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7/27/2019 Chapter7 Alkenes Rxn& Synthesis
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7. Alkenes: Reactions
and Synthesis
Based on McMurry’s Organic Chemistry , 7th edition
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Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful
products by addition (often through special reagents)
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Why this chapter? To begin a systematic description of major
functional groups
Begin to focus on general principles and
patterns of reactivity that tie organicchemistry
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7.1 Preparation of Alkenes: A Preview
of Elimination Reactions Alkenes are commonly made by
elimination of HX from alkyl halide(dehydrohalogenation)
Uses heat and KOH
elimination of H-OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 ºC)
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7.2 Addition of Halogens to
Alkenes Bromine and chlorine add to alkenes to give 1,2-dihaldes,
an industrially important process
F2 is too reactive and I2 does not add
Cl2 reacts as Cl+ Cl-
Br 2 is similar
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Addition of Br 2 to Cyclopentene Addition is exclusively trans
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Mechanism of Bromine
Addition Br + adds to an alkene producing a cyclic ion
Bromonium ion, bromine shares charge with carbon
Gives trans addition
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Bromonium Ion Mechanism 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
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The Reality of Bromonium Ions Bromonium ions were postulated more than 60 years ago
to explain the stereochemical course of the addition (togive the trans-dibromide from a cyclic alkene
Olah showed that bromonium ions are stable in liquid SO2with SbF5 and can be studied directly
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7.3 Addition of Hypohalous Acids
to Alkenes: Halohydrin Formation This is formally the addition of HO-X to an alkene to give a
1,2-halo alcohol, called a halohydrin
The actual reagent is the dihalogen (Br 2 or Cl2 in water in
an organic solvent)
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Mechanism of Formation of a
Bromohydrin Br 2 forms bromonium ion,
then water adds
Orientation toward stable
C+ species
Aromatic rings do not
react
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An Alternative to Bromine Bromine is a difficult reagent to use for this reaction
N-Bromosuccinimide (NBS) produces bromine in organicsolvents and is a safer source
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7.4 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 industrialprocesses: ethylene is converted to ethanol
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Oxymercuration Intermediates For laboratory-scale hydration of an
alkene
Use mercuric acetate in THF followedby sodium borohydride
Markovnikov orientation via mercurinium ion
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7.5 Addition of Water to
Alkenes: Hydroboration Herbert Brown (HB) invented hydroboration (HB)
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Addition of H-BH2 (from BH3-THF complex) to threealkenes gives a trialkylborane
Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene
Hydroboration-Oxidation Forms
an Alcohol from an Alkene
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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 sameface of the alkene (opposite of anti addition)
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Mechanism of Hydroboration Borane is a Lewis acid
Alkene is Lewis base
Transition state involves
anionic development on B
The components of BH3
are added across C=C
More stable carbocation is
also consistent with steric
preferences
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7.6 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 add symmetrically across double bonds to form
cyclopropanes
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Formation of Dichlorocarbene Base removes proton
from chloroform
Stabilized carbanionremains
Unimolecular elimination of Cl- giveselectron deficientspecies,
dichlorocarbene
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Reaction of Dichlorocarbene Addition of dichlorocarbene is stereospecific cis
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Simmons-Smith Reaction Equivalent of addition of CH2:
Reaction of diiodomethane with zinc-copper alloyproduces a carbenoid species
Forms cyclopropanes by cycloaddition
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7.7 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)
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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
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Mechanism of Catalytic
Hydrogenation Heterogeneous – reaction between phases
Addition of H-H is syn
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7.8 Oxidation of Alkenes:
Epoxidation and Hydroxylation Epoxidation results in a cyclic ether with an oxygen atom
Stereochemistry of addition is syn
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Osmium Tetroxide Catalyzed
Formation of Diols Hydroxylation - converts to syn-diol
Osmium tetroxide, then sodium bisulfate
Via cyclic osmate di-ester
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7.9 Oxidaton of Alkenes:Cleavage to
Carbonyl Compounds Ozone, O3, adds to alkenes to form molozonide
Reduce molozonide to obtain ketones and/or aldehydes
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Examples of Ozonolysis of
Alkenes Used in determination of structure of an unknown alkene
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Permangante 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
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Cleavage of 1,2-diols Reaction of a 1,2-diol with periodic ( per-iodic ) acid, HIO4 ,
cleaves the diol into two carbonyl compounds
Sequence of diol formation with OsO4 followed by diol
cleavage is a good alternative to ozonolysis
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7.10 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
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Free Radical Polymerization:
Initiation Initiation - a few radicals are generated by the reaction of
a molecule that readily forms radicals from a nonradical
molecule
A bond is broken homolytically
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Polymerization: Propagation Radical from initiation adds to alkene to generate alkene
derived radical
This radical adds to another alkene, and so on many times
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Polymerization: Termination Chain propagation ends when two radical chains combine
Not controlled specifically but affected by reactivity andconcentration
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Other Polymers Other alkenes give other common polymers
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7.11 Biological Additions of
Radicals to Alkenes Severe limitations to the usefulness of radical addition
reactions in the lab
In contrast to electrophilic additions, reactive intermediate
is not quenched so it reacts again and again uncontrollably
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Biological Reactions Biological reactions different than in the laboratory
One substrate molecule at a time is present in the
active site of an enzyme
Biological reactions are more controlled, morespecific than other reactions
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