Chapter 11
Radical 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 11. Each of the sentences below appears verbatim in the section
entitled Review of Concepts and Vocabulary.
• Radical mechanisms utilize fishhook arrows, each of which represents the flow
of _____________________.
• Every step in a radical mechanism can be classified as initiation, ____________,
or termination.
• A radical initiator is a compound with a weak bond that readily undergoes
________________________.
an alkane.
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 11. The answers appear in the section entitled
SkillBuilder Review.
11.1 Drawing Resonance Structures of Radicals
DRAW A RESONANCE STRUCTURE OF THE RADICAL BELOW:
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231
11.3 Identifying a Radical Pattern and Drawing Fishhook Arrows
DRAW THE CURVED ARROWS FOR EACH OF THE SIX STEPS SHOWN BELOW:
X X XHRRXX
X
X
X
X X RX HXR X
XX X
Coupling
Addition
to a Pi Bond
Halogen
Abstraction
Hydrogen
Abstraction
Homolytic
Cleavage Elimination
X
X
11.5 Predicting the Regiochemistry of Radical Bromination
Br
2
DRAW THE EXPECTED PRODUCTS OF THE FOLLOWING MONOBROMINATION REACTION:
hv
232
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11.7 Predicting the Products of Allylic Bromination
NBS
hv
DRAW THE EXPECTED PRODUCTS OF THE FOLLOWING REACTION:
Review of Reactions
Identify the reagents necessary to achieve each of the following transformations. To
verify that your answers are correct, look in your textbook at the end of Chapter 11. The
answers appear in the section entitled Review of Synthetically Useful Radical Reactions.
B
r
B
r
B
r
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233
Solutions
11.1.
a) The tertiary radical is the most stable and the primary radical is the least stable.
Increasing Stability
11.2.
a)
b)
234
CHAPTER 11
11.3. This radical is highly stabilized by resonance:
11.4.
H This hydrogen atom is removed
11.5.
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235
11.6. Draw the resonance structures of the radical that is formed when H
a
is abstracted,
and then draw the resonance structures of the radical that is formed when H
b
is
abstracted:
H
a
Hydrogen
Abstraction
11.7.
a)
H
+Br
H Br Hydrogen
Abstraction
b)
B
Br Addition
to a Pi Bond
e)
+
Elimination
236
CHAPTER 11
11.8.
11.10.
a) Chlorination of methylene chloride to produce chloroform:
Cl Cl
Cl Cl
Initiation
hv
CREATES TWO
CHLORINE RADICALS
HOMOLYTIC CLEAVAGE
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237
b) Chlorination of chloroform to produce carbon tetrachloride:
Cl Cl
Cl Cl
Initiation
hv
CREATES TWO
CHLORINE RADICALS
HOMOLYTIC CLEAVAGE
c) Chlorination of ethane to produce ethyl chloride
Cl Cl
Cl Cl
Initiation
hv
CREATES TWO
HOMOLYTIC CLEAVAGE
238
CHAPTER 11
d) Chlorination of 1,1,1-trichloroethane to produce 1,1,1,2-tetrachloroethane:
C
H
C
H
HCl
Cl
Cl
Cl Cl
Cl C
H
C
H
Cl
Cl
Cl
Cl Cl
H Cl
Initiation
Propagation
hv
CREATES TWO
CHLORINE RADICALS
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
FORMS A CARBON RADICAL
e) Chlorination of 2,2-dichloropropane to produce 1,2,2-trichloropropane:
C
H
C
H
HC
Cl
Cl
H
H
H
Cl Cl
Cl
Cl Cl
C
H
C
H
C
Cl
Cl
H
H
H
H Cl
Initiation
Propagation
hv
CREATES TWO
CHLORINE RADICALS
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
FORMS A CARBON RADICAL
CHAPTER 11
239
11.11. During the chlorination of methane, methyl radicals are generated. Two of these
methyl radicals can couple together to form ethane:
C
H
H
H
C
H
H
H
CC
H
H
H
H
H
H
11.12.
a)
Br
b)
Br
c)
B
11.13.
a)
11.14.
a)
Br
no chirality center
240
CHAPTER 11
c)
Br Br
+
11.15.
SH
NaSH
Br
Br
2
Br Br
S
N
2
Compound A
(
S
)-2-bromopentane
h
ν
11.16.
a)
NBS
B
Br
hv +
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241
11.17.
NBS Br Br
Br Br
Br
++ +
h
ν
(racemic mixture)
+
11.18.
11.19.
V
i
t
a
m
i
n
E
O
OR
H
11.20.
a)
HBr
R
O
O
R
Br Br
+
e)
HBr
ROOR Br
(no chirality centers)
242
CHAPTER 11
11.21.
a) One chemical entity is being converted into two chemical entities, which increases the
11.22.
a)
d)
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243
e)
11.23.
11.24.
a)
Increasing Stability
primary secondary tertiary tertiary allylic
11.25.
OOO O
244
CHAPTER 11
11.26. The benzylic position is selectively brominated because the benzylic C-H bond is
the weakest bond. The benzylic hydrogen atom is the only hydrogen atom that can be
abstracted to generate a resonance-stabilized radical.
N
O
O
Br
N
O
O
Br
Initiation
HOMOLYTIC CLEAVAGE
hv
+
11.27.
Cl
2
hv
Cl
11.28. Selective bromination at the benzylic position generates a new chirality center.
11.29.
a) These radicals are tertiary, and they are stabilized by resonance.
b) Loss of nitrogen gas would result in the formation of vinyl radicals, which are too
unstable to form under normal conditions:
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245
11.30.
OO H
11.31.
H
Br
N
O
O
Br
N
O
O
Br
Initiation
Propagation
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
hv
+
246
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11.32.
Increasing Stability
primary secondary tertiary
11.33.
a)
Br
2
h
v
Br
e)
NBS
h
v
Br
Br
+
f)
Br
2
hv
B
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247
11.35.
Br
2
h
v
+
Br Br
11.36.
a)
NBS
hv
Br
Br
+ +
Br
Br
+
+ En + En + En
c)
NBS
hv
Br
Br Br
++ + +
11.37.
OO O
O OR
COUPLING
248
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11.38. When Compound B is treated with a sterically hindered base, the Hofmann
product (Compound D) is favored. When treated with sodium ethoxide, the Zaitsev
product (Compound B) is favored:
11.39.
NBS
hv
Br
Br
(racemic mixture) (racemic mixture)
+
11.40.
a)
Br
2
h
v
Br
b)
Br
c)
Br Br
Br Br
Initiation
hv
HOMOLYTIC CLEAVAGE
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249
d)
Br Br
Br Br
Initiation
hv
HOMOLYTIC CLEAVAGE
11.41.
a) The two products are diastereomers
Br
2
hv
Br
Br
+
250
CHAPTER 11
11.43.
a) 1 b) 4 c) 3 d) 4 e) 2 f) 1 g) 5 h) 1 i) 3 j) 3
11.45.
a)
Cl
Cl
2
h
v
b)
I
1) Br
2
,
hv
2) NaI
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251
11.46. cis-1,2-Dimethylcyclopentane produces six pairs of compounds, where each pair
have a diastereomeric relationship. In contrast, trans-dimethylcyclopentane produces
only six different compounds, as shown below:
Cl2
hv
Cl
Cl
Cl Cl
Cl Cl
Cl2
Cl
Cl
Cl
Cl
Cl
Cl
hv
11.47. The first propagation step in a bromination process is generally slow and
selective. In fact, this is the source of the regioselectivity for this reaction. A pathway
via a tertiary radical will be significantly lower in energy than a pathway via a secondary
or primary radical. As a result, bromination occurs predominantly at the more substituted
252
CHAPTER 11
11.48.
O
O
O
O
O
O
– CO
2
RH