7. Alkenes: Reactions and
Synthesis
Based on McMurry’s Organic Chemistry, 6th edition
©2003 Ronald Kluger
Department of Chemistry
University of Toronto
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)
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
2
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)
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
3
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 Br2 is similar
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
4
Addition of Br2 to Cyclopentene
 Addition is exclusively trans
+
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
5
Mechanism of Bromine Addition
 Br+ adds to an alkene producing a cyclic ion
 Bromonium ion, bromine shares charge with carbon

Gives trans addition
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
6
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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
7
The Reality of Bromonium Ions
 Bromonium were postulated more than 60 years ago to
expain the stereochemical course of the addition (to give
the trans-dibromide from a cyclic alkene
 Olah showed that bromonium ions are stable in liquid SO2
with SbF5 and can be studied directly
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
8
7.3 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)
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
9
Mechanism of Formation of a
Bromohydrin
 Br2 forms bromonium ion, then water adds


Orientation toward stable C+ species
Aromatic rings do not react
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
10
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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
11
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 industrial
processes: ethylene is converted to ethanol
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
12
Oxymercuration Intermediates
 For laboratory-scale hydration of an
alkene
 Use mercuric acetate in THF followed
by sodium borohydride
 Markovnikov orientation
 via mercurinium ion
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
13
7.5 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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
14
BH3 Is a Lewis Acid
 Six electrons in outer shell
 Coordinates to oxygen electron pairs in ethers
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
15
Hydroboration-Oxidation Forms an Alcohol
from an Alkene
 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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
16
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)

McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
17
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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
18
Hydroboration: Orientation in
Addition Step
 Addition in least crowded orientation, syn
 Addition also is via most stable carbocation
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
19
Hydroboration, Electronic Effects Give
Non-Markovnikov
 More stable carbocation is also consistent with steric
preferences
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
20
Hydroboration - Oxygen Insertion
Step
 H2O2, OH- inserts OH in place of B
 Retains syn orientation
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
21
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 symmetrically across double bonds to form
cyclopropanes
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
22
Formation of Dichlorocarbene
 Base removes proton
from chloroform
 Stabilized carbanion
remains
 Unimolecular
Elimination of Cl- gives
electron deficient
species,
dichlorocarbene
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
23
Simmons-Smith Reaction
 Equivalent of addition of CH2:
 Reaction of diiodomethane with zinc-copper alloy
produces a carbenoid species
 Forms cyclopropanes by cycloaddition
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
24
Reaction of Dichlorocarbene
 Addition of dichlorocarbene is stereospecific cis
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
25
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)
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
26
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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
27
Mechanism of Catalytic
Hydrogenation
 Heterogeneous – reaction between phases
 Addition of H-H is syn
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
28
7.8 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)
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
29
Osmium Tetroxide Catalyzed Formation
of Diols
 Hydroxylation - converts to syn-diol
 Osmium tetroxide, then sodium bisulfate
 Via cyclic osmate di-ester
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
30
Alkene Cleavage: Ozone
 Ozone, O3, adds to alkenes to form molozonide
 Reduce molozonide to obtain ketones and/or aldehydes
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
31
Examples of Ozonolysis of Alkenes
 Used in determination of structure of an unknown alkene
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
32
Structure Elucidation With Ozone
 Cleavage products reveal an alkene’s structure
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
33
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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
34
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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
35
Mechanism of Periodic Acid
Oxidation
 Via cyclic periodate intermediate
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
36
7.9 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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
37
Biological Hydration Example
 Fumarate to malate catalyzed by fumarase
 Specific for trans isomer
 Addition of H, OH is anti
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
38
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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
39
Free Radical Polymerization of
Alkenes
 Alkenes combine many times to give polymer

Reactivity induced by formation of free radicals
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
40
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
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
41
Polymerization: Propagation
 Radical from intiation adds to alkene to generate alkene
derived radical
 This radical adds to another alkene, and so on many times
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
42
Polymerization: Termination
 Chain propagation ends when two radical chains combine
 Not controlled specifically but affected by reactivity and
concentration
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
43
Other Polymers
 Other alkenes give other common polymers
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
44
Unsymmetrical Monomers
 If alkene is unsymmetrical, reaction is via more highly
substituted radical
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
45
Chain Branching During
Polymerization
 During radical propagation chain can develop forks leading
to branching
 One mechanism of branching is short chain branching in
which an internal hydrogen is abstracted
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
46
Long Chain Branching
 In long chains, a hydrogen from another chain is
abstracted
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
47
Cationic Polymerization
 Vinyl monomers react with Brønsted or Lewis
acid to produce a reactive carbocation that
adds to alkenes and propagates via
lengthening carbocations
McMurry Organic Chemistry 6th edition Chapter
7 (c) 2003
48