Chapter 17
Conjugated Pi Systems and Pericyclic Reactions
Review of Concepts
Fill in the blanks below. To verify that your answers are correct, look in your textbook at
the end of Chapter 17. Each of the sentences below appears verbatim in the section
entitled Review of Concepts and Vocabulary.
• Reactions induced by light are called _______________ reactions.
• When butadiene is treated with HBr, two major products are observed, resulting
from ______-addition and ______-addition.
• Conjugated dienes that undergo addition at low temperature are said to be under
____________ control. Conjugated dienes that undergo addition at elevated
temperature are said to be under _________________ control.
is obtained, and the _____ cycloadduct is favored over the _____ cycloadduct.
• Conservation of orbital symmetry determines whether an electrocyclic reaction
occurs in a ___________ fashion or a ____________ fashion.
• A [ _________ ] sigmatropic rearrangement is called a Cope rearrangement
when all six atoms of the cyclic transition state are carbon atoms.
CHAPTER 17
381
Review of Skills
Fill in the blanks and empty boxes below. To verify that your answers are correct, look
in your textbook at the end of Chapter 17. The answers appear in the section entitled
SkillBuilder Review.
17.1 Proposing the Mechanism and Predicting the Products of Electrophilic Addition to Conjugated
Dienes
17.2 Predicting the Major Product of an Electrophilic Addition to Conjugated Dienes
HBr
0 C
DRAW THE MAJOR PRODUCTS OF THE FOLLOWING REACTION:
17.3 Predicting the Product of a Diels–Alder Reaction
382
CHAPTER 17
17.5 Using Woodward–Fieser Rules to Estimate λ
max
Review of Reactions
Predict the Products for each of the following transformations. To verify that your
answers are correct, look in your textbook at the end of Chapter 17. The answers appear
in the section entitled Review of Reactions.
Preparation of Dienes
Br t-BuOK
Br
B
r
t-BuOK
Electrophilic Addition
H Br
Br
2
Diels–Alder Reaction
CHAPTER 17
383
X
+
X
Electrocyclic Reactions
heat
heat
Sigmatropic Rearrangements
Cope Rearrangement
heat
384
CHAPTER 17
Solutions
17.1.
a)
HO
O
HO
O
OH
O
conjugated
conjugated
CHAPTER 17
385
17.2.
Br
2
hv
Br NaOEt Br
2
Br
Br
t-BuOK
+
E
n
17.3.
17.4.
17.5. In the compound below, all three π bonds are conjugated:
386
CHAPTER 17
17.6.
HOMO
ψ
5
ψ
4
E
GROUND
STATE
HOMO
ψ
5
ψ
4
EXCITED
STATE
light
LUMO
ψ
8
ψ
8
CHAPTER 17
387
17.7.
a)
H
Cl
H
Cl
b)
H
Cl
H
Cl
388
CHAPTER 17
c)
H
Cl
d)
H
H
CHAPTER 17
389
e)
H
Br
f)
H
Br
390
CHAPTER 17
17.8. The first diene can be protonated either at C1 or at C4. Each of these pathways
produces a resonance stabilized carbocation. And each of these carbocations can be
17.9.
HBr
(racemic)
Br
17.10.
a)
c)
HBr
0 C
+
major minor
Br
Br
17.11. In this case, the π bond in the 1,2-adduct is more substituted than the π bond in
CHAPTER 17
391
17.12. In this case, 1,2-addition and 1,4-addition yield the same product.
HBr
Br
17.13.
17.14.
a)
O
O
O
O
+O
O
O
O
(meso)
b)
O
+
OO
O
O
392
CHAPTER 17
e)
+O
O
OO
O
(meso)
O
h)
+
O
OH
OH
O
CHAPTER 17
393
17.16.
OO
O
OO
O
17.17. The 2E,4E isomer is expected to react more rapidly as a diene in a Diels–Alder
reaction, because it can readily adopt an s-trans conformation.
17.18.
Reactivity in Diels-Alder reactions
locked in an
s-trans
c
o
n
f
o
r
m
a
t
i
o
n
locked in an
s-cis
c
o
n
f
o
r
m
a
t
i
o
n
17.19.
a)
394
CHAPTER 17
c)
+
O
HH+ En
H
O
f)
O +
O
O O O
O
O
O
(meso)
17.20. We first consider the HOMO of one molecule of butadiene and the LUMO of
another molecule of butadiene. The phases of these MOs do not align, so a thermal
CHAPTER 17
395
17.21.
a)
heat
(meso)
17.22.
a)
light
+ En
b)
396
CHAPTER 17
17.23.
a)
Et
Et
Et
E
t
heat
17.24.
a) a meso compound
17.25.
a)
O O
heat
[3,3] Sigmatropic rearrangement
b) [3,3] Sigmatropic rearrangement
c) The ring strain associated with the three-membered ring is alleviated. The reverse
process would involve forming a high-energy, three-membered ring. The equilibrium
disfavors the reverse process.
CHAPTER 17
397
17.27.
a)
Oheat O
d)
heat
OO
taut.
OOH
17.28.
heat
17.29.
a)
398
CHAPTER 17
b)
Base = 217
c)
Base = 217
Additional double bonds = +30
Auxochromic alkyl groups = +30
17.30
17.31.
17.32.
CHAPTER 17
399
17.33.
O
17.34.
a) These drawings represent two different conformations of the same compound: the s-
cis conformation and the s-trans conformation. These two conformations are in
equilibrium at room temperature.
17.35.
HBr
(racemic)
Br
400
CHAPTER 17
17.38.
H
Br
H
Br
17.39. An increase in temperature allowed the system to reach equilibrium
concentrations, which are determined by the relative stability of each product.
Under these conditions, the 1,4-adducts predominate. Once at equilibrium,
lowering the temperature will not cause a decrease in the concentration of the 1,4-
adducts.
CHAPTER 17
401
17.40.
a) The tert-butyl groups provide significant steric hinderance that prevents the compound
17.41.
O
O
O O
Reactivity in Diels-Alder reactions
17.42. The π bonds in 1,2-butadiene are not conjugated, and λ
max
is therefore lower than
17.43.
a)
COOH
HOOC
+
C
O
O
H
COOH
H
H+ En
402
CHAPTER 17
d)
S + O
O
O
S
O
O
O
(meso)
17.44.
a)
COOH
C
O
O
H
COOH
COOH
+
d)
O
O
O
O
O
+
O
O
O
CHAPTER 17
403
e)
O
O
H
H
+
O
O
h)
O
O
+
OO
17.45.
17.46.
O
O
+O
O
404
CHAPTER 17
17.47.
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl HH
Cl
Cl Cl
Cl
Cl
c
h
l
o
r
d
a
n
e
+
17.49.
17.51. Two of the π bonds are homoannular in this compound, which adds +39 nm
according to Woodward-Fieser rules.
17.52.
Base = 217
17.53. Each of these transformations can be explained with a [1,5] sigmatropic
rearrangement:
CHAPTER 17
405
17.54. This transformation can be explained with a [1,5] sigmatropic rearrangement:
heat
D
DD
D
17.55.
heat light
17.56.
a)
heat
(meso)
17.57. The compound on the right has a π bond in conjugation with the aromatic ring,
while the compound on the left does not. Therefore, the compound on the right side of
the equilibrium is expected to be more stable, and the equilibrium will favor the
compound that is lower in energy.
406
CHAPTER 17
17.58.
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
heat
Not formed
Methyl groups are too crowded
+
17.59.
a)
heat
OO
taut
O OH
17.60.
a) α-Terpinene has two double bonds.
b)
OO H
HO
1) O
3
2) DMS O+
17.61.
Et
Et
Me
Me MeMe
Me
M
e
Et
17.63. In each case, the non-conjugated isomer will be higher in energy:
a) b)
408
CHAPTER 17
17.65.
O
Oheat O
17.66. The diene is electron-rich in one specific location, as seen in the second resonance
structure below:
MeO MeO
The diene is
electron rich
i
n
t
h
i
s
l
o
c
a
t
i
o
n
These two compounds will join in such a way that the electron-poor center lines up with
the electron-rich center:
MeO
O
MeO
O
heat
δ
–
δ
+
CHAPTER 17
409
17.68.
17.69. The nitrogen atom in divinyl amine is sp
2
hybridized. The lone pair is
delocalized, and joins the two neighboring π bonds into one conjugated system. As such,