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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
________________________.
• A __________________, also called a radical scavenger, is a compound that
prevents a chain process from either getting started or continuing.
• _________________ is more selective than chlorination.
• When a new chirality center is created during a radical halogenation process, a
____________ mixture is obtained.
• ________________ can undergo allylic bromination, in which bromination
occurs at the allylic position.
• Organic compounds undergo oxidation in the presence of atmospheric oxygen to
produce hydroperoxides. This process, called __________________, is believed
to proceed via a ____________ mechanism.
• Antioxidants, such as BHT and BHA, are used as food preservatives to prevent
autooxidation of ________________ oils.
• When vinyl chloride is polymerized, _______________________ is obtained.
• Radical halogenation provides a method for introducing _______________ into
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:
CHAPTER 11 231
11.2 Identifying the Weakest C-H Bond in a Compound
IDENTIFY THE WEAKEST C-H BOND IN THE FOLLOWING COMPOUND:
11.3 Identifying a Radical Pattern and Drawing Fishhook Arrows
DRAW THE CURVED ARROWS FOR EACH OF THE SIX STEPS SHOWN BELOW:
X X X H R R X XX
XX
X X RX H XR XX
X X
CouplingAddition
to a Pi BondHalogen
AbstractionHydrogen
Abstraction
HomolyticCleavage Elimination
X
X
11.4 Drawing a Mechanism for Radical Halogenation
INITIATION
C
R
R
R
X C
R
R
R
XX X
C
R
R
R
H X C
R
R
R
H X
X XX X XC
R
R
R
C
R
R
R
X
HALOGEN ABSTRACTION
HYDROGEN ABSTRACTION
hv
DRAW CURVED
ARROWS FOR THE
INITIATION STEP
BELOW:
DRAW CURVED ARROWS FOR THE PROPAGATION
STEPS BELOW:
PROPAGATION
DRAW CURVED ARROWS FOR THE
TERMINATION STEP BELOW:
TERMINATION
11.5 Predicting the Regiochemistry of Radical Bromination
Br2
DRAW THE EXPECTED PRODUCTS OF THE FOLLOWING MONOBROMINATION REACTION:
hv
232 CHAPTER 11
11.6 Predicting the Stereochemical Outcome of Radical Bromination
Br2
DRAW THE EXPECTED PRODUCTS OF THE FOLLOWING REACTION:
hv
11.7 Predicting the Products of Allylic Bromination
NBS
hv
DRAW THE EXPECTED PRODUCTS OF THE FOLLOWING REACTION:
11.8 Predicting the Products for Radical Addition of HBr
HBr
ROOR
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.
Br
Br
Br
CHAPTER 11 233
Solutions
11.1. a) The tertiary radical is the most stable and the primary radical is the least stable.
Increasing Stability
b) Increasing Stability
11.2.
a)
b)
c)
d)
234 CHAPTER 11
11.3. This radical is highly stabilized by resonance:
11.4.
H This hydrogen atom is removed
11.5.
a)
H
b)
H
c) H
d)
H
CHAPTER 11 235
11.6. Draw the resonance structures of the radical that is formed when Ha is abstracted,
and then draw the resonance structures of the radical that is formed when Hb is
abstracted:
HbHydrogen
Abstraction
HaHydrogen
Abstraction
Compare the resonance structures in each case. Specifically, look at the middle
resonance structure in each case. When Hb is abstracted, the middle resonance structure
is tertiary, and the methyl group stabilizes the radical via electron donation. This
stabilizing factor is not present when Ha is abstracted. Therefore, we expect the C-Hb
bond to be slightly weaker than the C-Ha bond.
11.7.
a)
H+ BrH Br
Hydrogen
Abstraction
b)
Br
BrAddition
to a Pi Bond
c)
CH3
Coupling
d)
+ HBrBr
H
Hydrogen
Abstraction
e)
+Elimination
f)
O O
R
O O
ROR
O2
Homolytic
Bond Cleavage
236 CHAPTER 11
11.8.
11.9.
Coupling
11.10. a) Chlorination of methylene chloride to produce chloroform:
C
Cl
H
Cl
C
Cl
H
Cl
C
Cl
H
Cl
H
Cl Cl
Cl Cl
Cl
Cl C
Cl
H
Cl
Cl Cl
C
Cl
H
Cl
Cl
C
Cl
H
Cl
Cl
H Cl
Cl
Initiation
Propagation
Termination
hv
CREATES TWO
CHLORINE RADICALS
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGEN ABSTRACTION
FORMS A CARBON RADICAL
GIVES THE PRODUCT
AND REGENERATES
A CHLORINE RADICAL
COUPLING
DESTROYS ONE CARBON
RADICAL AND ONE
CHLORINE RADICAL
CHAPTER 11 237
b) Chlorination of chloroform to produce carbon tetrachloride:
C
Cl
Cl
Cl
C
Cl
Cl
Cl
C
Cl
Cl
Cl
H
Cl Cl
Cl Cl
Cl
Cl C
Cl
Cl
Cl
Cl Cl
C
Cl
Cl
Cl
Cl
C
Cl
Cl
Cl
Cl
H Cl
Cl
Initiation
Propagation
Termination
hv
CREATES TWO
CHLORINE RADICALS
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGEN ABSTRACTION
FORMS A CARBON RADICAL
GIVES THE PRODUCT
AND REGENERATES
A CHLORINE RADICAL
COUPLING
DESTROYS ONE CARBON
RADICAL AND ONE
CHLORINE RADICAL
c) Chlorination of ethane to produce ethyl chloride
C
H
C
H
H
H
H
C
H
C
H
H
H
H
C
H
C
H
HH
H
H
Cl Cl
Cl Cl
Cl
Cl C
H
C
H
H
H
H
Cl Cl
C
H
C
H
ClH
H
H
C
H
C
H
ClH
H
H
H Cl
Cl
Initiation
Propagation
Termination
hv
CREATES TWO
CHLORINE RADICALS
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGEN ABSTRACTION
FORMS A CARBON RADICAL
GIVES THE PRODUCT
AND REGENERATES
A CHLORINE RADICAL
COUPLING
DESTROYS ONE CARBON
RADICAL AND ONE
CHLORINE RADICAL
238 CHAPTER 11
d) Chlorination of 1,1,1-trichloroethane to produce 1,1,1,2-tetrachloroethane:
C
H
C
H
Cl
Cl
Cl
C
H
C
H
Cl
Cl
Cl
C
H
C
H
HCl
Cl
Cl
Cl Cl
Cl Cl
Cl
Cl C
H
C
H
Cl
Cl
Cl
Cl Cl
C
H
C
H
ClCl
Cl
Cl
C
H
C
H
ClCl
Cl
Cl
H Cl
Cl
Initiation
Propagation
Termination
hv
CREATES TWO
CHLORINE RADICALS
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGEN ABSTRACTION
FORMS A CARBON RADICAL
GIVES THE PRODUCT
AND REGENERATES
A CHLORINE RADICAL
COUPLING
DESTROYS ONE CARBON
RADICAL AND ONE
CHLORINE RADICAL
e) Chlorination of 2,2-dichloropropane to produce 1,2,2-trichloropropane:
C
H
C
H
C
Cl
Cl
H
H
H
C
H
C
H
C
Cl
Cl
H
H
H
C
H
C
H
HC
Cl
Cl
H
H
H
Cl Cl
Cl Cl
Cl
Cl
Cl Cl
C
H
C
H
C
Cl
Cl
H
H
H
C
H
C
H
ClC
Cl
Cl
H
H
H
C
H
C
H
ClC
Cl
Cl
H
H
H
H Cl
Cl
Initiation
Propagation
Termination
hv
CREATES TWO
CHLORINE RADICALS
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGEN ABSTRACTION
FORMS A CARBON RADICAL
GIVES THE PRODUCT
AND REGENERATES
A CHLORINE RADICAL
COUPLING
DESTROYS ONE CARBON
RADICAL AND ONE
CHLORINE 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 Ethane can then undergo hydrogen abstraction, followed by halogen abstraction to
generate ethyl chloride:
C
H
C
H
H
H
H
C
H
C
H
HH
H
H
Cl Cl
Cl C
H
C
H
H
H
H
C
H
C
H
ClH
H
H
H Cl
Cl
11.12.
a)
Br
b)
Br
c) Br
11.13.
a)
b)
Cl
Cl
Cl
Cl
c)
Br
11.14.
a)
Br
no chirality center
b) BrBr
+
240 CHAPTER 11
c)
Br Br
+
d)
Br
+ Br
11.15. SH
NaSHBr
Br2Br Br
SN2Compound A
(S)-2-bromopentane
hν
11.16.
a)
NBS
Br
Brhv+
b)
NBS
hv
Br
c)
NBS
hv
+Br
Br
d)
NBS
hvBr
Br+
CHAPTER 11 241
11.17.
NBS BrBr
Br Br
Br
++ +hν
(racemic mixture)
+
11.18.
+ O2
O
O3
O2+
+
+
N ONO O
N OO NO
11.19.
Vitamin E
O
O R
H
11.20.
a)
HBr
ROORBr Br+
b)
HBr
ROOR
Br
(no chirality centers)
c)
HBr
ROORBr Br
+
d)
HBr
ROOR+
Br Br
e)
HBr
ROOR Br
(no chirality centers)
f)
HBr
ROORBr
(no chirality centers)
242 CHAPTER 11
11.21.
a) One chemical entity is being converted into two chemical entities, which increases the
entropy of the system.
b) Recall that ∆G has two components: (∆H) and (-T∆S). The magnitude of the latter
term is dependent on the temperature. At high temperature, the latter term dominates
over the former, and the reaction is thermodynamically favorable. However, at low
temperature, the first term (enthalpy) dominates, and the reaction is no longer
thermodynamically favored.
11.22.
a)
b)
c)
d)
CHAPTER 11 243
e)
11.23.
Ha
Hc
Hb
Ha > Hb > Hc
Increasing bond strength
abstraction of Ha generates an unstable vinyl radical
abstraction of Hc generates a resonance-stabilized radical
11.24.
a) Increasing Stability
primary secondary tertiary tertiary allylic
b) Increasing Stability
primary secondary tertiary
11.25.
O O O 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.
H
Br
Br
N
O
O
Br
Br Br
N
O
O
Br
Br
Br
Br
Initiation
Propagation
Termination
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGEN ABSTRACTION
COUPLING
hv+
+
11.27.
Cl2
hv
Cl
11.28. Selective bromination at the benzylic position generates a new chirality center.
The intermediate benzylic radical is expected to be attacked from either face of the planar
radical with equal likehood, giving rise to a racemic mixture of enantiomers:
NBS
hv
Br Br
+
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:
CHAPTER 11 245
11.30.
a)
OO H
b) Hydrogen abstraction leads to an exceptionally stable radical, with many,
many resonance structures (see problem 11.3).
c) Phenol acts as a radical scavenger, thereby preventing the chain process from
taking off.
11.31.
H
Br
Br
N
O
O
Br
Br
N
O
O
Br
Br
Br
Br
Br
Br
Br
Br
Br
Initiation
Propagation
Termination
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGENABSTRACTION
COUPLING
hv+
COUPLING
246 CHAPTER 11
11.32. Increasing Stability
primary secondary tertiary
11.33.
a) Br2
hv
Br
b) I2
hvno reaction
c)
Cl2
hv
Cl
Cl
Cl(racemic mixture)
+ +
d)
NBS
hv
Br
e)
NBS
hv
Br
Br+
f)
Br2
hvBr
11.34.
Cl2
hv
Cl Cl Cl Cl Cl
Cl ClCl
Cl
Cl
Cl Cl
Cl Cl
Cl
+
+
+
+
+
CHAPTER 11 247
11.35.
Br2
hv+
Br Br
11.36.
a)
NBS
hv
Br
Br
+ +
Br
Br
+
+
Br
Br
+
+ En + En + En
+ En+ En
b) NBS
hvBr
c)
NBS
hv
Br Br
Br
Br Br+ + + +
d)
NBS
hv Br Br+
11.37.
O O O
O O
ORH
O OR
O OH
OR
HYDROGEN ABSTRACTION
COUPLING
248 CHAPTER 11
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:
Br
A B C D
Br2
StrongBase
hv+
NaOEt
t-BuOK
11.39.
NBS
hv
Br
Br
(racemic mixture) (racemic mixture)
+
11.40.
a)
Br2
hv
Br
b) Br
c)
H
Br Br
Br Br
Br
Br
Br
Br Br
Br
H Br
Br
Initiation
Propagation
Termination
hv
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGEN ABSTRACTION
COUPLING
CHAPTER 11 249
d)
H
Br Br
Br Br
Br
Br
Br
Br Br
Br
H Br
Br
Initiation
Propagation
Termination
hv
HYDROGEN ABSTRACTION
HOMOLYTIC CLEAVAGE
HALOGEN ABSTRACTION
COUPLING
e) The minor product is only formed via a primary radical, which does not readily form
under bromination conditions. The tertiary radical is selectively formed, which leads to
the tertiary alkyl bromide as the major product.
11.41.
a) The two products are diastereomers
Br2
hv
BrBr
+
b)
Br+
Br
Br
Br+
Br
BrBr
Br
enantiomers a meso compound There are two chirality centers, so we might expect four products. However, one product
is a meso compound, so there are only three products, rather than four. For a review of
meso compounds, see Section 5.6.
250 CHAPTER 11
11.42. Methyl radicals are less stable and less hindered than tert-butyl radicals.
11.43.
a) 1 b) 4 c) 3 d) 4 e) 2 f) 1 g) 5 h) 1 i) 3 j) 3
11.44.
Br2
hv
NaOEtBr
HBr
ROORBr
1) NaNH2
2) Br
H2SO4, H2O
HgSO4
O
11.45.
a)
ClCl2
hv b)
I1) Br2, hv
2) NaI
c)
1) Cl2, hv
2) NaOEt d) Br
Br
+ En1) Cl2, hv
2) NaOEt
3) Br2
e)
1) Br2, hv
2) NaOEt
CHAPTER 11 251
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
ClCl
Cl Cl
Cl Cl
Cl Cl
Cl2
Cl
Cl
Cl
Cl
Cl
Cl
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
position. However, when chlorine is present, chlorine radicals can perform the first
propagation step (hydrogen abstraction) very rapidly, and with little selectivity. Under
these conditions, secondary and primary radicals are formed almost as easily as tertiary
radicals. The resulting radicals then react with bromine in the second propagation step to
yield monobrominated products. Therefore, in the presence of chlorine, the selectivity
normally observed for bromination is lost.
252 CHAPTER 11
11.48.
O
O
O
O
O
O
- CO2
RH
RH
RH
