℓ =
absorbance
molar absorptivity
concentration
path length
● e.g. 2,5-Dimethyl-2,4-hexadiene
lmax(methanol) 242.5 nm
(e = 13,100)
Ch. 13 - 41
9C. Absorption Maxima for Nonconjugated
and Conjugated Dienes
Ch. 13 - 42
Acetone
O
n
Ground state

p
lmax = 280 nm
emax = 15
O
p* Excited state
O
n
p
lmax = 324 nm,emax = 24
p
p
lmax = 219 nm,emax = 3600
Ch. 13 - 43
9D. Analytical Uses of UV–Vis Spectroscopy



UV–Vis spectroscopy can be used in the
structure elucidation of organic molecules to
indicate whether conjugation is present in a
given sample
A more widespread use of UV–Vis
spectroscopy, however, has to do with
determining the concentration of an
unknown sample
Quantitative analysis using UV–Vis
spectroscopy is routinely used in
biochemical studies to measure the rates of
enzymatic reactions
Ch. 13 - 44
10. Electrophilic Attack on Conjugated
Dienes: 1,4 Addition
4
2
1
H
Cl
25oC
3
Cl
H
(78%)
(1,2-Addition)
+
H
Cl
(22%)
(1,4-Addition)
Ch. 13 - 45

Cl
Mechanism
H
X
H
+
H
(a)
Cl
H
(a)
(a)
Cl
H
(b)
H
+
+
(b)
Cl
H
(b)
Ch. 13 - 46
10A. Kinetic Control versus
Thermodynamic Control of a
Chemical Reaction
-80oC
Br
+
Br
(20%)
(80%)
+
HBr
Br
+
40oC
(20%)
Br
(80%)
Ch. 13 - 47
Br
o
40 C, HBr
Br
1,2-Addition
product
1,4-Addition
product
Ch. 13 - 48
Ch. 13 - 49
11. The Diels–Alder Reaction:
A 1,4-Cycloaddition Reaction
of Dienes
+
(diene)
(dienophile)
[4p+2p]
(adduct)
Ch. 13 - 50

e.g.
O
+
O
O
benzene
O
100oC
O
1,3-Butadiene Maleic
(diene)
anhydride
(dienophile)
O
Adduct
(100%)
Ch. 13 - 51
11A. Factors Favoring the Diels–Alder
Reaction
Type A
EDG
EDG
EWG
EWG
+
EWG
Type B
EWG
+
EDG
EDG
● Type A and Type B are normal Diels-Alder
reactions
Ch. 13 - 52
Type C
EWG
EWG
EDG
EDG
+
EDG
Type D
EDG
+
EWG
EWG
● Type C and Type D are Inverse Demand
Diels-Alder reactions
Ch. 13 - 53

Relative rate
O
Diene +
O
30oC
D.A. cycloadduct
O
OMe
>
Diene
t1/2
20 min.
>
70 min.
4 h.
Ch. 13 - 54

Relative rate
20oC
+ Dienophile
NC
CN
NC
CN
Dienophile
t1/2
0.002 sec.
>
D.A. cycloadduct
CN
>
CN
CN
20 min.
28 h.
Ch. 13 - 55

Steric effects
>
Dienophile:
COOEt
Relative rate:
1
>
COOEt
0.14
COOEt
0.007
Ch. 13 - 56
11B. Stereochemistry of the
Diels–Alder Reaction
1. The Diels–Alder reaction is stereospecific:
The reaction is a syn addition, and the
configuration of the dienophile is retained
in the product
O
H
+
H
H
O
OMe
OMe
OMe
OMe
O
Dimethyl maleate
(a cis-dienophile)
H
O
Dimethyl cyclohex-4-enecis-1,2-dicarboxylate
Ch. 13 - 57
O
+
H
H
MeO
O
OMe
OMe
H
O
Dimethyl fumarate
(a trans -dienophile)
OMe
H
O
Dimethyl cyclohex4-ene-trans -1,2dicarboxylate
Ch. 13 - 58
2. The diene, of necessity, reacts in the
s-cis rather than in the s-trans
conformation
s-cis Configuration
s-trans Configuration
O
+
R
X
O
R
Highly strained
Ch. 13 - 59

e.g.
COOMe
+
heat
COOMe
(diene locked
in s-cis
conformation)
COOMe
+
(diene locked
in s-trans
conformation)
heat
No Reaction
Ch. 13 - 60


Cyclic dienes in which the double bonds are
held in the s-cis conformation are usually
highly reactive in the Diels–Alder reaction
Relative rate
O
Diene +
O
30oC
D.A. cycloadduct
O
>
Diene
t1/2
11 sec.
>
130 sec.
4 h.
Ch. 13 - 61
3. The Diels–Alder reaction occurs primarily in
an endo rather than an exo fashion when
the reaction is kinetically controlled
H
H
R
longest bridge
H
H
R is exo
H
H
R
R is endo
Ch. 13 - 62

Alder-Endo Rule
● If a dienophile contains activating
groups with p bonds they will prefer
an ENDO orientation in the
transition state
H
H
X
X
X
X
Ch. 13 - 63

e.g.
+
O
O
O

H
H
100% endo
O
O
O
Ch. 13 - 64

Stereospecific reaction
(i)
X
X
X
X
+
X
X
+
X
X
Ch. 13 - 65

Stereospecific reaction
(ii)
Y
Y
+
Y
Y
Y
Y
Y
+
Y
Ch. 13 - 66

(A)
Examples
Me
Me
NC
CN
+
NC
(B)
CN
CN
D.A.
CN
CN
CN
Me
Me
NC
CN
Me +
NC
CN
D.A.
Me
CN
CN
CN
CN
Ch. 13 - 67

Diene A reacts 103 times faster than
diene B even though diene B has two
electron-donating methyl groups
Me
Me
Me
Me
H
(s-cis)
(s-trans)
Ch. 13 - 68

Examples
(C)
O
+
O
D.A.
O
H
O
(D)
O
+
O
O
O
H
H
D.A.
O
O
O
H
O
Ch. 13 - 69

Examples
(E)
O
+
O
D.A.
No Reaction
O
● Rate of Diene C > Diene D (27 times),
but Diene D >> Diene E
● In Diene C, tBu group  electron
donating group  increase rate
● In Diene E, 2 tBu group  steric effect,
cannot adopt s-cis conformation
Ch. 13 - 70
 END OF CHAPTER 13 
Ch. 13 - 71