Chapter 6
Reactions of Alkenes:
Addition Reactions
Dr. Wolf's CHM 201 & 202
6-1
Reactions of Alkenes
The characteristic reaction of alkenes
is addition to the double bond.
C
C
Dr. Wolf's CHM 201 & 202
+
A—B
A
C
C
B
6-2
Hydrogenation of Alkenes
Dr. Wolf's CHM 201 & 202
6-3
Hydrogenation of ethylene
H
C
H

+
C
H

H
H—H
H
H

H
C
C
H
H

H
exothermic H° = –136 kJ/mol
catalyzed by finely divided Pt, Pd, Rh, Ni
Dr. Wolf's CHM 201 & 202
6-4
Example
H3C
CH2
H3C
H2, Pt
CH3
H3C
H
H3C
(73%)
Dr. Wolf's CHM 201 & 202
6-5
Problem 6.1
What three alkenes yield 2-methylbutane on
catalytic hydrogenation?
Dr. Wolf's CHM 201 & 202
6-6
Problem 6.1
What three alkenes yield 2-methylbutane on
catalytic hydrogenation?
H2, Pt
Dr. Wolf's CHM 201 & 202
6-7
Mechanism of catalytic hydrogenation.
Figure 6.1
B
H H
A
Dr. Wolf's CHM 201 & 202
H H
Y
C
C
X
6-8
Mechanism of catalytic hydrogenation.
Figure 6.1
B
A
H
Y
C
C
X
H
H
H
Dr. Wolf's CHM 201 & 202
6-9
Mechanism of catalytic hydrogenation.
Figure 6.1
B
H
Y
A
H
C
X
C
H
H
Dr. Wolf's CHM 201 & 202
6-10
Mechanism of catalytic hydrogenation.
Figure 6.1
B
H
Y
A
H
C
X
C
H
H
Dr. Wolf's CHM 201 & 202
6-11
Mechanism of catalytic hydrogenation.
Figure 6.1
B
H
A
C
H
Dr. Wolf's CHM 201 & 202
Y
X
C
H
H
6-12
Mechanism of catalytic hydrogenation.
Figure 6.1
B
Y
A
C
H
Dr. Wolf's CHM 201 & 202
H
X
C
H
H
6-13
Heats of Hydrogenation
can be used to measure relative stability
of isomeric alkenes
correlation with structure is same as
when heats of combustion are measured
Dr. Wolf's CHM 201 & 202
6-14
126
119
115
CH3CH2CH2CH3
Dr. Wolf's CHM 201 & 202
6-15
Heats of Hydrogenation (kJ/mol)
Ethylene
136
Monosubstituted
125-126
cis-Disubstituted
117-119
trans-Disubstituted
114-115
Terminally disubstituted
116-117
Trisubstituted
112
Tetrasubstituted
110
Dr. Wolf's CHM 201 & 202
6-16
Problem 6.2
Match each alkene of Problem 6.1 with its correct
heat of hydrogenation.
126 kJ/mol
118 kJ/mol
112 kJ/mol
Dr. Wolf's CHM 201 & 202
6-17
Problem 6.2
Match each alkene of Problem 6.1 with its correct
heat of hydrogenation.
126 kJ/mol
highest heat of
hydrogenation;
least stable isomer
118 kJ/mol
112 kJ/mol
Dr. Wolf's CHM 201 & 202
lowest heat of
hydrogenation;
most stable isomer
6-18
Stereochemistry of
Alkene Hydrogenation
Dr. Wolf's CHM 201 & 202
6-19
Two spatial (stereochemical) aspects of
alkene hydrogenation:
• syn addition of both H atoms to double bond
• hydrogenation is stereoselective,
corresponding to addition to less crowded
face of double bond
Dr. Wolf's CHM 201 & 202
6-20
syn-Addition versus anti-Addition
syn addition
Dr. Wolf's CHM 201 & 202
anti addition
6-21
Example of syn-Addition
CO2CH3
CO2CH3
H
CO2CH3
H2, Pt
CO2CH3
H
(100%)
Dr. Wolf's CHM 201 & 202
6-22
Stereoselectivity
A reaction in which a single starting material
can give two or more stereoisomeric products
but yields one of them in greater amounts than
the other (or even to the exclusion of the other)
is said to be stereoselective.
Dr. Wolf's CHM 201 & 202
6-23
H3C
CH3
H
H3C
H3C
H
H
H3C
H
Dr. Wolf's CHM 201 & 202
Example of
stereoselective reaction
H2, cat
CH3
Both products
correspond to
syn addition
of H2.
H3C
H
H
CH3
H
CH3
6-24
H3C
CH3
H
H3C
H3C
H
H
H3C
Example of
stereoselective reaction
H2, cat
CH3
But only this
one is formed.
H
Dr. Wolf's CHM 201 & 202
6-25
H3C
CH3
H
H3C
Example of
stereoselective reaction
H2, cat
Top face of double
bond blocked by
this methyl group
H3C
H
CH3
H
H3C
H
Dr. Wolf's CHM 201 & 202
6-26
H3C
CH3
H
H3C
H3C
H
Example of
stereoselective reaction
H2, cat
CH3
H
H3C
H
Dr. Wolf's CHM 201 & 202
H2 adds to
bottom face of
double bond.
6-27
Electrophilic Addition of
Hydrogen Halides to
Alkenes
Dr. Wolf's CHM 201 & 202
6-28
General equation for electrophilic addition
C
C
Dr. Wolf's CHM 201 & 202
 –
+ E—Y
E
C
C
Y
6-29
When EY is a hydrogen halide
C
C
Dr. Wolf's CHM 201 & 202
 –
+ H—X
H
C
C
X
6-30
Example
CH2CH3
CH3CH2
C
H
HBr
C
H
CHCl3, -30°C
CH3CH2CH2CHCH2CH3
Br
(76%)
Dr. Wolf's CHM 201 & 202
6-31
Mechanism
Electrophilic addition of hydrogen halides
to alkenes proceeds by rate-determining
formation of a carbocation intermediate.
Electrons flow from the  system of the
alkene (electron rich) toward the positively
polarized proton of the hydrogen halide.
Dr. Wolf's CHM 201 & 202
6-32
Mechanism
+C
C
H
.. –
:
:X
..
H
C
..
X:
..
C
Dr. Wolf's CHM 201 & 202
6-33
Mechanism
+C
C
H
.. –
:
:X
..
H
C
C
Dr. Wolf's CHM 201 & 202
..
X:
..
..
:X
..
C
C
H
6-34
Regioselectivity of
Hydrogen Halide Addition:
Markovnikov's Rule
Dr. Wolf's CHM 201 & 202
6-35
Markovnikov's Rule
When an unsymmetrically substituted
alkene reacts with a hydrogen halide,
the hydrogen adds to the carbon that
has the greater number of hydrogen
substituents, and the halogen adds to
the carbon that has the fewer hydrogen
substituents.
Dr. Wolf's CHM 201 & 202
6-36
Markovnikov's Rule
CH3CH2CH
CH2
HBr
acetic acid
CH3CH2CHCH3
Br
(80%)
Example 1
Dr. Wolf's CHM 201 & 202
6-37
Markovnikov's Rule
CH3
CH3
H
C
HBr
C
CH3
H
acetic acid
CH3
C
CH3
Br
(90%)
Example 2
Dr. Wolf's CHM 201 & 202
6-38
Markovnikov's Rule
CH3
HCl
CH3
0°C
Cl
(100%)
Example 3
Dr. Wolf's CHM 201 & 202
6-39
Mechanistic Basis
for
Markovnikov's Rule
Protonation of double bond occurs in
direction that gives more stable of two
possible carbocations.
Dr. Wolf's CHM 201 & 202
6-40
Mechanistic Basis for Markovnikov's Rule:
Example 1
CH3CH2CH
CH2
HBr
CH3CH2CHCH3
acetic acid
Br
Dr. Wolf's CHM 201 & 202
6-41
Mechanistic Basis for Markovnikov's Rule:
Example 1
+
CH3CH2CH—CH3 + Br –
HBr
CH3CH2CH
CH2
CH3CH2CHCH3
Br
Dr. Wolf's CHM 201 & 202
6-42
Mechanistic Basis for Markovnikov's Rule:
Example 1
+
CH3CH2CH2—CH2
primary carbocation is less stable: not formed
+
CH3CH2CH—CH3 + Br –
HBr
CH3CH2CH
CH2
CH3CH2CHCH3
Br
Dr. Wolf's CHM 201 & 202
6-43
Mechanistic Basis for
Markovnikov's Rule:
Example 3
H
CH3
Dr. Wolf's CHM 201 & 202
HCl
CH3
0°C
Cl
6-44
Mechanistic Basis for
Markovnikov's Rule:
Example 3
H
H
+
H
Cl –
HCl
CH3
Dr. Wolf's CHM 201 & 202
CH3
HCl
CH3
0°C
Cl
6-45
Mechanistic Basis for
Markovnikov's Rule:
Example 3
secondary carbocation
is less stable:
not formed
H
+
H
H
CH3
+
H
H
Cl –
HCl
CH3
Dr. Wolf's CHM 201 & 202
CH3
HCl
CH3
0°C
Cl
6-46
Carbocation Rearrangements in
Hydrogen Halide Addition
to Alkenes
Dr. Wolf's CHM 201 & 202
6-47
Rearrangements sometimes occur
H2C
CHCH(CH3)2
HCl, 0°C
H
+
CH3CHCH(CH3)2
+
CH3CHC(CH3)2
CH3CHCH(CH3)2
CH3CH2C(CH3)2
Cl
Dr. Wolf's CHM 201 & 202
(40%)
(60%)
Cl
6-48
Addition of Sulfuric Acid
to Alkenes
Dr. Wolf's CHM 201 & 202
6-49
Addition of H2SO4
CH3CH
CH2
HOSO2OH
CH3CHCH3
OSO2OH
Isopropyl
hydrogen sulfate
follows Markovnikov's rule:
yields an alkyl hydrogen sulfate
Dr. Wolf's CHM 201 & 202
6-50
Mechanism
CH3CH
CH2 + H
..
O
..
SO2OH
slow
+
CH3CH
CH3
..–
+ :O
..
fast
Dr. Wolf's CHM 201 & 202
SO2OH
CH3CHCH3
: OSO
.. 2OH
6-51
Alkyl hydrogen sulfates undergo
hydrolysis in hot water
CH3CHCH3
O
+
H—OH
SO2OH
heat
CH3CHCH3
O
Dr. Wolf's CHM 201 & 202
+
HO—SO2OH
H
6-52
Application: Coversion of alkenes to alcohols
OH
1. H2SO4
2. H2O, heat
(75%)
Dr. Wolf's CHM 201 & 202
6-53
But...
not all alkenes yield alkyl hydrogen sulfates
on reaction with sulfuric acid
these do:
H2C=CH2, RCH=CH2, and RCH=CHR'
these don't:
R2C=CH2, R2C=CHR, and R2C=CR2
(These form polymers, sec 6.21)
Dr. Wolf's CHM 201 & 202
6-54
Acid-Catalyzed Hydration of Alkenes
Dr. Wolf's CHM 201 & 202
6-55
Acid-Catalyzed Hydration of Alkenes
C
+
C
H—OH
reaction is acid
catalyzed; typical
hydration medium is
50% H2SO4-50% H2O
H
Dr. Wolf's CHM 201 & 202
C
C
OH
6-56
Follows Markovnikov's Rule
H3C
CH3
H
C
50% H2SO4
C
H3C
CH3
50% H2O
CH3
C
CH2CH3
OH
(90%)
Dr. Wolf's CHM 201 & 202
6-57
Follows Markovnikov's Rule
CH2
50% H2SO4
CH3
50% H2O
OH
(80%)
Dr. Wolf's CHM 201 & 202
6-58
Mechanism
involves a carbocation intermediate
is the reverse of acid-catalyzed dehydration
of alcohols to alkenes
CH3
H3C
H+
C
CH2 + H2O
H3C
Dr. Wolf's CHM 201 & 202
CH3
C
CH3
OH
6-59
Mechanism
Step (1) Protonation of double bond
H
H3C
C
CH2
+
+
O:
H
H3C
H
slow
H3C
H3C
Dr. Wolf's CHM 201 & 202
H
+
C
CH3 +
: O:
H
6-60
Mechanism
Step (2) Capture of carbocation by water
H3C
H
+
C
H3C
fast
CH3
CH3
C
CH3
Dr. Wolf's CHM 201 & 202
: O:
CH3 +
H
H
+
O:
H
6-61
Mechanism
Step (3) Deprotonation of oxonium ion
CH3
CH3
C
H
:
+O
CH3
H
+
: O:
H
H
fast
CH3
CH3
Dr. Wolf's CHM 201 & 202
C
CH3
..
O:
H
H
+
H
+
O:
H
6-62
Relative Rates
Acid-catalyzed hydration
ethylene
CH2=CH2
1.0
propene
CH3CH=CH2
1.6 x 106
2-methylpropene
(CH3)2C=CH2
2.5 x 1011
The more stable the carbocation, the faster
it is formed, and the faster the reaction rate.
Dr. Wolf's CHM 201 & 202
6-63
Principle of microscopic reversibility
CH3
H3C
H+
C
CH2 + H2O
H3C
CH3
C
CH3
OH
In an equilibrium process, the same intermediates
and transition states are encountered in the
forward direction and the reverse, but in the
opposite order.
Dr. Wolf's CHM 201 & 202
6-64
Thermodynamics of Addition-Elimination
Equilibria
Dr. Wolf's CHM 201 & 202
6-65
Hydration-Dehydration
Equilibrium
H
C
H
H+
H
+ H2O
C
H
C
C
OH
H
How do we control the position of the
equilibrium and maximize the product?
Dr. Wolf's CHM 201 & 202
6-66
Le Chatelier’s
Principle
A system at equilibrium adjusts so to
minimize any stress applies to it.
For the hydration-dehydration equilibria, the
key stress is water.
Adding water pushes the equilibrium toward
more product (alcohol).
Removing water pushes the equilibrium
toward more reactant (alkene).
Dr. Wolf's CHM 201 & 202
6-67
Hydroboration-Oxidation of Alkenes
Dr. Wolf's CHM 201 & 202
6-68
Synthesis
Suppose you wanted to prepare 1-decanol
from 1-decene?
OH
Dr. Wolf's CHM 201 & 202
6-69
Synthesis
Suppose you wanted to prepare 1-decanol
from 1-decene?
Needed: a method for hydration of
alkenes with a regioselectivity opposite to
Markovnikov's rule.
OH
Dr. Wolf's CHM 201 & 202
6-70
Synthesis
Two-step reaction sequence called hydroborationoxidation converts alkenes to alcohols with a
regiochemistry opposite to Markovnikov's rule.
1. hydroboration
2. oxidation
OH
Dr. Wolf's CHM 201 & 202
6-71
Hydroboration step
C
C
+
H—BH2
H
C
C
BH2
Hydroboration can be viewed as the addition of
borane (BH3) to the double bond. But BH3 is
not the reagent actually used.
Dr. Wolf's CHM 201 & 202
6-72
Hydroboration step
C
+
C
H—BH2
H
C
C
BH2
Hydroboration reagents:
H
BH2
H2B
H
Dr. Wolf's CHM 201 & 202
Diborane (B2H6)
normally used in an
ether- like solvent
called "diglyme"
6-73
Hydroboration step
C
+
C
H—BH2
H
C
C
BH2
Hydroboration reagents:
Borane-tetrahydrofuran
complex (H3B-THF)
..
+O
– BH
3
Dr. Wolf's CHM 201 & 202
6-74
Oxidation step
H2O2, HO–
H
C
C
BH2
H
C
C
OH
Organoborane formed in the hydroboration
step is oxidized with hydrogen peroxide.
Dr. Wolf's CHM 201 & 202
6-75
Example
1. B2H6, diglyme
2. H2O2, HO–
OH
(93%)
Dr. Wolf's CHM 201 & 202
6-76
Example
H3C
CH3
C
C
H3C
H
1. H3B-THF
2. H2O2, HO–
CH3
H
OH
C
C
CH3
CH3 H
(98%)
Dr. Wolf's CHM 201 & 202
6-77
Example
OH
1. B2H6, diglyme
2. H2O2, HO–
(no rearrangement)
(82%)
Dr. Wolf's CHM 201 & 202
6-78
Stereochemistry of
Hydroboration-Oxidation
Dr. Wolf's CHM 201 & 202
6-79
Features of HydroborationOxidation
• hydration of alkenes
• regioselectivity opposite to Markovnikov's rule
• no rearrangement
• stereospecific syn addition
Dr. Wolf's CHM 201 & 202
6-80
syn-Addition
•H and OH become attached to same
face of double bond
H
CH3
1. B2H6
2. H2O2, NaOH
H
CH3
HO
H
only product is trans-2-methylcyclopentanol
(86%) yield
Dr. Wolf's CHM 201 & 202
6-81
Mechanism of
Hydroboration-Oxidation
Dr. Wolf's CHM 201 & 202
6-82
Hydroboration-oxidation
 electrons from alkene flow into 2P orbital of boron
Dr. Wolf's CHM 201 & 202
6-83
Hydroboration-oxidation
H atom with 2 electrons migrates to C with
greatest (+) charge (most highly substituted). Syn add.
Dr. Wolf's CHM 201 & 202
6-84
Hydroboration-oxidation
Anion of hydrogen peroxide attacks boron
Dr. Wolf's CHM 201 & 202
6-85
Hydroboration-oxidation
Carbon with pair of electrons migrates to oxygen,
displacing hydroxide. Oxygen on same side as boron
Dr. Wolf's CHM 201 & 202
6-86
Hydroboration-oxidation
Hydrolysis cleaves boron-oxygen bond resulting
in anti-Markovnikov addition
Dr. Wolf's CHM 201 & 202
6-87
1-Methylcyclopentene + BH3
•syn addition of H
and B to double
bond
•B adds to less
substituted carbon
Dr. Wolf's CHM 201 & 202
6-88
Organoborane
intermediate
Dr. Wolf's CHM 201 & 202
6-89
Add hydrogen
peroxide
•OH replaces B on
same side
Dr. Wolf's CHM 201 & 202
6-90
trans-2Methylcyclopentanol
Dr. Wolf's CHM 201 & 202
6-91
Addition of Halogens
to Alkenes
Dr. Wolf's CHM 201 & 202
6-92
C
C
+ X2
X
C
C
X
electrophilic addition to double bond
forms a vicinal dihalide
Dr. Wolf's CHM 201 & 202
6-93
Example
CH3CH
CHCH(CH3)2
Br2
CHCl3
0°C
CH3CHCHCH(CH3)2
Br Br
(100%)
Dr. Wolf's CHM 201 & 202
6-94
Scope
Limited to Cl2 and Br2
F2 addition proceeds with explosive violence
I2 addition is endothermic:
vicinal diiodides dissociate to an alkene and I2
Dr. Wolf's CHM 201 & 202
6-95
Stereochemistry of Halogen Addition
Anti-addition
Dr. Wolf's CHM 201 & 202
6-96
Example
H
H
H
Br2
Br
Br
H
trans-1,2-Dibromocyclopentane
80% yield; only product
Dr. Wolf's CHM 201 & 202
6-97
Example
H
H
Cl
Cl2
H
H
Cl
trans-1,2-Dichlorocyclooctane
73% yield; only product
Dr. Wolf's CHM 201 & 202
6-98
Mechanism of Halogen Addition
to Alkenes:
Halonium Ions
Dr. Wolf's CHM 201 & 202
6-99
Mechanism is electrophilic addition
Br2 is not polar, but it is polarizable
two steps involved
(1) formation of bromonium ion
(2) nucleophilic attack on
bromonium ion by bromide
Dr. Wolf's CHM 201 & 202
6-100
Relative Rates
Bromination
ethylene
H2C=CH2
1
propene
CH3CH=CH2
2-methylpropene
(CH3)2C=CH2
2,3-dimethyl-2-butene
(CH3)2C=C(CH3)2 920,000
61
5400
More highly substituted double bonds react faster.
Alkyl groups on the double bond make it
more “electron rich.”
Dr. Wolf's CHM 201 & 202
6-101
Mechanism?
H2C
CH2 +
Br2
BrCH2CH2Br
?
C
: Br :
..
Dr. Wolf's CHM 201 & 202
+
C
.. –
+ : Br :
..
No obvious explanation for
anti addition provided by
this mechanism.
6-102
Mechanism
H2C
CH2 +
Br2
C
C
: Br :
+
Dr. Wolf's CHM 201 & 202
BrCH2CH2Br
.. –
+ : Br :
..
Cyclic bromonium ion
6-103
Formation of bromonium ion
Br
Mutual polarization
of electron distributions
of Br2 and alkene
Dr. Wolf's CHM 201 & 202
Br
6-104
Formation of bromonium ion
Electrons flow
from alkene toward Br2
Br
–
Br
+
Dr. Wolf's CHM 201 & 202
+
6-105
Formation of bromonium ion
–
Br
 electrons of alkene
displace Br– from Br
+
Br
Dr. Wolf's CHM 201 & 202
6-106
Stereochemistry
.. –
: Br :
..
..
Br +
..
..
: Br
..
attack of Br– from side opposite
C—Br bond of bromonium ion gives
anti addition
Dr. Wolf's CHM 201 & 202
..
Br :
..
6-107
Example
H
H
H
Br2
Br
Br
H
trans-1,2-Dibromocyclopentane
80% yield; only product
Dr. Wolf's CHM 201 & 202
6-108
Cyclopenten
e +Br2
Dr. Wolf's CHM 201 & 202
6-109
Bromonium
ion
–
Dr. Wolf's CHM 201 & 202
6-110
–
–
Dr. Wolf's CHM 201 & 202
Bromide ion attacks
the bromonium ion
from side opposite
carbon-bromine bond
6-111
trans-Stereochemistry in
vicinal dibromide
Dr. Wolf's CHM 201 & 202
6-112
Conversion of Alkenes
to Vicinal Halohydrins
Dr. Wolf's CHM 201 & 202
6-113
C
C
+ X2
X
C
C
X
alkenes react with X2 to form vicinal dihalides
Dr. Wolf's CHM 201 & 202
6-114
C
C
+ X2
X
C
C
X
alkenes react with X2 to form vicinal dihalides
alkenes react with X2 in water to give vicinal
halohydrins
C
C
+ X2 + H2O
X
C
C
OH
+ H—X
Dr. Wolf's CHM 201 & 202
6-115
Examples
H2O
H2C
CH2 +
Br2
BrCH2CH2OH
(70%)
H
H
Cl2
OH
H2O
H
Cl
H
anti addition: only product
Dr. Wolf's CHM 201 & 202
6-116
Mechanism
O:
..
..
Br +
..
+
• bromonium ion is
intermediate
• water is nucleophile that
attacks bromonium ion
Dr. Wolf's CHM 201 & 202
O
..
..
Br :
..
6-117
Examples
H2O
H2C
CH2 +
Br2
BrCH2CH2OH
(70%)
H
H
Cl2
OH
H2O
H
Cl
H
anti addition: only product
Dr. Wolf's CHM 201 & 202
6-118
Cyclopentene
+ Cl2
Dr. Wolf's CHM 201 & 202
6-119
Chloronium
ion
Dr. Wolf's CHM 201 & 202
6-120
Water attacks
chloronium ion
from side
opposite
carbon-chlorine
bond
Dr. Wolf's CHM 201 & 202
6-121
transStereochemistry
in oxonium ion
Dr. Wolf's CHM 201 & 202
6-122
trans-2-Chlorocyclopentanol
Dr. Wolf's CHM 201 & 202
6-123
Regioselectivity
CH3
H3C
C
H3C
CH2
Br2
H2O
CH3
C
CH2Br
OH
(77%)
Markovnikov's rule applied to
halohydrin formation: the halogen
adds to the carbon having the
greater number of hydrogens.
Dr. Wolf's CHM 201 & 202
6-124
Explanation
H
H
H
..
 O
..
O 
H3C
H3C

C
CH2
: Br :

H3C
H3C
H

CH2
C
: Br :

transition state for attack of water on bromonium ion has
carbocation character; more stable transition state (left)
has positive charge on more highly substituted carbon
Dr. Wolf's CHM 201 & 202
6-125
Free-radical Addition
of HBr to Alkenes
The "peroxide effect"
Dr. Wolf's CHM 201 & 202
6-126
Markovnikov's Rule
CH3CH2CH
CH2
HBr
acetic acid
CH3CH2CHCH3
Br
(80%)
Example 1
Dr. Wolf's CHM 201 & 202
6-127
Addition of HBr to 1-Butene
CH3CH2CH
CH2
HBr
CH3CH2CHCH3
Br
only product in
absence of peroxides
Dr. Wolf's CHM 201 & 202
CH3CH2CH2CH2Br
only product when
peroxides added to
reaction mixture
6-128
Addition of HBr to 1-Butene
CH3CH2CH
CH2
HBr
addition opposite to
Markovnikov's rule
occurs with HBr (not
HCl or HI)
Dr. Wolf's CHM 201 & 202
CH3CH2CH2CH2Br
only product when
peroxides added to
reaction mixture
6-129
CH2 + HBr
h
CH2Br
H
(60%)
Addition of HBr with a regiochemistry opposite
to Markovnikov's rule can also occur when
initiated with light with or without added peroxides.
Dr. Wolf's CHM 201 & 202
6-130
Mechanism
Addition of HBr opposite to Markovnikov's rule
proceeds by a free-radical chain mechanism.
Initiation steps:
R
..
O
..
Dr. Wolf's CHM 201 & 202
..
O
..
R
R
..
..
. + .O
O
..
..
R
6-131
Mechanism
Addition of HBr opposite to Markovnikov's rule
proceeds by a free-radical chain mechanism.
Initiation steps:
R
R
..
O
..
..
O
..
..
. + H
O
..
Dr. Wolf's CHM 201 & 202
R
..
:
Br
..
R
R
..
..
. + .O
O
..
..
R
..
..
.
+
:
H
O
Br
..
..
6-132
Propagation steps:
..
CH2 + . Br :
..
CH3CH2CH
.
CH3CH2CH
CH2
..
:
Br
..
Gives most stable free radical
CH3CH2CH
.
H
Dr. Wolf's CHM 201 & 202
CH2
..
:
Br
..
..
Br
.. :
..
:
CH3CH2CH2CH2 Br
..
+
..
. Br :
..
6-133
Epoxidation of Alkenes
Dr. Wolf's CHM 201 & 202
6-134
Epoxides
• are examples of heterocyclic compounds
• three-membered rings that contain oxygen
ethylene oxide
CH2
H2C
O
Dr. Wolf's CHM 201 & 202
propylene oxide
CHCH3
H2C
O
6-135
Epoxide Nomenclature
Substitutive nomenclature:
•named as epoxy-substituted alkanes.
•“epoxy” precedes name of alkane
1,2-epoxypropane
2-methyl-2,3-epoxybutane
H3C 1
2
O
Dr. Wolf's CHM 201 & 202
H3C
4
CHCH3
C
CHCH3
H2C
3
O
6-136
Problem 6.17 Give the IUPAC name, including
stereochemistry, for disparlure.
H O H
cis-2-Methyl-7,8-epoxyoctadecane
Dr. Wolf's CHM 201 & 202
6-137
Epoxidation of Alkenes
O
C
C
+
RCOOH
peroxy acid
O
C
C
+
RCOH
O
Dr. Wolf's CHM 201 & 202
6-138
Example
O
+ CH3COOH
O
+ CH3COH
O
(52%)
Dr. Wolf's CHM 201 & 202
6-139
Epoxidation of Alkenes
O
C
C
+
RCOOH
syn addition
O
C
C
+
RCOH
O
Dr. Wolf's CHM 201 & 202
6-140
Relative Rates
Epoxidation
ethylene
H2C=CH2
1
propene
CH3CH=CH2
22
2-methylpropene
(CH3)2C=CH2
484
2-methyl-2-butene
(CH3)2C=CHCH3
6526
More highly substituted double bonds react faster.
Alkyl groups on the double bond make it
more “electron rich.”
Dr. Wolf's CHM 201 & 202
6-141
Mechanism of Epoxidation
Dr. Wolf's CHM 201 & 202
6-142
Mechanism of Epoxidation
Dr. Wolf's CHM 201 & 202
6-143
Mechanism of Epoxidation
Dr. Wolf's CHM 201 & 202
6-144
Mechanism of Epoxidation
Dr. Wolf's CHM 201 & 202
6-145
Mechanism of Epoxidation
Dr. Wolf's CHM 201 & 202
6-146
Ozonolysis of Alkenes
Ozonolyis has both synthetic and analytical
applications.
• synthesis of aldehydes and ketones
• identification of substituents on the
double bond of an alkene
Dr. Wolf's CHM 201 & 202
6-147
Ozonolysis of Alkenes
First step is the reaction of the alkene with
ozone. The product is an ozonide.
C
C
+ O3
O
C
O
Dr. Wolf's CHM 201 & 202
C
O
6-148
Ozonolysis of Alkenes
Second step is hydrolysis of the ozonide.
Two aldehydes, two ketones, or an aldehyde
and a ketone are formed.
C
C
O
+ O3
C
O
C
O
H2O, Zn
C
O
Dr. Wolf's CHM 201 & 202
+
O
C
6-149
Ozonolysis of Alkenes
As an alternative to hydrolysis, the ozonide
can be treated with dimethyl sulfide.
C
C
O
+ O3
C
O
C
O
(CH3)2S
C
O
Dr. Wolf's CHM 201 & 202
+
O
C
6-150
Example
CH2CH3
CH3
C
C
H
CH2CH3
1. O3
2. H2O, Zn
CH2CH3
CH3
C
H
Dr. Wolf's CHM 201 & 202
O
(38%)
+
O
C
(57%)
CH2CH3
6-151
Introduction to Organic
Chemical Synthesis
Dr. Wolf's CHM 201 & 202
6-152
Prepare cyclohexane from cyclohexanol
OH
devise a synthetic plan
reason backward from the target molecule
always use reactions that you are sure will
work
Dr. Wolf's CHM 201 & 202
6-153
Prepare cyclohexane from cyclohexanol
OH
ask yourself the key question
"Starting with anything, how can I make
cyclohexane in a single step by a reaction I
am sure will work?"
Dr. Wolf's CHM 201 & 202
6-154
Prepare cyclohexane from cyclohexanol
OH
H2
Pt
The only reaction covered so far for preparing
alkanes is catalytic hydrogenation of alkenes.
This leads to a new question. "Starting with
anything, how can I prepare cyclohexene in a
single step by a reaction I am sure will work?"
Dr. Wolf's CHM 201 & 202
6-155
Prepare cyclohexane from cyclohexanol
OH
H2SO4
H2
heat
Pt
Alkenes can be prepared by dehydration of
alcohols.
The synthesis is complete.
Dr. Wolf's CHM 201 & 202
6-156
Prepare 1-bromo-2-methyl-2-propanol
from tert-butyl alcohol
(CH3)3COH
(CH3)2CCH2Br
OH
"Starting with anything, how can I make the
desired compound in a single step by a reaction
I am sure will work?"
The desired compound is a vicinal bromohydrin.
How are vicinal bromohydrins prepared?
Dr. Wolf's CHM 201 & 202
6-157
Prepare 1-bromo-2-methyl-2-propanol
from tert-butyl alcohol
(CH3)2C
CH2
Br2
H2O
(CH3)2CCH2Br
OH
Vicinal bromohydrins are prepared by treatment
of alkenes with Br2 in water.
How is the necessary alkene prepared?
Dr. Wolf's CHM 201 & 202
6-158
Prepare 1-bromo-2-methyl-2-propanol
from tert-butyl alcohol
(CH3)3COH
H2SO4
heat
(CH3)2C
CH2
Br2
H2O
(CH3)2CCH2Br
OH
2-Methylpropene is prepared from tert-butyl
alcohol by acid-catalyzed dehydration.
The synthesis is complete.
Dr. Wolf's CHM 201 & 202
6-159
Reactions of Alkenes with Alkenes:
Polymerization
Dr. Wolf's CHM 201 & 202
6-160
Polymerization of alkenes
cationic polymerization
free-radical polymerization
coordination polymerization
Dr. Wolf's CHM 201 & 202
6-161
Cationic Polymerization
Dimerization of 2-methylpropene
monomer
(C4H8)
(CH3)2C
CH2
H2SO4
two dimers
(C8H16)
CH3
CH3CCH
Dr. Wolf's CHM 201 & 202
CH3
CH3
C(CH3)2 + CH3CCH2C
CH3 CH3
CH2
6-162
Mechanism of Cationic Polymerization
CH3
H2C
CH3C +
C
CH3
Dr. Wolf's CHM 201 & 202
H+
CH3
CH3
6-163
Mechanism of Cationic Polymerization
CH3
CH3C +
CH3
+
H2C
C
CH3
CH3
CH3
+
CH3CCH2C
Dr. Wolf's CHM 201 & 202
CH3 CH3
CH3
6-164
Mechanism of Cationic Polymerization
CH3
CH3CCH
CH3
C(CH3)2 + CH3CCH2C
CH3
CH2
CH3 CH3
CH3
+
CH3CCH2C
Dr. Wolf's CHM 201 & 202
CH3 CH3
CH3
6-165
Free-Radical Polymerization of Ethylene
H2C
CH2
200 °C
2000 atm
CH2
CH2
CH2
CH2
O2
peroxides
CH2
CH2
CH2
polyethylene
Dr. Wolf's CHM 201 & 202
6-166
..
•
RO
..
H2C
Dr. Wolf's CHM 201 & 202
Mechanism
CH2
6-167
..
RO:
H2C
Dr. Wolf's CHM 201 & 202
Mechanism
CH2
•
6-168
..
RO:
Mechanism
H2C
CH2
•
H2C
Dr. Wolf's CHM 201 & 202
CH2
6-169
..
RO:
H2C
Mechanism
CH2
H2C
Dr. Wolf's CHM 201 & 202
CH2
•
6-170
..
RO:
H2C
Mechanism
CH2
H2C
CH2
•
H2C
Dr. Wolf's CHM 201 & 202
CH2
6-171
..
RO:
H2C
Mechanism
CH2
H2C
CH2
H2C
Dr. Wolf's CHM 201 & 202
CH2
•
6-172
..
RO:
H2C
Mechanism
CH2
H2C
CH2
H2C
CH2
•
H2C
Dr. Wolf's CHM 201 & 202
CH2
6-173
Free-Radical Polymerization of Propene
H2C
CHCH3
CH
CH
CH
CH
CH
CH
CH
H
CH3
H
CH3
H
CH3
H
polypropylene
Dr. Wolf's CHM 201 & 202
6-174
..
•
RO
..
H2C
Dr. Wolf's CHM 201 & 202
Mechanism
CHCH3
6-175
..
RO:
H2C
Dr. Wolf's CHM 201 & 202
Mechanism
CHCH3
•
6-176
..
RO:
Mechanism
H2C
CHCH3
•
H2C
Dr. Wolf's CHM 201 & 202
CHCH3
6-177
..
RO:
H2C
Mechanism
CHCH3
H2C
Dr. Wolf's CHM 201 & 202
CHCH3
•
6-178
..
RO:
H2C
Mechanism
CHCH3
H2C
CHCH3
•
H2C
Dr. Wolf's CHM 201 & 202
CHCH3
6-179
..
RO:
H2C
Mechanism
CHCH3
H2C
CHCH3
H2C
Dr. Wolf's CHM 201 & 202
CHCH3
•
6-180
..
RO:
H2C
Mechanism
CHCH3
H2C
CHCH3
H2C
CHCH3
•
H2C
Dr. Wolf's CHM 201 & 202
CHCH3
6-181
H2C=CHCl
 polyvinyl chloride
H2C=CHC6H5  polystyrene
F2C=CF2
 Teflon
Dr. Wolf's CHM 201 & 202
6-182
End of Chapter 6
Dr. Wolf's CHM 201 & 202
6-183