Chapter 7. Alkenes: Reactions
and Synthesis
Diverse Reactions of Alkenes
• Alkenes react with many electrophiles to give useful
products by addition (often through special reagents)
–
–
–
–
–
–
–
alcohols (add H-OH)
alkanes (add H-H)
halohydrins (add HO-X)
dihalides (add X-X)
halides (add H-X)
diols (add HO-OH)
cyclopropane (add :CH2)
2
Reactions of Alkenes
H
OH
H
X
H
OH
Alcohol
Alkane
Halohydrin
X
HO
OH
1,2-Diol
X
C C
1,2-Dihalide
alkene
O
H
Carbonyl Compound
X
Halide
O
Epoxide
C
Cyclopropane
3
Preparation of Alkenes
Addition
Y
X
C
C
+X
Y
C
C
Elimination
4
Part 1 - Synthesis of Alkenes
• These reactions are used to produce
alkenes.
5
Synthesis of Alkenes: Synthesis 1#
• Alkenes are commonly made by elimination of HX from
alkyl halide (dehydrohalogenation)
• Uses heat and KOH
H
H
H
Br
KOH
CH3CH2OH
H
H
+ KBr +
H2O
6
Synthesis of Alkenes: Synthesis 2#
– elimination of H-OH from an alcohol (dehydration)
• require strong acids (sulfuric acid, 50 ºC)
CH3
OH
H2SO4, H2O
THF, 50 oC
CH3
+
H2O
7
Part 2 - Reaction of Alkenes
• These reactions react alkenes to form a
series of alkane products.
8
Addition of Halogens to Alkenes
• Bromine and chlorine add to alkenes to give 1,2dihaldes
• F2 is too reactive and I2 does not add.
Cl
H
H
C C
H
H
Cl
Cl Cl
H C C H
H H
9
Example: Mechanism of Bromine Addition
• Electrophilic addition of bromine to give a cation is
followed by cyclization to give a bromonium ion.
• This bromoniun ion is a reactive electrophile and
bromide ion is a good nucleophile.
– Gives trans addition.
Br
Br
+
C C
Br
C C
+ Br-
Br
C C
Br-
Br
C C
Br
10
Example: Addition of Br2 to Cyclopentene
found
H
Br
Br
H
• Addition is exclusively trans
H
H Br Br
Br
Br
H
H
not found
11
Halohydrin Formation
• This is formally the addition of HO-X to an alkene (with +OH
as the electrophile) to give a 1,2-halo alcohol, called a
halohydrin.
• The actual reagent is the dihalogen (Br2 or Cl2 in water in
an organic solvent)
C C
Alkene
X2
H2O
X
C C
HO
+ HX
Halohydrin
12
An Alternative to Bromine
• Bromine is a difficult reagent to use for this reaction
• N-Bromosuccinimide (NBS) produces bromine in organic
solvents and is a safer source.
O
N Br
(NBS)
O
H2O, CH3SOCH 3(DMSO)
OH
Br
2-Bromo-1-phenylethanol (76%)
13
Addition of Water to Alkenes:
Oxymercuration
• Hydration of an alkene is the addition of H-OH to to
give an alcohol
• Acid catalysts are used in high temperature industrial
processes: ethylene is converted to ethanol
H
O H
+
H
H3PO4
HO -
C C
250oC
H
H
H
H
H
HO
H
H
H
H
Intermediate
14
Addition of Water to Alkenes:
Oxymercuration
H
CH3
Hg(OAc) 2
HgOAc
CH3
H2O
H HgOAc
CH3
OH
NaBH 4
H H
CH3
OH
Hg(OAc)2 is used as an electrophillic sink. The double bond is
then attacked by the water creating an alchohol. This is then
REDUCED by NaBH4 that adds an H to the molecule.
15
Addition of Water to Alkenes:
Hydroboration
• Herbert Brown (HB) invented hydroboration (HB)
• Borane (BH3) is electron deficient is a Lewis acid.
• Borane adds to an alkene to give an organoborane.
H
H
B
+
C C
H
BH2
H
Borane
Organoborane
16
BH3 Is a Lewis Acid
• Six electrons in outer shell
• Coordinates to oxygen electron pairs in ethers
17
Hydroboration-Oxidation Alcohol Formation
from Alkenes
• Addition of H-BH2 (from BH3-THF complex) to three
alkenes gives a trialkylborane
• Oxidation with alkaline hydrogen peroxide in water
produces the alcohol derived from the alkene
18
Orientation in Hydration via Hydroboration
• Regiochemistry is opposite to Markovnikov orientation
– OH is added to carbon with most H’s
• H and OH add with syn stereochemistry, to the same face
of the alkene (opposite of anti addition)
H
H B H
CH 3
BH3
THF
1-methylcycopentene
H
CH 3
Alkylborane intermediate
-
OH
H2O2
H OH
H
CH 3
trans-2-methylcyclopentanol
(85%)
19
Mechanism of Hydroboration
• Borane is a Lewis acid
• Alkene is Lewis base
• Transition state involves
anionic development on
B
• The components of BH3
are across C=C
20
Hydroboration, Electronic Effects Give NonMarkovnikov
• More stable carbocation is also consistent with
steric preferences
21
Hydroboration - Oxygen Insertion Step
• H2O2, OH- inserts OH in place of B
• Retains syn orientation
22
Addition of Carbenes to Alkenes
• The carbene functional group is “half of an alkene”
• Carbenes are electrically neutral with six electrons in
the outer shell
• They symmetrically across double bonds to form
cyclopropanes
+
Alkene
R1
C
R2
Carbene
R1
R2
C
Cyclopropane
23
Formation of Dichlorocarbene
• Base removes proton
from chloroform
• Stabilized carbanion
remains
• Unimolecular
Elimination of Clgives electron
deficient species,
dichlorocarbene
24
Simmons-Smith Reaction
• Equivalent of addition of CH2:
• Reaction of diiodomethane with zinc-copper alloy
produces a carbenoid species
• Forms cyclopropanes by cycloaddition
25
Reaction of Dichlorocarbene
• Addition of dichlorocarbene is stereospecific cis
26
Reduction of Alkenes: Hydrogenation
• Addition of H-H across C=C
• Reduction in general is addition of H2 or its
equivalent
• Requires Pt or Pd as powders on carbon and H2
• Hydrogen is first adsorbed on catalyst
• Reaction is heterogeneous (process is not in
solution)
27
Hydrogen Addition- Selectivity
• Selective for C=C. No reaction with C=O, C=N
• Polyunsaturated liquid oils become solids
• If one side is blocked, hydrogen adds to other
28
Mechanism of Catalytic Hydrogenation
• Heterogeneous – reaction between phases
• Addition of H-H is syn
29
Oxidation of Alkenes: Hydroxylation and
Cleavage
•
•
•
•
Hydroxylation adds OH to each end of C=C
Catalyzed by osmium tetroxide
Stereochemistry of addition is syn
Product is a 1,2-dialcohol or diol (also called a glycol)
C C
1. OsO4
HO
OH
2. NaHSO3
Alkene
A 1,2-diol
30
Osmium Tetroxide Catalyzed Formation of
Diols
• Hydroxylation - converts to syn-diol
• Osmium tetroxide, then sodium bisulfate
• Via cyclic osmate di-ester
Intermediate
CH 3
CH 3
1. OsO4
pyridine
NAME THIS MOLECULE
CH 3
O O
Os
O O
CH 3
A cyclic osmate
2. NaHSO3
H2O
CH3
OH
OH
CH 3
A 1,2-diol (87%)
NAME THIS MOLECULE
31
Section 3: Breakdown of Alkenes
• These Reactions are used to breakdown
alkenes into two products.
32
Alkene Cleavage: Ozone
• Ozone, O3, adds to alkenes to form molozonide
• Reduce molozonide to obtain ketones and/or
aldehydes
33
Examples of Ozonolysis of Alkenes
• Used in determination of structure of an unknown
alkene
34
Structure Elucidation With Ozone
• Cleavage products reveal an alkene’s structure
35
Permanganate Oxidation of Alkenes
• Oxidizing reagents other than ozone also cleave
alkenes
• Potassium permanganate (KMnO4) can produce
carboxylic acids and carbon dioxide if H’s are
present on C=C
O
+ KMnO4
O
O
+
H
H
O
36
Cleavage of 1,2-diols
• Reaction of a 1,2-diol with periodic (per-iodic) acid,
HIO4 , cleaves the diol into two carbonyl compinds
• Sequence of diol formation with OsO4 followed by
diol cleavage is a good alternative to ozonolysis
37
Mechanism of Periodic Acid Oxidation
• Via cyclic periodate intermediate
38
Biological Alkene Addition Reactions
• Living organisms convert organic molecules using
enzymes as catalysts
• Many reactions are similar to organic chemistry
conversions, except they occur in neutral water
• Usually much specific for reactant and
stereochemistry
39
Biological Hydration Example
• Fumarate to malate catalyzed by fumarase
• Specific for trans isomer
• Addition of H, OH is anti
O
O
O
O
O
H OH
Fumarase
H
O
O
OH
O
40
Addition of Radicals to Alkenes: Polymers
• A polymer is a very large molecule consisting of
repeating units of simpler molecules, formed by
polymerization
• Alkenes react with radical catalysts to undergo
radical polymerization
• Ethylene is polymerized to poyethylene, for
example
41
Free Radical Polymerization of Alkenes
• Alkenes combine many times to give polymer
– Reactivity induced by formation of free radicals
42
Free Radical Polymerization:
Initiation
• Initiation - a few radicals are generated by the
reaction of a molecule that readily forms radicals
from a non-radical molecule
• A bond is broken homolytically
43
Polymerization: Propagation
• Radical from intiation adds to alkene to generate
alkene derived radical
• This radical adds to another alkene, and so on many
times
44
Polymerization: Termination
• Chain propagation ends when two radical chains
combine
• Not controlled specifically but affected by reactivity
and concentration
45
Other Polymers
• Other alkenes give other common polymers
46
Cationic Polymerization
• Vinyl monomers react with Brønsted or
Lewis acid to produce a reactive
carbocation that adds to alkenes and
propagates via lengthening carbocations
47
Take Home Message
• Learn the REACTIONS (ALL OF THEM)
48
Synthesis of Alkenes
1) dehydrohalogenation
H
H
H
Br
H
KOH
H
CH3CH2OH
+ KBr +
H2O
2) dehydration
CH3
OH
H2SO4, H2O
o
THF, 50 C
CH3
+
H2O
49
Part 2 - Reaction of Alkenes
1) Addition of Halogens to Alkenes
X2
H2O
C C
X
C C
HO
+ HX
2) Halohydrin Formation
H
H
C C
H
H
+ Cl
Cl
Cl Cl
H C C H
H H
O
N Br
(NBS)
O
H2O, CH3SOCH 3(DMSO)
OH
Br
2-Bromo-1-phenylethanol (76%)
50
Part 2 - Reaction of Alkenes
3) Addition of Water to Alkenes
H
H B H
BH3
CH 3
H
CH 3
THF
1-methylcycopentene
-
H OH
OH
H
CH 3
H2O2
Alkylborane intermediate
trans-2-methylcyclopentanol
(85%)
4) Hydroboration-Oxidation Alcohol Formation
C C
H
+ O H
H
HO
H
H3PO4
250oC
H
H
H
5) Carbene Formation – Cyclopropane synthesis
+
Alkene
R1
C
R2
Carbene
R1
R2
C
Cyclopropane
51
Part 2 - Reaction of Alkenes
6) Catalytic Hydrogenation
7) Hydroxylation and Cleavage
C C
1. OsO4
HO
OH
2. NaHSO3
Alkene
A 1,2-diol
52
Part 3 - Breakdown of Alkenes
1) Ozonolysis
2) Permangante Oxidation
O
+ KMnO4
O
O
+
H
H
O
3) Periodic Acid Oxidation, Cleavage of 1,2-diols
53