Title: Chapter 6 Reactions of Alkenes: Addition Reactions
1Chapter 6Reactions of AlkenesAddition Reactions
26.1Hydrogenation of Alkenes
3Reactions of Alkenes
- The characteristic reaction of alkenes is
addition to the double bond.
AB
4Hydrogenation of Ethylene
?
?
?
?
HH
- exothermic ?H 136 kJ/mol
- catalyzed by finely divided Pt, Pd, Rh, Ni
5Example
H2, Pt
(73)
6Problem 6.1
- What three alkenes yield 2-methylbutane on
catalytic hydrogenation?
76.2Heats of Hydrogenation
- can be used to measure relative stability of
isomeric alkenes - correlation with structure is same as when heats
of combustion are measured
8Mechanism of Catalytic Hydrogenation Figure 6.1
9Mechanism of Catalytic Hydrogenation Figure 6.1
B
Y
C
C
A
X
10Mechanism of Catalytic Hydrogenation Figure 6.1
11Mechanism of Catalytic Hydrogenation Figure 6.1
B
Y
A
X
C
C
12Mechanism of Catalytic Hydrogenation Figure 6.1
13Mechanism of Catalytic Hydrogenation Figure 6.1
14Heats of Hydrogenation of Isomers
126
119
115
CH3CH2CH2CH3
15Heats 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
16Problem 6.2
Match each alkene of Problem 6.1 with its
correctheat of hydrogenation.
176.3Stereochemistry of Alkene Hydrogenation
18Two 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
19Two spatial (stereochemical) aspects ofalkene
hydrogenation
- (1) syn addition of both H atoms to double bond
20syn-Additon versus anti-Addition
syn addition
anti addition
21Example of Syn Addition
CO2CH3
CO2CH3
(100)
22Two 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
23Two 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.
24Example of Stereoselective Reaction
H2, cat
Both productscorrespond tosyn additionof H2.
25Example of Stereoselective Reaction
H2, cat
But only thisone is formed.
26Example of Stereoselective Reaction
H2, cat
Top face of doublebond blocked bythis methyl
group
27Example of Stereoselective Reaction
H2, cat
H2 adds to bottom face of double bond.
286.4Electrophilic Addition of Hydrogen Halides
to Alkenes
29General equation for electrophilic addition
??
?
EY
30When E is a hydrogen halide
??
?
HX
31Example
CH3CH2
CH2CH3
HBr
CHCl3, -30C
H
H
32Mechanism
- Electrophilic addition of hydrogen halides to
alkenes proceeds by rate-determining formation
of a carbocation intermediate.
33Mechanism
- Electrons flow fromthe ? system of thealkene
(electron rich) toward the positivelypolarized
proton of the hydrogen halide.
34Mechanism
X
H
356.5Regioselectivity of Hydrogen Halide
AdditionMarkovnikov's Rule
36Markovnikov'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.
37Markovnikov's Rule
HBr
acetic acid
(80)
Example 1
38Markovnikov's Rule
CH3
H
HBr
acetic acid
CH3
H
(90)
Example 2
39Markovnikov's Rule
HCl
0C
(100)
Example 3
406.6Mechanistic Basis for Markovnikov's Rule
- Protonation of double bond occurs in direction
that gives more stable of two possible
carbocations.
41Mechanistic Basis for Markovnikov's RuleExample
1
42Mechanistic Basis for Markovnikov's RuleExample
1
HBr
43Mechanistic Basis for Markovnikov's RuleExample
3
HCl
0C
44Cl
HCl
456.7Carbocation Rearrangements in Hydrogen Halide
Addition to Alkenes
46Rearrangements sometimes occur
HCl, 0C
476.8Free-radical Addition of HBr to Alkenes
48Markovnikov's Rule
HBr
acetic acid
(80)
49Addition of HBr to 1-Butene
HBr
CH3CH2CH2CH2Br
50Addition 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
51Photochemical 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.
52Mechanism
- Addition of HBr opposite to Markovnikov's rule
proceeds by a free-radical chain mechanism. - Initiation steps
53Mechanism
Propagation steps
546.9Addition of Sulfuric Acid to Alkenes
55Addition of H2SO4
HOSO2OH
Isopropylhydrogen sulfate
- follows Markovnikov's rule
- yields an alkyl hydrogen sulfate
56Mechanism
..
SO2OH
O
H
..
57Alkyl hydrogen sulfates undergo hydrolysis in hot
water
HOH
58Application Conversion of alkenes to alcohols
1. H2SO4
2. H2O, heat
(75)
59But...
- not all alkenes yield alkyl hydrogen sulfateson
reaction with sulfuric acid - these do H2CCH2, RCHCH2, and RCHCHR'
- these don't R2CCH2, R2CCHR, and R2CCR2
606.10Acid-Catalyzed Hydration of Alkenes
61Acid-Catalyzed Hydration of Alkenes
HOH
- reaction is acid catalyzed typical hydration
medium is 50 H2SO4-50 H2O
62Follows Markovnikov's Rule
(90)
63Follows Markovnikov's Rule
(80)
64Mechanism
- involves a carbocation intermediate
- is the reverse of acid-catalyzed dehydrationof
alcohols to alkenes
H
H2O
65Mechanism
Step (1) Protonation of double bond
66Mechanism
Step (2) Capture of carbocation by water
67Mechanism
Step (3) Deprotonation of oxonium ion
68Relative 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.
69Principle 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.
706.11Thermodynamics of Addition-Elimination
Equlibria
71Hydration-Dehydration Equilibrium
How do we control the position of the equilibrium
and maximize the product?
72Le 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).
73Le 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
74Le 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
75Le 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.
766.12Hydroboration-Oxidation of Alkenes
77Synthesis
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.
78Synthesis
Two-step reaction sequence called
hydroboration-oxidation converts alkenes to
alcohols with a regiochemistry opposite to
Markovnikov's rule.
79Hydroboration Step
HBH2
Hydroboration can be viewed as the addition
ofborane (BH3) to the double bond. But BH3 is
not the reagent actually used.
80Hydroboration Step
HBH2
Hydroboration reagents
H
Diborane (B2H6)normally used in an ether- like
solventcalled "diglyme"
BH2
H2B
H
81Hydroboration Step
HBH2
Hydroboration reagents
Borane-tetrahydrofurancomplex (H3B-THF)
82Oxidation Step
H2O2, HO
Organoborane formed in the hydroborationstep is
oxidized with hydrogen peroxide.
83Example
(93)
84Example
H3C
CH3
H3C
H
(98)
85Example
1. B2H6, diglyme
2. H2O2, HO
(82)
866.13Stereochemistry of Hydroboration-Oxidation
87Features of Hydroboration-Oxidation
- hydration of alkenes
- regioselectivity opposite to Markovnikov's rule
- no rearrangement
- stereospecific syn addition
88syn-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
896.14Mechanism of Hydroboration-Oxidation
901-Methylcyclopentene BH3
- syn addition of H and B to double bond
- B adds to less substituted carbon
91Organoborane intermediate
92Add hydrogen peroxide
- OH replaces B on same side
93trans-2-Methylcyclopentanol
946.15Addition of Halogensto Alkenes
95General features
X2
X
X
- electrophilic addition to double bond
- forms a vicinal dihalide
96Example
Br2
CH3CHCHCH(CH3)2
CH3CH
CHCH(CH3)2
CHCl3 0C
Br
Br
(100)
97Scope
limited to Cl2 and Br2
- F2 addition proceeds with explosive violence
- I2 addition is endothermic vicinal
diiodidesdissociate to an alkene and I2
986.16Stereochemistry of Halogen Addition
99Example
Br2
trans-1,2-Dibromocyclopentane80 yield only
product
100Example
H
Cl2
H
trans-1,2-Dichlorocyclooctane73 yield only
product
1016.17Mechanism of Halogen Addition to Alkenes
Halonium Ions
102Mechanism 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
103Relative 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.
104Mechanism?
BrCH2CH2Br
Br2
H2C
CH2
?
- No obvious explanation for anti addition provided
by this mechanism.
105Mechanism
BrCH2CH2Br
Br2
H2C
CH2
..
Br
C
C
106Formation of Bromonium Ion
Br
Mutual polarizationof electron distributionsof
Br2 and alkene
Br
107Formation of Bromonium Ion
?
Electrons flow from alkenetoward Br2
Br
Br
?
?
108Formation of Bromonium Ion
Br
? electrons ofalkene displaceBr from Br
109Stereochemistry
Br
..
Br
..
attack of Br from side oppositeCBr bond of
bromonium ion givesanti addition
..
Br
..
110Example
Br2
trans-1,2-Dibromocyclopentane80 yield only
product
111Cyclopentene Br2
112Bromonium ion
113Bromide ion attacks the bromonium ion from side
opposite carbon-bromine bond
114trans-Stereochemistry in vicinal dibromide
1156.18Conversion 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
118Examples
H2O
H2C
BrCH2CH2OH
Br2
CH2
(70)
Cl2
H2O
anti addition only product
119Mechanism
- bromonium ion is intermediate
- water is nucleophile that attacks bromonium ion
..
Br
..
120Examples
H2O
H2C
BrCH2CH2OH
Br2
CH2
(70)
Cl2
H2O
anti addition only product
121Cyclopentene Cl2
122Chloronium ion
123Water attacks chloronium ion from side opposite
carbon-chlorine bond
124trans-Stereochemistry in oxonium ion
125trans-2-Chlorocyclopentanol
126Regioselectivity
(77)
- Markovnikov's rule applied to halohydrin
formation the halogen adds to the carbon having
the greater number of hydrogens.
127Explanation
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
1286.19Epoxidation of Alkenes
129Epoxides
- are examples of heterocyclic compounds
- three-membered rings that contain oxygen
- ethylene oxide propylene oxide
130Epoxide 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
131Problem 6.21 Give the IUPAC name, including
stereochemistry, for disparlure.
- cis-7,8-Epoxy-2-methyloctadecane
132Epoxidation of Alkenes
peroxy acid
133Example
134Stereochemistry of Epoxidation
syn addition
135Problem 6.22 Give the structure of the
alkene,including stereochemistry, that you
wouldchoose as the starting material in a
preparationof synthetic disparlure.
peroxy acid
136Relative 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.
137Mechanism of Epoxidation
138(No Transcript)
139(No Transcript)
1406.20Ozonolysis of Alkenes
- Ozonolysis has both synthetic and analytical
applications. - synthesis of aldehydes and ketones
- identification of substituents on the double
bond of an alkene
141Ozonolysis of Alkenes
- First step is the reaction of the alkene with
ozone.The product is an ozonide.
O3
142Ozonolysis 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
143Ozonolysis of Alkenes
- As an alternative to hydrolysis, the ozonide
canbe treated with dimethyl sulfide.
O3
(CH3)2S
144Example
1. O3
2. H2O, Zn
1456.21Introduction to OrganicChemical Synthesis
146Prepare cyclohexane from cyclohexanol
- devise a synthetic plan
- reason backward from the target molecule
- always use reactions that you are sure will work
147Prepare 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?"
148Prepare 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?"
149Prepare cyclohexane from cyclohexanol
H2
Pt
- Alkenes can be prepared by dehydration of
alcohols. - The synthesis is complete.
150Prepare 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?
151Prepare 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?
152Prepare 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.
1536.22Reactions of Alkenes with AlkenesPolymeriza
tion
154Polymerization of Alkenes
- cationic polymerization
- free-radical polymerization
- coordination polymerization
155Cationic Polymerization
Dimerization of 2-methylpropene
monomer(C4H8)
H2SO4
156Mechanism of Dimerization
157Mechanism of Dimerization
158Free-Radical Polymerization of Ethylene
H2C
159Mechanism
160Mechanism
H2C
CH2
161Mechanism
H2C
CH2
162Mechanism
H2C
CH2
H2C
CH2
163Mechanism
H2C
CH2
H2C
CH2
164Mechanism
H2C
CH2
H2C
CH2
H2C
CH2
165Mechanism
H2C
CH2
H2C
CH2
H2C
CH2
166Free-Radical Polymerization of Propene
H2C
167Mechanism
H2C
CHCH3
168Mechanism
H2C
CHCH3
169Mechanism
H2C
CHCH3
170Mechanism
H2C
CHCH3
H2C
CHCH3
171Mechanism
H2C
CHCH3
H2C
CHCH3
172Mechanism
H2C
CHCH3
H2C
CHCH3
H2C
CHCH3
173Mechanism
H2C
CHCH3
H2C
CHCH3
H2C
CHCH3
174Likewise...
- H2CCHCl ?? polyvinyl chloride
- H2CCHC6H5 ?? polystyrene
- F2CCF2 ?? Teflon