Chapter 6 Reactions of Alkenes: Addition Reactions

1
Chapter 6Reactions of AlkenesAddition Reactions
2
6.1Hydrogenation of Alkenes
3
Reactions of Alkenes

• The characteristic reaction of alkenes is
  addition to the double bond.

AB
4
Hydrogenation of Ethylene
?
?
?
?
HH

• exothermic ?H 136 kJ/mol
• catalyzed by finely divided Pt, Pd, Rh, Ni

5
Example
H2, Pt
(73)
6
Problem 6.1

• What three alkenes yield 2-methylbutane on
  catalytic hydrogenation?

7
6.2Heats of Hydrogenation

• can be used to measure relative stability of
  isomeric alkenes
• correlation with structure is same as when heats
  of combustion are measured

8
Mechanism of Catalytic Hydrogenation Figure 6.1
9
Mechanism of Catalytic Hydrogenation Figure 6.1
B
Y
C
C
A
X
10
Mechanism of Catalytic Hydrogenation Figure 6.1
11
Mechanism of Catalytic Hydrogenation Figure 6.1
B
Y
A
X
C
C
12
Mechanism of Catalytic Hydrogenation Figure 6.1
13
Mechanism of Catalytic Hydrogenation Figure 6.1
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Heats of Hydrogenation of Isomers
126
119
115
CH3CH2CH2CH3
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

16
Problem 6.2
Match each alkene of Problem 6.1 with its
correctheat of hydrogenation.
17
6.3Stereochemistry of Alkene Hydrogenation
18
Two spatial (stereochemical) aspects ofalkene
hydrogenation

• (1) syn addition of both H atoms to double bond
• (2) hydrogenation is stereoselective,
  corresponding to addition to less crowded face of
  double bond

19
Two spatial (stereochemical) aspects ofalkene
hydrogenation

• (1) syn addition of both H atoms to double bond

20
syn-Additon versus anti-Addition
syn addition
anti addition
21
Example of Syn Addition
CO2CH3
CO2CH3
(100)
22
Two spatial (stereochemical) aspects ofalkene
hydrogenation

• (1) syn addition of both H atoms to double bond
• (2) hydrogenation is stereoselective,
  corresponding to addition to less crowded face of
  double bond

23
Two spatial (stereochemical) aspects ofalkene
hydrogenation

• (1) syn addition of both H atoms to double bond
• (2) hydrogenation is stereoselective,
  corresponding to addition to less crowded face of
  double bond

A reaction in which a single starting
materialcan give two or more stereoisomeric
productsbut yields one of them in greater
amounts thanthe other (or even to the exclusion
of the other)is said to be stereoselective.
24
Example of Stereoselective Reaction
H2, cat
Both productscorrespond tosyn additionof H2.
25
Example of Stereoselective Reaction
H2, cat
But only thisone is formed.
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Example of Stereoselective Reaction
H2, cat
Top face of doublebond blocked bythis methyl
group
27
Example of Stereoselective Reaction
H2, cat
H2 adds to bottom face of double bond.
28
6.4Electrophilic Addition of Hydrogen Halides
to Alkenes
29
General equation for electrophilic addition
??
?
EY
30
When E is a hydrogen halide
??
?
HX
31
Example
CH3CH2
CH2CH3
HBr
CHCl3, -30C
H
H
32
Mechanism

• Electrophilic addition of hydrogen halides to
  alkenes proceeds by rate-determining formation
  of a carbocation intermediate.

33
Mechanism

• Electrons flow fromthe ? system of thealkene
  (electron rich) toward the positivelypolarized
  proton of the hydrogen halide.

34
Mechanism

X
H
35
6.5Regioselectivity of Hydrogen Halide
AdditionMarkovnikov's Rule
36
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.

37
Markovnikov's Rule
HBr
acetic acid
(80)
Example 1
38
Markovnikov's Rule
CH3
H
HBr
acetic acid
CH3
H
(90)
Example 2
39
Markovnikov's Rule
HCl
0C
(100)
Example 3
40
6.6Mechanistic Basis for Markovnikov's Rule

• Protonation of double bond occurs in direction
  that gives more stable of two possible
  carbocations.

41
Mechanistic Basis for Markovnikov's RuleExample
1
42
Mechanistic Basis for Markovnikov's RuleExample
1
HBr
43
Mechanistic Basis for Markovnikov's RuleExample
3
HCl
0C
44
Cl
HCl
45
6.7Carbocation Rearrangements in Hydrogen Halide
Addition to Alkenes
46
Rearrangements sometimes occur
HCl, 0C
47
6.8Free-radical Addition of HBr to Alkenes

• The "peroxide effect"

48
Markovnikov's Rule
HBr
acetic acid
(80)
49
Addition of HBr to 1-Butene
HBr
CH3CH2CH2CH2Br
50
Addition of HBr to 1-Butene
HBr
addition opposite to Markovnikov's rule occurs
with HBr (not HCl or HI)
CH3CH2CH2CH2Br
only product when peroxides added to reaction
mixture
51
Photochemical Addition of HBr
h?
HBr
(60)

• Addition of HBr with a regiochemistry oppositeto
  Markovnikov's rule can also occur wheninitiated
  with light with or without added peroxides.

52
Mechanism

• Addition of HBr opposite to Markovnikov's rule
  proceeds by a free-radical chain mechanism.
• Initiation steps

53
Mechanism
Propagation steps
54
6.9Addition of Sulfuric Acid to Alkenes
55
Addition of H2SO4
HOSO2OH
Isopropylhydrogen sulfate

• follows Markovnikov's rule
• yields an alkyl hydrogen sulfate

56
Mechanism
..

SO2OH
O
H
..
57
Alkyl hydrogen sulfates undergo hydrolysis in hot
water
HOH

58
Application Conversion of alkenes to alcohols
1. H2SO4
2. H2O, heat
(75)
59
But...

• not all alkenes yield alkyl hydrogen sulfateson
  reaction with sulfuric acid
• these do H2CCH2, RCHCH2, and RCHCHR'
• these don't R2CCH2, R2CCHR, and R2CCR2

60
6.10Acid-Catalyzed Hydration of Alkenes
61
Acid-Catalyzed Hydration of Alkenes

HOH

• reaction is acid catalyzed typical hydration
  medium is 50 H2SO4-50 H2O

62
Follows Markovnikov's Rule
(90)
63
Follows Markovnikov's Rule
(80)
64
Mechanism

• involves a carbocation intermediate
• is the reverse of acid-catalyzed dehydrationof
  alcohols to alkenes

H
H2O
65
Mechanism
Step (1) Protonation of double bond

66
Mechanism
Step (2) Capture of carbocation by water
67
Mechanism
Step (3) Deprotonation of oxonium ion


68
Relative Rates
Acid-catalyzed hydration

• ethylene CH2CH2 1.0
• propene CH3CHCH2 1.6 x 106
• 2-methylpropene (CH3)2CCH2 2.5 x 1011
• The more stable the carbocation, the fasterit
  is formed, and the faster the reaction rate.

69
Principle of Microscopic Reversibility
H
H2O

• In an equilibrium process, the same intermediates
  and transition states are encountered in the
  forward direction and the reverse, but in the
  opposite order.

70
6.11Thermodynamics of Addition-Elimination
Equlibria
71
Hydration-Dehydration Equilibrium
How do we control the position of the equilibrium
and maximize the product?
72
Le Chateliers 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).
73
Le Chateliers Principle
At constant temperature and pressure a reaction
proceeds in a direction which is spontaneous or
decreases free energy (G). The sign of G is
always positive, but ?G can be positive or
negative. ?G Gproducts Greactants Spontaneo
us when ?G lt 0
74
Le Chateliers Principle
For a reversible reaction aA bB
cC dD The relationship between ?G and
?Go is
R 8.314 J/(mol.K) and T is the temperature in K
75
Le Chateliers Principle
At equilibrium ?G 0 and the following becomes
true
Substituting Keq into the previous equation
gives ?Go - RT lnKeq Reactions
for ?Go positive are endergonic and for ?Go
negative are exergonic.
76
6.12Hydroboration-Oxidation of Alkenes
77
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.

78
Synthesis
Two-step reaction sequence called
hydroboration-oxidation converts alkenes to
alcohols with a regiochemistry opposite to
Markovnikov's rule.
79
Hydroboration Step
HBH2
Hydroboration can be viewed as the addition
ofborane (BH3) to the double bond. But BH3 is
not the reagent actually used.
80
Hydroboration Step
HBH2
Hydroboration reagents
H
Diborane (B2H6)normally used in an ether- like
solventcalled "diglyme"
BH2
H2B
H
81
Hydroboration Step
HBH2
Hydroboration reagents
Borane-tetrahydrofurancomplex (H3B-THF)
82
Oxidation Step
H2O2, HO
Organoborane formed in the hydroborationstep is
oxidized with hydrogen peroxide.
83
Example
(93)
84
Example
H3C
CH3
H3C
H
(98)
85
Example
1. B2H6, diglyme
2. H2O2, HO
(82)
86
6.13Stereochemistry of Hydroboration-Oxidation
87
Features of Hydroboration-Oxidation

• hydration of alkenes
• regioselectivity opposite to Markovnikov's rule
• no rearrangement
• stereospecific syn addition

88
syn-Addition

• H and OH become attached to same face of double
  bond

1. B2H6
2. H2O2, NaOH
only product is trans-2-methylcyclopentanol(86)
yield
89
6.14Mechanism of Hydroboration-Oxidation
90
1-Methylcyclopentene BH3

• syn addition of H and B to double bond
• B adds to less substituted carbon

91
Organoborane intermediate
92
Add hydrogen peroxide

• OH replaces B on same side

93
trans-2-Methylcyclopentanol
94
6.15Addition of Halogensto Alkenes
95
General features
X2
X
X

• electrophilic addition to double bond
• forms a vicinal dihalide

96
Example
Br2
CH3CHCHCH(CH3)2
CH3CH
CHCH(CH3)2
CHCl3 0C
Br
Br
(100)
97
Scope
limited to Cl2 and Br2

• F2 addition proceeds with explosive violence
• I2 addition is endothermic vicinal
  diiodidesdissociate to an alkene and I2

98
6.16Stereochemistry of Halogen Addition

• anti addition

99
Example
Br2
trans-1,2-Dibromocyclopentane80 yield only
product
100
Example
H
Cl2
H
trans-1,2-Dichlorocyclooctane73 yield only
product
101
6.17Mechanism of Halogen Addition to Alkenes
Halonium Ions
102
Mechanism is electrophilic addition

• Br2 is not polar, but it is polarizable
• two steps (1) formation of bromonium ion (2)
  nucleophilic attack on bromonium ion by
  bromide

103
Relative Rates of Bromination

• ethylene H2CCH2 1
• propene CH3CHCH2 61
• 2-methylpropene (CH3)2CCH2 5400
• 2,3-dimethyl-2-butene (CH3)2CC(CH3)2 920,000
• More highly substituted double bonds react
  faster.Alkyl groups on the double bond make
  itmore electron rich.

104
Mechanism?

BrCH2CH2Br
Br2
H2C
CH2
?

• No obvious explanation for anti addition provided
  by this mechanism.

105
Mechanism

BrCH2CH2Br
Br2
H2C
CH2
..


Br
C
C

• Cyclic bromonium ion

106
Formation of Bromonium Ion
Br
Mutual polarizationof electron distributionsof
Br2 and alkene
Br
107
Formation of Bromonium Ion
?
Electrons flow from alkenetoward Br2
Br
Br
?
?
108
Formation of Bromonium Ion

Br
? electrons ofalkene displaceBr from Br
109
Stereochemistry

Br

..


Br
..
attack of Br from side oppositeCBr bond of
bromonium ion givesanti addition
..

Br
..
110
Example
Br2
trans-1,2-Dibromocyclopentane80 yield only
product
111
Cyclopentene Br2
112
Bromonium ion

113
Bromide ion attacks the bromonium ion from side
opposite carbon-bromine bond
114
trans-Stereochemistry in vicinal dibromide
115
6.18Conversion of Alkenes to Vicinal Halohydrins
116
X2
X
X
alkenes react with X2 to form vicinal dihalides
117
X2
X
X
alkenes react with X2 to form vicinal dihalides
alkenes react with X2 in water to give vicinal
halohydrins
H2O
X2
X
OH
HX
118
Examples
H2O

H2C
BrCH2CH2OH
Br2
CH2
(70)
Cl2
H2O
anti addition only product
119
Mechanism

• bromonium ion is intermediate
• water is nucleophile that attacks bromonium ion

..

Br
..
120
Examples
H2O

H2C
BrCH2CH2OH
Br2
CH2
(70)
Cl2
H2O
anti addition only product
121
Cyclopentene Cl2
122
Chloronium ion
123
Water attacks chloronium ion from side opposite
carbon-chlorine bond
124
trans-Stereochemistry in oxonium ion
125
trans-2-Chlorocyclopentanol
126
Regioselectivity
(77)

• Markovnikov's rule applied to halohydrin
  formation the halogen adds to the carbon having
  the greater number of hydrogens.

127
Explanation
H3C
H3C
??
??
H3C
H3C
CH2
CH2
C
C






Br
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

128
6.19Epoxidation of Alkenes
129
Epoxides

• are examples of heterocyclic compounds
• three-membered rings that contain oxygen
• ethylene oxide propylene oxide

130
Epoxide Nomenclature

• Substitutive nomenclature named as
  epoxy-substituted alkanes.
• epoxy precedes name of alkane
• 1,2-epoxypropane 2,3-epoxy-2-methylbutane

1
4
2
3
131
Problem 6.21 Give the IUPAC name, including
stereochemistry, for disparlure.

• cis-7,8-Epoxy-2-methyloctadecane

132
Epoxidation of Alkenes

peroxy acid
133
Example
134
Stereochemistry of Epoxidation

syn addition
135
Problem 6.22 Give the structure of the
alkene,including stereochemistry, that you
wouldchoose as the starting material in a
preparationof synthetic disparlure.
peroxy acid
136
Relative Rates of Epoxidation

• ethylene H2CCH2 1
• propene CH3CHCH2 22
• 2-methylpropene (CH3)2CCH2 484
• 2-methyl-2-butene (CH3)2CCHCH3 6526
• More highly substituted double bonds react
  faster.Alkyl groups on the double bond make
  itmore electron rich.

137
Mechanism of Epoxidation
138
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139
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140
6.20Ozonolysis of Alkenes

• Ozonolysis has both synthetic and analytical
  applications.
• synthesis of aldehydes and ketones
• identification of substituents on the double
  bond of an alkene

141
Ozonolysis of Alkenes

• First step is the reaction of the alkene with
  ozone.The product is an ozonide.

O3
142
Ozonolysis of Alkenes

• Second step is hydrolysis of the ozonide. Two
  aldehydes, two ketones, or an aldehyde and a
  ketone are formed.

O3
H2O, Zn

O
C
143
Ozonolysis of Alkenes

• As an alternative to hydrolysis, the ozonide
  canbe treated with dimethyl sulfide.

O3
(CH3)2S

144
Example
1. O3
2. H2O, Zn
145
6.21Introduction to OrganicChemical Synthesis
146
Prepare cyclohexane from cyclohexanol

• devise a synthetic plan
• reason backward from the target molecule
• always use reactions that you are sure will work

147
Prepare cyclohexane from cyclohexanol
H2
Pt

• 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?"

148
Prepare cyclohexane from cyclohexanol
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?"

149
Prepare cyclohexane from cyclohexanol
H2
Pt

• Alkenes can be prepared by dehydration of
  alcohols.
• The synthesis is complete.

150
Prepare 1-bromo-2-methyl-2-propanol from
tert-butyl alcohol
(CH3)3COH

• "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?

151
Prepare 1-bromo-2-methyl-2-propanol from
tert-butyl alcohol
Br2
H2O

• Vicinal bromohydrins are prepared by treatment of
  alkenes with Br2 in water.
• How is the necessary alkene prepared?

152
Prepare 1-bromo-2-methyl-2-propanol from
tert-butyl alcohol
(CH3)3COH
H2SO4
heat

• 2-Methylpropene is prepared from tert-butyl
  alcohol by acid-catalyzed dehydration.
• The synthesis is complete.

153
6.22Reactions of Alkenes with AlkenesPolymeriza
tion
154
Polymerization of Alkenes

• cationic polymerization
• free-radical polymerization
• coordination polymerization

155
Cationic Polymerization
Dimerization of 2-methylpropene
monomer(C4H8)
H2SO4
156
Mechanism of Dimerization
157
Mechanism of Dimerization
158
Free-Radical Polymerization of Ethylene
H2C
159
Mechanism

160
Mechanism
H2C
CH2


161
Mechanism
H2C
CH2

162
Mechanism
H2C
CH2
H2C
CH2


163
Mechanism
H2C
CH2
H2C
CH2


164
Mechanism
H2C
CH2
H2C
CH2
H2C
CH2


165
Mechanism
H2C
CH2
H2C
CH2
H2C
CH2


166
Free-Radical Polymerization of Propene
H2C
167
Mechanism

H2C
CHCH3
168
Mechanism
H2C
CHCH3


169
Mechanism
H2C
CHCH3

170
Mechanism
H2C
CHCH3
H2C
CHCH3


171
Mechanism
H2C
CHCH3
H2C
CHCH3


172
Mechanism
H2C
CHCH3
H2C
CHCH3
H2C
CHCH3


173
Mechanism
H2C
CHCH3
H2C
CHCH3
H2C
CHCH3


174
Likewise...

• H2CCHCl ?? polyvinyl chloride
• H2CCHC6H5 ?? polystyrene
• F2CCF2 ?? Teflon