CHAPTER 12 Reactions of Alkenes

1
CHAPTER 12Reactions of Alkenes
2
Why Addition Reactions Proceed Thermodynamic
Feasibility
12-1
Because the C-C ? bond is relatively weak, alkene
chemistry is dominated by its reactions. Addition
of a reagent, A-B, to give a saturated compound
is the most common transformation of an alkene.
?Ho for the above reaction can be estimated from
the relevant bond energies ?Ho (DHo? bond
DHoA-B) (DHoC-A DHoC-B)
3
Most additions to alkenes should proceed to
products with the release of energy.
4
Catalytic Hydrogenation
12-2

• Hydrogenation takes place on the surface of a
  heterogeneous catalyst.
• In the absence of a catalyst, hydrogenations of
  alkenes, although exothermic, do not
  spontaneously occur, even at high temperatures.
• In the presence of a catalyst, the same
  hydrogenations proceed at a steady rate, even at
  room temperature.
• The most frequently used catalysts for
  hydrogenation reactions are
• Palladium dispersed on carbon (Pd-C)
• Collodial platinum (Adams catalyst, PtO2)
• Nickel (Raney nickel, Ra-Ni)

5
The primary function of a catalyst in
hydrogenation reactions is to provide metal-bound
hydrogen atoms on the catalyst surface.
Common solvents used for hydrogenations include
methanol, ethanol, acetic acid, and ethyl acetate.
6
Hydrogenation is stereospecific. During a
hydrogenation reaction, both atoms of hydrogen
are added to the same face of the double bond
(syn addition). In the absence of steric
hindrance, addition to either face of the double
bond can occur with equal probability which
results in a racemic mixture of products.
7
Nucleophilic Character of the Pi Bond
Electrophilic Addition of Hydrogen Halides
12-3
The ? electrons of a double bond are more loosely
held than those of the ? bond. As a result, the ?
electrons which extend above and below the
molecular plane of the alkene, can act as a
nucleophile in a manner similar to that of more
typical Lewis bases.
2,3-dimethylbutene
The electrophilic addition reactions of alkenes
can be both regioselective and stereospecific.
8
Electrophilic attack by protons gives
carbocations. A strong acid may add a proton to a
double bond to give a carbocation. This
reaction is simply the reverse of the last step
in an E1 elimination reaction, and has the same
transition state. At low temperatures, and with a
good nucleophile, an electrophilic addition
product is formed.
Typically, the gaseous HX (HCl, HBr, or HI) is
bubbled through the pure or dissolved alkene.
The reaction can also be carried out in a solvent
such as acetic acid.
9
The Markovnikov rule predicts regioselectivity in
electrophilic additions. The only product formed
during the reaction of propene with HCl is
2-chloropropane
Other addition reactions show similar results
10
If the carbon atoms participating in the double
bond are not equally substituted, the proton from
the hydrogen halide attaches itself to the less
substituted carbon. The halogen, as a result
attaches to the more substituted carbon. This
result is known as Markovnikovs rule and is
based on the stability of the carbocation formed
by the addition of the proton.
11
Markovnikovs rule can also be stated HX adds to
unsymmetric alkenes in a way that the initial
protonation gives the more stable
carbocation. Product mixures will be formed from
alkenes that are similarly substituted at both
sp2 carbon atoms. If addition to an achiral
alkene generates a chiral product, a racemic
mixure will be obtained.
12
Carbocation rearrangements may follow
electrophilic addition. In the absence of a good
nucleophile, a rearrangement of the carbocation
may occur prior to the addition of the
nucleophile. An example of such a rearrangement
is the addition of trifluoroacetic acid to
3-methyl-1-butene, where a hydride shift converts
a secondary carbocation into a more stable
tertiary carbocation
13

• The extent of carbocation rearrangement depends
  upon
• Alkene structure
• Solvent
• Strength and concentration of nucleophile
• Temperature
• Rearrangements are generally favored under
  strongly acidic, nucleophile-deficient conditions.

14
Alcohol Synthesis by Electrophilic Hydration
Thermodynamic Control
12-4
When other nucleophiles are present, they may
also attack the intermediate carbocation. Electrop
hilic hydration results when an alkene is exposed
to an aqueous solution of sulfuric acid (HSO4- is
a poor nucleophile).
15
The addition of water by electrophilic hydration
follows Markovnikovs rule, however carbocation
rearrangements can occur because water is a poor
nucleophile. The electrophilic hydration process
is the reverse of the acid induced elimination of
water (dehydration) of alcohols previously
discussed.
16
Alkene hydration and alcohol dehydration are
equilibrium processes.
All steps are reversible in the hydration of
alkenes. The proton serves as a catalyst only
it is regenerated in the reaction.
In the absence of protons, alkenes are stable in
water. The position of the equilibrium in the
hydration reaction can be changed by adjusting
the reaction conditions.
17
The reversibility of alkene protonation leads to
alkene equilibration. Protonation-deprotonation
reactions may interconvert related alkenes and
produce an equilibrium mixture of isomers. Under
these conditions a reactions is said to be under
thermodynamic control.
18
This mechanism can convert less stable alkenes
into their more stable isomers
19
Electrophilic Addition of Halogens to Alkenes
12-5
Halogen molecules also act as electrophiles with
alkenes giving vicinal dihalides.
The reaction with bromine results in a color
change from red to colorless, which is sometimes
used as a test for unsaturation. Halogenations
are best carried out at or below room temperature
and in inert halogenated solvents (i.e. -
halomethanes)
20
Electrophilic Addition of Halogens to Alkenes
12-5
Bromination takes place through anti
addition. Consider the bromination of
cyclohexene. No cis-1,2-dibromocyclohexane is
formed.
Only anti addition is observed. The product is
racemic since the initial attack of bromine can
occur with equal probability at either face of
the cyclohexene.
21
With acyclic alkenes the reaction is cleanly
stereospecific
22
Cyclic bromonium ions explain the
stereochemistry. The polarizability of the Br-Br
bond allows heterolytic cleavage when attacked by
a nucleophile, forming a cyclic bromonium ion
The bridging bromine atoms serves as the leaving
group as the bromonium ion is attacked from the
bottom by a Br- ion.
In symmetric bromonium ions, attack is equally
probable at either carbon atom leading to racemic
or meso products.
23
The Generality of Electrophilic Addition
12-6
The bromonium ion can be trapped by other
nucleophiles. Bromonation of cyclopentene using
water as the solvent gives the vicinal
bromoalcohol (bromohydrin).
The water molecule is added anti to the bromine
atom and the other product is HBr.
24
Vicinal haloalcohols are useful synthetic
intermediates.
25
Vicinal haloethers can be produced if an alcohol
is used as the solvent, rather than water.
26
Halonium ion opening can be retioselective. Mixed
additions to double bonds can be regioselective
The nucleophile attacks the more highly
substituted carbon of the bromonium ion, because
it is the more positively polarized.
27
Electrophilic additions of unsymmetric reagents
add in a Markovnikov-like fashion the
electrophilic unit becomes attached to the less
substituted carbon of the double bond. Mixtures
of products are formed only when the two carbons
are not sufficiently differentiated.
28
Reagents of the type A-B, in which A acts as the
electrophile, A, and B the nucleophile, B-, can
undergo stereo- and regiospecific addition
reactions to alkenes
29
Oxymercuration-Demercuration A special
Electrophilic Addition
12-7
The electrophilic addition of a mercuric salt to
an alkene is called mercuration. The product
formed is known as an alkylmercury derivative. A
reaction sequence known as oxymercuration-demercur
ation is a useful alternative to acid-catalyzed
hydration
30
Oxymercuration is anti stereospecific and
regioselective. The alcohol obtained from
oxymercuration-demercuration is the same as that
obtained from Markovnikov hydration, however
since no carbocation is involved in the reaction
mechanism, rearrangements of the transition state
do not occur.
31
Oxymercuration-demercuration in an alcohol
solvent yields an ether
32
Hydroboration-Oxidation A Stereospecific
Anti-Markovnikov Hydration
12-8
The boron-hydrogen bond adds across double
bonds. Borane, BH3, adds to double bonds without
catalytic activation
The borane is commercially available in an
ether-tetrahydrofuran solvent
33
Because the borane is electron poor, and the
alkene is electron rich, an initial Lewis
acid-base complex similar to the bromonium ion
can form
Because of the four center transition state, the
addition reaction is syn. All three B-H bonds
can react.
34
Hydroboration is regioselective as well as
stereospecific (syn addition). Here, steric
factors are mover important than electronic
factors. The boron binds to the less hindered
(substituted) carbon.
35
The oxidation of alkylboranes gives alcohols. The
oxidation of a trialkylborane by hydrogen
peroxide produces an alcohol in which the
hydroxyl group has replaced the boron atom. In
this reaction, the hydroxyl group ends up at the
less substituted carbon an anti-Markovnikov
addition.
36
During the oxidation, an alkyl group migrates,
with its electron pair (with retention of
configuration) to the neighboring oxygen atom.
After all three alkyl groups have migrated to
oxygen atoms, the trialkyl borate is hydrolyzed
by base to the alcohol and sodium borate.
37
Hydroboration-oxidation of alkenes allows
stereospecific and regioselective synthesis of
alcohols. The reaction sequence exhibits
anti-Marovnikov regioselectivity which
complements acid-catalyzed hydration and
oxymercuration-demercuration. The reaction
mechanism does not involve a carbocation and thus
rearrangements are not observed.
38
Diazomethane, Carbenes, and Cyclopropane Synthesis
12-9
Cyclopropanes can be readily prepared by the
addition of a carbene to the double bond of an
alkene. A carbene has the general structure
R2C, in which the central carbon is surrounded
by six electrons (sextet), and is thus electron
deficient. The electron deficient carbene readily
adds to an electron rich alkene.
39
Diazomethane forms methylene, which converts
alkenes into cyclopropanes. The highly reactive
species methylene H2C (the simplest carbene)
can be produced from the decomposition of
diazomethane
40
When methylene is generated in the presence of an
alkene, an addition reaction occurs producing a
cyclopropane. This reaction is usually
stereospecific, with retention of the original
double bond configuration.
41
Halogenated carbenes and carbenoids also give
cyclepropanes. Halogenated carbenes, prepared
from halomethanes, can also be used to synthesize
cyclopropanes. Treatment of trichloromethane
(chloroform) with strong base causes an
elimination reaction in which both a proton and a
chlorine atom are removed from the same carbon.
The resulting product is a dichlorocarbene which
reacts with alkenes to produce cyclopropanes.
42
To avoid the hazards associated with diazomethane
preparation, an alternate route using
diiodomethane and zinc (Simmons-Smith reagent) to
produce ICH2ZnI is used. This substance is an
example of a carbenoid a carbenelike substance
that converts alkenes into cyclopropanes
stereospecifically.
43
Oxacyclopropane (Epoxide) Synthesis Epoxidation
by Peroxycarboxylic Acids
12-10
Oxacyclopropanes contain a single oxygen atom
connected to two carbons to form a three membered
ring. Oxacyclopropanes may be converted into
vicinal anti diols.
44
Peroxycarboxylic acids deliver oxygen atoms to
double bonds. Peroxycarboxylic acids have the
general formula
These compounds react with double bonds because
one of the oxygen atoms is electrophilic. The
resulting products are an oxacyclopropane and a
carboxylic acid.
45
This reaction is referred to as an epoxidation
the older common name of an oxacyclopropane was
an epoxide. Commonly used peroxycaraboxylic acids
for this reaction are meta-chloroperoxybenzoic
acid (MCPBA) which is somewhat shock sensitive,
and magnesium monoperoxyphthalate (MMPP).
46
The mechanism of this epoxidation reaction
involves a cyclic transition state
The peroxycarboxylic acid reactivity with double
bounds increases with alkyl substitution,
allowing for selective oxidations
47
Hydrolysis of oxacyclopropanes furnishes the
products of anti dihydroxylation of an
alkene. Ring opening of oxacyclopropanes with
water produces anti vicinal diols.
48
Vicinal Syn Dihydroxylation with Osmium Tetroxide
12-11
The reaction of osmium tetroxide with alkenes
yields syn vicinal diols in a two step process
49
The reaction mechanism involves the concerted
addition of the osmium tetroxide to the ? bond of
the alkene
Catalytic amounts of osmium tetroxide in the
presence of an oxidizing agent (H2O2) to
regenerate the spent osmium tetroxide are often
used, due to the expense and toxicity of OsO4.
50
An older reagent for vicinal syn dihydroxylation
of alkenes is KMnO4.
This reagent is less useful than OsO4 because of
its tendency towards overoxidation. The deep
purple KMnO4 is converted into a brown
precipitate, (MnO2) during the reaction which can
serve as a useful test for the presence of
alkenes.
51
Oxidative Cleavage Ozonolysis
12-12
The mildest reagent capable of breaking both the
? and ? bonds in a double bond is ozone, O3.
This process is known as ozonolysis. Ozone is
produced by an electrical discharge in dry oxygen
in a instrument called an ozonator. The initial
product of the reaction of ozone with an alkene
is an ozonide which is then directly reduced to
two carbonyl products.
52
The mechanism of ozonolysis proceeds through a
molozonide which breaks apart into two fragments
which then recombine to form the ozonide
53
Radical Additions Anti-Markovnikov Product
Formation
12-13
Hydrogen bromide can add to alkenes in
anti-Markovnikov fashion a change in
mechanism. The reaction products from the
treatment of 1-butene with HBr depend upon the
presence or absence of molecular oxygen in the
reaction mixture
54
In the presence of oxygen, a radical chain
sequence mechanism leads to the anti-Markovnikov
product. Small amounts of peroxides (RO-OR) are
formed in alkene samples stored in the presence
of air (O2). The peroxides initiate the radical
chain sequence mechanism, which is much faster
than the ionic mechanism operating in the absence
of peroxides.
55
The halogens attack is regioselective,
generating the more stable secondary radical
rather than the primary one. The alkyl radical
subsequently abstracts a hydrogen from HBr which
regenerates the chain-carrying bromine atom. Both
propagation steps are exothermic. Termination is
by radical recombination or by some other removal
of the chain carriers. Commonly used peroxides
for initiating radical additions include
56
Are radical additions general? HCl and HI do not
give anit-Markovnikov addition products with
alkenes. The chain propogation steps involving
these hydrogen halides are endothermic which
leads to very slow reactions and chain
termination. HCl and HI give Markovnikov products
by ionic mechanisms irregardless of the presence
of radicals. Other reagents, such as thiols, do
lundergo successful radical additions to alkenes
57
Dimerization, Oligomerization, and Polymerization
of Alkenes
12-14
Alkenes can react with one another in the
presence of an appropriate catalyst an acid, a
radical, a base, or a transition metal. Polymer
synthesis is of great industrial importance
58
Carbocations attack pi bonds. Protonation of
2-methylpropene by hot aqueous sulfuric acid
leads to the formation of two dimers
59
The initial protonation produces a
1,1-dimethylethyl (tert-butyl) cation which then
attacks the double bond of a second
2-methylpropene molecule.
The cation addition proceeds according to the
Markovnikov rule to generate the more stable
carbocation. Deprotonation of the addition
product from either adjacent carbon leads to a
mixture of two products.
60
Repeated attack can lead to oligomerization and
polymerization. When 2-methylpropene is treated
with mineral acid under more stringent
conditions, higher oligiomers can through
obtained repeated addition reactions
61
At higher temperatures, polymers containing many
subunits are formed.
62
Synthesis of Polymers
12-15
Polymerization reactions can be categorized as
cationic, radical, anionic, and metal
catalyzed. Acid catalyzed cationic
polymerizations have already been covered.
Initiators include H2SO4, HF, and BF3.
63
Radical polymerizations lead to commercially
useful materials. The polymerization of ethene in
the presence of an organic peroxide at high
pressures and temperatures proceeds by a radical
polymerization process.
64
Polyethene (polyethylene) polymerized in this way
is actually a branched polymer. Branching occurs
as a result of hydrogen abstraction along the
growing chain by another radical center.
The average molecular weight of polyethene is
almost 1 million.
65
Polychloroethene (PVC or polyvinylchloride) is a
polymer of chloroethene (vinyl chloride). The
peroxide initiator and the intermediate radical
chains add only to the unsubstituted end of the
monomer (producing the most stable radical) which
results in a very regular head-to-tail structure
of molecular weight over 1.5 million.
Pure PVC is fairly hard and brittle. It can be
softened by the addition of carboxylic acid
esters (plasticizers) for use in elastic
materials such as vinyl leather, plastic covers,
and garden hoses.
66
Polypropenenitrile (polyacrylonitrile) can be
prepared from propenenitrile (acrylonitrile)
using hydrogen peroxide with FeSO4 as a
catalyst. Polypropenenitrile, -(CH2CHCN)n-, also
known as Orlon, is used to make fibers.
67
Anionic polymerizations require initiation by
bases. Anionic polymerizations are initiated by
strong bases such as alkyllithiums, amides,
alkoxides, and hydroxide. The adhesive properties
of Super Glue result from the hydroxide
initiated polymerization of 2-cyanopropenoate.
The electron withdrawing natures of the carbonyl
and nitrile groups create a partially positive
carbon center at which the hydroxide can
initially attack. The negative charge on the
resulting anion is then resonance stabilized by
both the carbonyl and nitrile groups.
68
Metal-catalyzed polymerizations produce highly
regular chains. Ziegler-Natta catalysts are
important initiators for metal-catalyzed
polymerizations. They are typically made from
titanium tetrachloride and a trialkylaluminum
such as Al(CH2CH3)3. Polymers produced using a
Ziegler-Natta catalyst are characterized by
regularity of construction and high linearity.
This results in much higher density and strength
than similar polymers obtained from radical
polymerization.
69
Ethene An Important Industrial Feedstock
12-16
Ethene is the basis for the production of
polyethene (polyethylene). The major source of
ethene is the pyrolysis of petroleum, or
hydrocarbons derived from natural gas. Ethene is
the starting material for the production of many
other industrial chemicals
70
Alkenes in Nature Insect Pheromones
12-17
Pheromones are chemical substances used for
communication within a living species.
Pheromones are used for sex, trail, alarm, and
defense signaling, to name a few uses.
71
The sex attractant for the male silkworm moth is
10-trans-12-cis-hexadecadien-1-ol (bombykol).
The natural pheromome is 10 billion times more
active in eliciting a response than is the
10-cis-12-trans isomer, and 10 trillion times
more active than the trans, trans isomer.
72
Important Concepts
12

• Double Bond Reactivity - Exothermic addition
  reactions leading to saturated products.
• Hydrogenation of Alkenes - Immeasurably slow
  unless a catalyst is used.
• Palladium on carbon, PtO2, Raney nickel.
• H2 preferentially added to the least hindered
  face of the double bond.
• ? Bond - Attacked by acid and electrophiles.
• If the initial intermediate is a carbocation, the
  more highly substituted carbocation is formed.
  (Markovnikovs Rule)
• If the initial intermediate is cyclic onium ion,
  nucleophilic ring opening is at the more
  substituted carbon. (Control of both regio- and
  stereochemistry)

73
Important Concepts
12

• Hydroboration - Mechanistically between
  hydrogenation and electrophilic addition.
• Step 1 ? complexation to boron.
• Step 2 concerted transfer of hydrogen to
  carbon.
• Hydroboration-oxidation Anti-Markovnikov
  hydration of alkenes.
• Carbenes and Carbenoids - Useful for synthesis
  of cyclopropanes from alkenes.
• Peroxycarboxylic Acids Contains oxygen atom
  transferable to alkenes to give oxacyclopropanes
  (epoxidation).
• Osmium Tetroxide Addition to alkenes in a
  concerted syn manner to give vicinal diols.

74
Important Concepts
12

• Ozonolysis - When followed by reduction, yields
  carbonyl compounds by cleavage of the double
  bond.
• Radical Chain Additions To Alkenes -
• Chain carrier adds to the ? bond to form the more
  highly substituted radical.
• Allows for anti-Markovnikov hydrobromination of
  alkenes.
• Allows for the addition of thiols and some
  halomethanes.
• Polymers - Alkenes react with themselves to form
  polymers.
• Initiation by charged species, radicals or some
  transition metals.