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11/1/2010
1
Chapter 6Addition Reactions of
AlkAlkenes
• The characteristic reaction of alkenes is addition to the double bond.
• Reverse of elimination, A—B represents: H—H; hydrogenation reaction H—OH; hydration reaction H—X; hydrohalogenation reaction
Reactions of Alkenes
; y g X—X; halogenation reaction
• Also epoxidation, formation of a 3-membered ring with an O
+ A—BC C A C C B
+ H—H
H C C
H H
H H
H
C C
H
H
H
H
Hydrogenation of Ethylene
• exothermic H° = –136 kJ/mol
• catalyzed by finely divided Pt, Pd, Rh, Ni
Example
H2, Pt
CH2H3C
H3C
(73%)
CH3
HH3C
H3C
• What three alkenes yield 2-methylbutane on catalytic hydrogenation?
Problem 6.1
H2, Pt
• can be used to measure relative stability of isomeric alkenes
• correlation with structure is same as when heats of combustion are measured
Heats of Hydrogenation
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H H HC C
A
B
X
Y
Mechanism of Catalytic Hydrogenation
Interaction with the metal catalyst is critical Hydrogen bonds to the surface of the metal
H
H H
Mechanism of Catalytic Hydrogenation
• Alkene uses its electrons to bond to the surface of the metal; the other carbon bonds to H
H HA
BXY
H HHCC
H
Mechanism of Catalytic Hydrogenation
H HH
CC
AB
XY
H HHH
Heats of Hydrogenation of Isomers
CH3CH2CH2CH3
119115
• Ethylene 136
• Monosubstituted 125-126
• cis-Disubstituted 117-119
Heats of Hydrogenation (kJ/mol)
• trans-Disubstituted 114-115
• Terminally disubstituted 116-117
• Trisubstituted 112
• Tetrasubstituted 110
Match each alkene of Problem 6.1 with its correctheat of hydrogenation.
126 kJ/molhighest heat ofhydrogenation;least stable isomer
Problem 6.2
118 kJ/mol
112 kJ/mollowest heat ofhydrogenation;most stable isomer
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Stereochemistry of Alkene Hydrogenation
• There are 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
• A reaction in which a single starting material• 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.
syn addition anti addition
syn Addition versus anti Addition
Example of syn-Addition
H2, PtCO2CH3
CO2CH3
CO2CH3
CO2CH3
H
(100%)CO2CH3
H
A single starting material can give two or morestereoisomeric products, but gives one of themin greater amounts than any other.
Stereoselective reaction
H3C CH3
H3C
H
H cat
Both products correspond to syn addition of H2.
Example of A Stereoselective Reaction
H2, cat
CH3H3C
H3C
HH
H
H3C
CH3H3C
H
H
H
But only this one is formed.
H3C CH3
H3C
H
Top face of doublebond blocked by
Example of A Stereoselective
Reaction
H2, catbond blocked bythis methyl group
H2 adds to bottom face of double bond.
HCH3
H3C
H3CH
H
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General Equation for Electrophilic Addition
• The electrons of the alkene can be used to form bonds in a reaction – it is a nucleophile.
• Alkenes “can add on” species that are electrophiles
• An hydrogen halide (acid) is an electrophilic
+ E—Y –
C C E YC C
y g ( ) pspecies – the electrons can be used to bond to H+
A Hydrogen Halide is an Electrophile
(E—Y is H—X in the reaction below)
+ H—X –
C C H XC C
CH3CH2 CH2CH3
H H
CHCl3, -30°CC CHBr
• Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocationintermediate.
• Electrons flow from the
Mechanism
• Electrons flow from the system of the alkene (electron rich) toward the positively polarized proton of the hydrogen halide.
Mechanism
..
..:X:–
HC C+
XH
C C
..
..:
HC C....X:
Markovnikov’s Rule: • When an unsymmetrically substituted alkene
reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number f h d b i d h h l dd
Regioselectivity of Hydrogen Halide Addition
of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.
Markovnikov's Rule
acetic acidBr
CH3CH2CHCH2HCH2CH3CH2CHHBr
(80%)
CH H
CH3
CH CC CHBr
CH3 H
CH2HCH3 C
Br (90%)
C Cacetic acid
CH3 H
(100%)HCl
CH3
CH3
Cl0ºC
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CH3CH2CH2—CH2
+
primary carbocation is less stable: not formed
Mechanistic Basis for Markovnikov's Rule
• Protonation of double bond occurs in the direction that gives more stable of two possible carbocations.
Br
CH3CH2CHCH3CH2CH3CH2CH
HBr
CH3CH2CH—CH3 + Br –+
primary carbocation is less stable: not formed
(CH3)2CH—CH2
+
primary carbocation is less stable: not formed
Mechanistic Basis for Markovnikov's Rule
(CH3)2C—CH3 + Br –+
HBr
( 3)2 3
CH3
CH3
CH3 C
Br
C C
CH3
CH3
H
H
Mechanistic Basis for Markovnikov's Rule
H H
CH3
H
+
H
secondary carbocation is less stable: not formed
HClH
CH3
CH3
Cl
CH3+ Cl–
HCl, 0°C
CH3CHCH(CH3)2
+
H2C CHCH(CH3)2
+CH CHC(CH )
H
Carbocation Rearrangements Can Occur
CH3CHCH(CH3)2 CH3CHC(CH3)2
CH3CHCH(CH3)2
Cl(40%)
CH3CH2C(CH3)2
Cl(60%)
CH3CHCH3
OSO2OH
CH3CH CH2HOSO2OH
Isopropylh d lf t
Addition of H2SO4 to Alkenes
• follows Markovnikov's rule • yields an alkyl hydrogen sulfate
hydrogen sulfate
slow
+CH3CH CH2 H..
SO2OH..O
+ ..–
Mechanism
CH3CH CH3 + SO2OH..:O
fastCH3CHCH3
OSO2OH..:
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Alkyl Hydrogen Sulfates Undergo Hydrolysis in Hot Water, Form Alcohols
H—OHCH3CHCH3
O SO2OH heat
HO SO OH+CH CHCH
+
HO—SO2OH+CH3CHCH3
O H
1. H2SO4
2. H2O, heat (75%)
OH
• not all alkenes yield alkyl hydrogen sulfateson reaction with sulfuric acid
• these do: H2C=CH2, RCH=CH2, RCH=CHR' • these don't: R2C=CH2, R2C=CHR, R2C=CR2
Reaction Selectivity
• The presence of one H on each carbon is critical to the addition of H2SO4.
• Does that mean the other alkenes cannot be hydrated to alcohols?
Acid-Catalyzed Hydration of Alkenes
H—OHC C + OHC CH
• reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O
(90%)
50% H2SO4
50% H2O
H3C
H3C CH3
H
C C
OH
C CH2CH3CH3
CH3
• Regioselectivity follows Markovnikov’s rule
Follows Markovnikov's Rule
50% H2SO4
50% H2O(80%)
OH
CH3CH2
• involves a carbocation intermediate • is the reverse of acid-catalyzed dehydration of
+ H2OH+
H3C
H3C
C CH2
OH
C CH3CH3
CH3
is the reverse of acid catalyzed dehydration of alcohols to alkenes
Step (1) Protonation of double bond
H3C
C CH2+ O
+H
H
:
Mechanism
H3C Hslow
+H3C
H3C
C CH3
H
+ O
H
::
Step (2) Capture of carbocation by water
+H3C
H3C
C CH3
H
+ O
H
::
fast
Mechanism
H3C H
+
H
O
H
:C
CH3
CH3
CH3
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Step (3) Deprotonation of oxonium ion
+
H
O
H
:C
CH3
CH
CH3 O
H
::
H
+
Mechanism
HCH3Hfast
O+
H
H
H
:+
H
O:C
CH3
CH3
CH3
..
• 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
Relative Rates – Acid Catalyzed Hydration
• The more stable the carbocation, the faster it is formed, and the faster the reaction rate.
+ H2OH+
H3C
H3C
C CH2
OH
C CH3CH3
CH3
Principle of Microscopic Reversibility
• In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order.
• Let’s examine the kinetics and thermodynamics…
Hydration-Dehydration Equilibrium
• How do we control the position of the equilibrium and maximize the product?
Le Chatelier’s Principle
• A system at equilibrium adjusts so to minimize any stress applied to it.
• For hydration-dehydration, the key stress is water.• Adding water pushes the equilibrium toward
product (alcohol); removing water pushes toward reactant (alkene)reactant (alkene)
• At constant T and P, reactions proceed to decrease free energy (G, spontaneous reaction)
• The sign of G is always positive, but G can be positive or negative.
• G = Gproduct – Greactant; spontaneous when G < 0
Le Chatelier’s Principle
For a reversible reaction: aA + bB cC + dD The relationship between G and Go is:
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 forGo negative are exergonic.
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Synthesis: Suppose you wanted to prepare 1-decanol from 1-decene?
Hydroboration Oxidation of Alkenes
• Needed: a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule.
OH
• Two-step reaction sequence called hydroboration-oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule.
Synthesis
1 h droboration1. hydroboration
2. oxidation
OH
Hydroboration Step
+ H—BH2C C H BH2C C
• Hydroboration can be viewed as the addition of borane (BH3) to the double bond.
• But BH3 is not the reagent actually used.
Hydroboration Step
Hydroboration reagents:
H2B
H
H
BH2
Diborane (B2H6)normally used in an ether-like solventcalled “diglyme”g y
Borane-tetrahydrofurancomplex (H3B-THF)+O
BH3–
••
Oxidation Step
H2O2, HO–
H BH2C C H OHC C
Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.
Examples
1. B2H6, diglyme
2. H2O2, HO–
(93%)OH
( %)
1. B2H6, diglyme
2. H2O2, HO–
(82%)
OH
11/1/2010
9
• hydration of alkenes • regioselectivity opposite to Markovnikov's rule • no rearrangement • stereospecific syn addition
Features of Hydroboration-Oxidation
(98%)
H3C
H3C
CH3
H
C C1. H3B-THF
2. H2O2, HO–
H
C CCH3
CH3
CH3
H OH
• H and OH become attached to same face of double bond
syn Addition
1. B2H6, diglymeCH3CH3
H
only product is trans-2-methylcyclopentanol(86%) yield
2. H2O2, NaOH
H HHO
Mechanism: 1-Methylcyclopentene + BH3
syn addition of H and B to double bond B adds to less substituted carbon
Organoborane Intermediate
Add hydrogen peroxide; OH replaces B on same side
Trans-2-methylcyclohexanol
+ X2 X XC CC C
Addition of Halogens to Alkenes
• Electrophilic addition to double bond forms a ect op c add t o to doub e bo d o s avicinal dihalide.
• limited to Cl2 and Br2
• F2 addition proceeds with explosive violence.• I2 addition is endothermic: vicinal diiodides
dissociate to an alkene and I2.
Examples
CHCl30ºC
CHCH(CH3)2CH3CHBr2
CH3CHCHCH(CH3)2
(100%)Br Br
The stereochemistry of halogen addition is anti.
H
trans-1,2-Dibromocyclopentane80% yield; only product
Br2H
H
Br
Br
H
H
• A halonium ion is the key intermediate in the halogenation of alkenes
• Br2 is not polar, but it is polarizable. • two steps
(1) formation of bromonium ion
Mechanism is electrophilic addition
(1) formation of bromonium ion(2) nucleophilic attack on bromonium ion by bromide
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10
ethylene H2C=CH2 1
propene CH3CH=CH2 61
2-methylpropene (CH ) C=CH 5400
Relative Rates of Bromination
2-methylpropene (CH3)2C=CH2 5400
2,3-dimethyl-2-butene (CH3)2C=C(CH3)2 920,000
More highly substituted double bonds react faster.Alkyl groups on the double bond make it more “electron rich.”
H2C CH2 BrCH2CH2Br+ Br2
?
+C C+ ..
: :Br–
Mechanism?
C C
Br: :..
: :..Br
• No obvious explanation for anti addition provided by this mechanism.
H2C CH2 BrCH2CH2Br+ Br2
+C C..
: :Br–
Mechanism
Cyclic bromonium ion
+C C
Br: :+
: :..Br Mutual polarization of
BrBr
Electrons flow from
BrBr
–
++
Formation of Bromonium Ion
Mutual polarization of electron distributions of Br2 and alkene
Electrons flow from alkene toward Br2.
electrons of alkene displace Br– from Br.
Br–
Br+
Stereochemistry of Anti Addition
+
–Br: :..
..
Br..
..
..
Br
Br..
:..
..::
• Attack of Br– from side opposite C—Br bond of bromonium ion gives anti addition.
Cyclopentene +Br2
B i i trans-Stereochemistry–Bromonium ion
––
––Bromide ion attacks the bromonium ion from side opposite carbon-bromine bond; anti addition – trans-1,2-dibromo-cyclopentane is the only product
trans-Stereochemistry in vicinal dibromide
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Anti Addition of Bromine to 2-Butene
Br2R
S R
Smeso
• Anti addition to trans 2 butene gives the meso
Br2R
R
50%
+
S
S
50%
• Anti addition to trans-2-butene gives the mesocompound; cis-2-butene gives a racemic mixture
+ X2 X XC CC C
• Alkenes react with X2 to form vicinal dihalides. • Alkenes react with X2 in water to give vicinal
Vicinal Halohydrins from Alkenes
+ H2O OH
+ H—X
+ X2 X C CC C
2 ghalohydrins.
Examples
H2C CH2 BrCH2CH2OH+ Br2
H2O
(70%)
Cl2H OH
H
trans-2-chlorocyclopentanol anti addition: only product
H2OH
H
OH
Cl H
Mechanism
+Br..
..
:..O
Br..
:..
..O
+
• Bromonium ion is the intermediate• Water is nucleophile that attacks
bromonium ion.
Cyclopentene + Cl2
Chloronium ion
Water attacks chloronium ion from side opposite carbon-chlorine bond.
trans-Stereochemistry in oxonium ion
(77%)
H3C
C CH2
H3C
CH3
OH
C CH2BrCH3
Br2
H2O
Regioselectivity
• Markovnikov's rule is applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens.
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12
H
OH ..
H
O H..
H3C CH2
H3C C H3C CH2
H3C
C
Explanation
• 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.
Br: :
Br: :
acetic acidBr
CH3CH2CHCH3CH2CH3CH2CHHBr
(80%)
Markovnikov's Rule
• Can HBr addition take place such that Br is bonded to C1 in this molecule? Yesbonded to C1 in this molecule? Yes.
• When HBr is added to an alkene in the presence of peroxides, an anti-Markovnikov addition is observed – called the “peroxide effect”
• The reaction occurs via the free radical mechanism…
C C C C
CH2CH3CH2CH
HBr
Addition of HBr to 1-Butene
CH3CH2CH2CH2Br
only product inabsence of peroxides
only product when peroxides added to reaction mixture
Br
CH3CH2CHCH3 C C C C
CH2CH3CH2CH
HBraddition opposite to Markovnikov's rule occurs with
Addition of HBr to 1-Butene
CH3CH2CH2CH2Bronly product when peroxides added to reaction mixture
rule – occurs with HBr (not HCl or HI)
• Addition of HBr with a regiochemistry opposite to
+ HBrh
CH2
(60%)
CH2Br
H
Photochemical Addition of HBr
Markovnikov's rule can also occur when initiated with light with or without added peroxides.
• Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism.
• Initiation steps:
Mechanism
..
..O RR O....
O .R ....
..
..O R.+
+O .R ....
+OR ....
H . Br..
..:..BrH
..:
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Propagation steps:
Mechanism
.
..CH3CH2CH CH2 Br:..
+CH3CH2CH CH2 ... Br
..:
..
..
+....
.CH3CH2CH CH2 Br:..
..
H Br:
....CH3CH2CH2CH2Br:
. Br:
H2C CH2 H2C CHCH3
Epoxides
• are examples of heterocyclic compounds • three-membered rings that contain oxygen
ethylene oxide propylene oxide
O O
H3C
Epoxide Nomenclature
• Substitutive nomenclature: named as epoxy-substituted alkanes.
• “epoxy” precedes name of alkane • 1,2-epoxypropane 2-methyl-2,3-epoxybutane
H2C CHCH3
O
CHCH3
O
C
3C
H3C1
2 3 4
O
RCOOH
Epoxidation of Alkenes
peroxy acid
C C +
O
+ CH3COOH
(52%)+ CH3COH
O
O
Stereochemistry of Epoxidation
C C +
syn addition
RCOOH
O
CC
O
+
O
RCOH
syn addition
Problem 6.23 Give the structure of the alkene, including stereochemistry, that you would choose as the starting material in a preparation of synthetic disparlure (6.22, nomenclature).
peroxy acid
OH H
H H
disparlure
(cis-2-Methyl-7,8-epoxyoctadecane)
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14
• ethylene H2C=CH2 1
• propene CH3CH=CH2 22
• 2-methylpropene (CH3)2C=CH2 484
2 th l 2 b t (CH ) C CHCH 6526
Relative Rates of Epoxidation
• 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.”
Mechanism of Epoxidation
• The peroxy acid reacts with the alkene to form a bicyclic ring intermediate.
• The ring dissociates to the epoxide. • Additon is syn; alkene stereochemistry is retained.
syn addition to trans-2-butene gives racemic
Epoxidation of 2-Butene
RCO3H RR S
S
50%
+
50%
syn addition to trans 2 butene gives racemic mixture; cis-2-butene gives the meso diastereomer
R
S R
S
RCO3H
meso
• Ozonolysis has both synthetic and analytical applications.
• Used for synthesis of aldehydes and ketones• Used for identification of substituents on the
double bond of an alkene• First step is the reaction of the alkene with ozone.
Ozonolysis of Alkenes
First step is the reaction of the alkene with ozone. • The product is an ozonide.
+ O3 CCO
O O
C C
• Second step is reduction of the ozonide. • Two aldehydes, two ketones, or an aldehyde
and a ketone are formed.
Ozonolysis of Alkenes
+ O3CC
O
C C
C O CO+ H2O, Zn
O O
• The ozonide can also be reduced with dimethyl sulfide.
1. O3
CH3
CH2CH3H
C C
CH2CH3
Example
1. O32. H2O, Zn
(38%) (57%)
CH2CH3
CO
CH2CH3
C O
CH3
H
+
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15
Polymerization of Alkenes
• Alkenes react with alkenes to form polymers by three mechanisms: cationic polymerization free-radical polymerization coordination polymerization
H2SO4
monomer(C4H8)
Dimerization of 2-methylpropene
(CH3)2C CH2
Cationic Polymerization
H2SO4
+
two dimers(C8H16)
CH3CCH
CH3
CH3
C(CH3)2 CH3CCH2C
CH3
CH3
CH2
CH3
Step (1) Protonation of double bond
H3C
H3C
C CH2+ O
+H
H
H
:
l
Mechanism of Dimerization
H3C Hslow
+H3C
H3C
C CH3
H
+ O
H
::
+
CH3
H2C C
CH3
CH3C
CH3
+
CH3
Mechanism of Dimerization
CH3CCH2C
CH3
CH3
CH3
CH3
+
The carbocation formed can lose a proton to form a dimer or react with another molecule of alkene to form a “polymer”
Mechanism of Dimerization
CH3CCH
CH3
CH3
C(CH3)2 CH3CCH2C
CH3
CH3
CH2
CH3
Two constitutional
CH3CCH2C
CH3
CH3
CH3
CH3
+
Two constitutional isomers are the products
200 ºC2000 atm
O2
peroxides
H2C CH2
CH CH CH CH CH CH CH
Free-Radical Polymerization of Ethylene
polyethylene
CH2 CH2 CH2 CH2 CH2 CH2 CH2
• Peroxides are used for free radical reactions as initiators;
• they readily form hydroxy or alkoxy radicals…
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16
..RO: •
..RO..
H2C CH2H2C CH2
•
CHH C
Mechanism
CH2H2C
H2C CH2
H2C CH2•
..RO:
The process continues…
H2C CHCH3
Free-Radical Polymerization of Propene
polypropylene
CH CH CHCHCHCH CH
CH3 CH3 CH3 CH3 CH3 CH3 CH3
•..
RO..
H2C CHCH3H2C CHCH3•
..RO:
CHCHH C
Mechanism
CHCH3H2C
H2C CHCH3
H2C CHCH3•
..RO:
As noted before, the process continues…
Likewise...
• H2C=CHCl polyvinyl chloride• H2C=CHC6H5 polystyrene• F2C=CF2 Teflon®