Download (substituted) carbon

CHAPTER 12
Reactions of Alkenes
12-1 Why Addition Reactions Proceed:
Thermodynamic Feasibility
Because the C-C  bond is relatively weak, alkene chemistry is
dominated by its reactions.
The 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)
2
Most additions
to alkenes
should proceed
to products with
the release of
energy.
12-2 Catalytic Hydrogenation
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 (Adam’s catalyst, PtO2)
• Nickel (Raney nickel, Ra-Ni)
4
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.
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.
6
12-3 Nucleophilic Character of the  Bond: Electrophilic
Addition of Hydrogen Halides
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.
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.
8
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:
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.
As a result, the halogen attaches to the more substituted carbon.
This result is known as “Markovnikov’s rule” and is based on the
stability of the carbocation formed by the addition of proton.
10
Markovnikov’s rule can also be stated:
HX adds to unsymmetric alkenes in a way that the initial
protonation gives the more stable carbocation.
Product mixtures 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 mixture will be obtained.
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:
12
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.
12-4 Alcohol Synthesis by Electrophilic Hydration:
Thermodynamic Control
When other nucleophiles are present, they may also attack the
intermediate carbocation.
Electrophilic hydration results when an alkene is exposed to an
aqueous solution of sulfuric acid (HSO4- is a poor nucleophile).
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The addition of water by electrophilic hydration follows
Markovnikov’s rule, however carbocation rearrangements can
occur because water is a poor nucleophile.
The electrophilic hydration process is the reverse of the acidinduced elimination of water (dehydration) of alcohols previously
discussed.
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.
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The reversibility of alkene protonation leads to
alkene equilibration.
Protonation-deprotonation reactions interconvert alkenes to give
an equilibrium mixture of isomers. Under these conditions, a
reaction is said to be under thermodynamic control.
This mechanism can convert less stable
alkenes into their more stable isomers:
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12-5 Electrophilic Addition of Halogens to Alkenes
Halogen acts 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)
12-5 Electrophilic Addition of Halogens to Alkenes
Bromination takes place through anti addition.
Bromination of cyclohexene: no cis-1,2-dibromocyclohexane
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.
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With acyclic alkenes the reaction is cleanly stereospecific:
Cyclic bromonium ions explain the stereochemistry.
The polarizability of the Br-Br bond allows heterolytic cleavage
when attacked by a nucleophile, forming 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.
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12-6 The Generality of Electrophilic Addition
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.
Vicinal haloalcohols are useful synthetic intermediates.
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Vicinal haloethers can be produced
if an alcohol is used as the
solvent, rather than water.
Halonium ion opening can be regioselective.
Mixed additions to double bonds can be regioselective:
The nucleophile attacks the more highly substituted carbon of the
bromonium ion, because it is more positively polarized.
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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.
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:
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12-7 Oxymercuration-Demercuration: A Special
Electrophilic Addition
Electrophilic addition of a mercuric salt to an alkene is called
mercuration. The product is known as an alkylmercury derivative.
A reaction sequence known as “oxymercuration-demercuration” is
a useful alternative to acid-catalyzed hydration:
Oxymercuration is anti
stereospecific and
regioselective.
Alcohol obtained from
oxymercurationdemercuration 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.
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Oxymercuration-demercuration in an alcohol solvent yields an
ether:
12-8 Hydroboration-Oxidation: A Stereospecific
anti-Markovnikov Hydration
The boron-hydrogen bond adds across double bonds.
Borane, BH3, adds to double bonds without catalytic activation:
The borane is commercially
available in an ethertetrahydrofuran solvent.
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Because the borane is electron poor, and the alkene is e-rich, an
initial Lewis acid-base complex similar to the bromonium ion form:
Because of the four center transition state, the addition reaction is
syn. All three B-H bonds can react.
Hydroboration is regioselective as well as stereospecific
(syn addition).
Here, steric factors are more important than electronic factors.
Boron binds to the less hindered (substituted) carbon.
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The oxidation of alkylboranes gives alcohols.
The oxidation of a trialkylborane by hydrogen peroxide produces
an alcohol where a hydroxyl group has replaced the boron atom.
In this reaction, the hydroxyl group ends up at the less
substituted carbon: an anti-Markovnikov addition.
During the oxidation, alkyl group migrates with its electron pair
(with retention of configuration) to the neighboring O atom.
After all three alkyl groups have migrated to oxygen atoms, the
trialkyl borate is hydrolyzed by base to the alcohol and sodium
borate.
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Hydroboration-oxidation of alkenes allows stereospecific and
regioselective synthesis of alcohols.
The reaction sequence exhibits anti-Markovnikov regioselectivity
which complements acid-catalyzed hydration and oxymercurationdemercuration.
The reaction mechanism does not involve a carbocation and
thus rearrangements are not observed.