Organic Chemistry
Second Edition
David Klein
Chapter 9
Addition Reactions and Alkenes
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e
9.1 Addition Reactions
• Addition is
the
opposite of
elimination
• A pi bond is
converted
to a sigma
bond
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9-2
Klein, Organic Chemistry 2e
9.1 Addition Reactions
• A pi bond will often act as a Lewis base (as a nucleophile
or as a Brønsted-Lowry base)
• Why are pi bonds more reactive in this sense than sigma
bonds?
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9-3
Klein, Organic Chemistry 2e
9.2 Addition / Elimination Equilibria
• Because an addition is the reverse of an elimination,
often the processes are at equilibrium
• An equilibrium is a thermodynamic expression
• We assess ΔG (the free energy) to determine which side
the equilibrium will favor
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9-4
Klein, Organic Chemistry 2e
9.2 Addition / Elimination Equilibria
• To determine which side the equilibrium will favor, we
must consider both enthalpy and entropy
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9-5
Klein, Organic Chemistry 2e
9.2 Addition / Elimination Equilibria
Bonds broken – bonds formed = 166 kcal/mol – 185 kcal/mol = –19 kcal/mol
• Typical addition reactions have a –ΔH. WHY?
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9-6
Klein, Organic Chemistry 2e
9.2 Addition / Elimination Equilibria
• Typical addition reactions have a –ΔH
• Will heat be absorbed by or released into the
surroundings?
• What will the sign (+/-) be for ΔSsurr?
• Will the enthalpy term favor the reactants or products?
• The heat change (ΔH) will remain roughly constant
regardless of temperature
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9-7
Klein, Organic Chemistry 2e
9.2 Addition / Elimination Equilibria
• Having a –ΔH (or a +ΔSsurr) favors the addition reaction
rather than the elimination reaction
• To get ΔG (or ΔStot) and make a complete assessment,
we must also consider the entropy of the system (ΔSsys)
• What will the sign (+/-) be for ΔSsys? WHY?
• What will the sign (+/-) be for -TΔSsys?
• Will the enthalpy term favor the reactants or products?
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9-8
Klein, Organic Chemistry 2e
9.2 Addition / Elimination Equilibria
• Plugging into the formula gives…
• To favor addition, a –ΔG (or a +ΔStot) is needed
• How can the temperature be adjusted to favor addition?
• To favor elimination (the reverse reaction in this
example), a +ΔG (or a –ΔStot) is needed
• How can the temperature be adjusted to favor
elimination?
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9-9
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation
• Note the temperature used in this addition reaction
• Does it matter whether the Br adds to the right side of
the C=C double bond or whether it adds to the left?
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9-10
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation
• Regiochemistry becomes important for asymmetrical
alkenes
• In 1869, Markovnikov showed that in general, H atoms
tend to add to the carbon already bearing more H atoms
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9-11
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation
• Markovnikov’s rule could also be stated by saying that in
general, halogen atoms tend to add to the carbon that is
more substituted with other carbon groups
• This is a regioselective reaction, because one
constitutional isomer is formed in greater quantity than
another
• Draw the structure of the minor product
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9-12
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation
• Anti-Markovnikov products are observed when
reactions are performed in the presence of peroxides
such as H2O2
• Why would some reactions follow Markovnikov’s rule,
while other reactions give Anti-Markovnikov products?
• The answer must be found in the mechanism
• Practice with conceptual checkpoint 9.1
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9-13
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation Mechanism
• The mechanism is a two step process
• Which step do you think is rate determining?
• Write a rate law for the reaction
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9-14
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation Mechanism
• Explain the
FREE energy
changes in
each step
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9-15
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation Mechanism
• Recall that there are two possible products,
Markovnikov and anti-Markovnikiv
• Which process looks more favorable? WHY?
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9-16
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation Mechanism
• Practice with SkillBuilder 9.1
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9-17
Klein, Organic Chemistry 2e
9.3 Stereochemical Aspects
• In many addition reactions, chirality centers are formed
• There are two possible Markovnikov products
• Which step in the mechanism determines the
stereochemistry of the product?
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9-18
Klein, Organic Chemistry 2e
9.3 Stereochemical Aspects
• Recall the geometry of the carbocation
• Practice with conceptual checkpoint 9.6
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9-19
Klein, Organic Chemistry 2e
9.3 Rearrangements
• Rearrangements (hydride or methyl shifts) occur for the
carbocation if the shift makes it more stable
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9-20
Klein, Organic Chemistry 2e
9.3 Rearrangements
• A mixture of products limits synthetic utility
• With an INTRAmolecular rearrangement, WHY isn’t the
rearrangement product an even greater percentage?
• How might [Cl-] be used to alter the ratio of products?
• Practice with SkillBuilder 9.2
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9-21
Klein, Organic Chemistry 2e
9.3 Hydrohalogenation Example
• Predict the major product(s) for the reaction below
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9-22
Klein, Organic Chemistry 2e
9.4 Hydration
• The components of water (-H and –OH) are added
across a C=C double bond
• The acid catalyst is often shown over the arrow, because
it is regenerated rather than being a reactant
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9-23
Klein, Organic Chemistry 2e
9.4 Hydration
• Given the data below, do you think the acid catalyzed
hydration goes through a mechanism that involves a
carbocation?
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9-24
Klein, Organic Chemistry 2e
9.4 Hydration Mechanism
• Why does the hydrogen add to this carbon of the
alkene?
• Mechanism continues on next slide
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9-25
Klein, Organic Chemistry 2e
9.4 Hydration Mechanism
• Could a stronger base help promote the last step?
• Practice with conceptual checkpoint 9.10
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9-26
Klein, Organic Chemistry 2e
9.4 Hydration Thermodynamics
• Similar to Hydrohalogenation, hydration reactions are
also at equilibrium
Addition
Elimination
• Explain HOW and WHY temperature could be used to
shift the equilibrium to the right or left
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9-27
Klein, Organic Chemistry 2e
9.4 Hydration Thermodynamics
• How could Le Châtelier’s principle be used to shift the
equilibrium to the right or left?
• Practice with conceptual checkpoint 9.11
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9-28
Klein, Organic Chemistry 2e
9.4 Hydration Thermodynamics
• Similar to Hydrohalogenation, the stereochemistry of
hydration reactions is controlled by the geometry of the
carbocation
• Draw the complete mechanism for the reaction above
to show WHY a racemic mixture is formed
• Practice with SkillBuilder 9.3
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9-29
Klein, Organic Chemistry 2e
9.4 Hydrations
• Ethanol is mostly produced from fermentation of sugar
using yeast, but industrial synthesis is also used to
produce ethanol through a hydration reaction
• Predict the major product(s) for the reaction below
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9-30
Klein, Organic Chemistry 2e
9.5 Oxymercuration-Demercuration
• Because rearrangements often produce a mixture of
products, the synthetic utility of Markovnikov hydration
reactions is somewhat limited
• Oxymercuration-demercuration is an alternative
process that can yeild Markovnikov products without
the possibility of rearrangement
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9-31
Klein, Organic Chemistry 2e
9.5 Oxymercuration-Demercuration
• Oxymercuration begins with mercuric acetate
• How would you classify the mercuric cation?
– As a nucleophile or an electrophile?
– As a Lewis acid or Lewis base?
• How might an alkene react with the mercuric cation?
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9-32
Klein, Organic Chemistry 2e
9.5 Oxymercuration-Demercuration
• Similar to how we saw the alkene attack a proton
previously, it can also attack the mercuric cation
• Resonance stabilizes the mercurinium ion and the
carbocation. Draw a reasonable resonance
hybrid
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9-33
Klein, Organic Chemistry 2e
9.5 Oxymercuration-Demercuration
• The mercurinium ion is also a good electrophile, and it
can easily be attacked by a nucleophile, even a weak
nucleophile such as water
• NaBH4 is generally used to replace the –HgOAc group
with a –H group via a free radical mechanism
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9-34
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• To achieve anti-Markovnikov hydration, HydroborationOxidation is often used
• Note that the process occurs in two steps
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9-35
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• Hydroboration-Oxidation reactions achieve syn addition
• Anti addition is NOT observed
• To answer WHY, we must investigate the mechanism
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9-36
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• Let’s examine how this new set of reagents might react
• The BH3 molecule is similar to a carbocation but not as
reactive, because it does not carry a formal charge
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9-37
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• Because of their broken octet, BH3 molecules undergo
intermolecular resonance to help fulfill their octets
• The hybrid that results from the resonance (diborane)
involves a new type of bonding called banana bonds
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9-38
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• In the hydroboration reaction, BH3•THF is used.
BH3•THF is formed when borane is stabilized with THF
(tetrahydrofuran)
• What general role do you think
BH3•THF is likely to play in a
reaction?
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9-39
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
Hydroboration
• Let’s examine the first step of the Hydroboration
mechanism on the next slide
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9-40
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• What evidence is there for a concerted addition of the
B-H bond across the C=C double bond?
• Use sterics and electronics to explain the regioselectivity
of the reaction
• Practice with conceptual checkpoint 9.17
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9-41
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
Oxidation
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9-42
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
Oxidation
Start
Here
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9-43
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• When ONE chirality center is formed, a racemic mixture
results
• WHY? What is the geometry of the alkene as the borane
attacks?
• The squiggle bond above shows two products, a 50/50
mixture of the R and the S enantiomer
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9-44
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• When TWO chirality centers are formed, a racemic
mixture results
• Why aren’t the other stereoisomers formed?
• Practice with SkillBuilder 9.4
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9-45
Klein, Organic Chemistry 2e
9.6 Hydroboration-Oxidation
• Predict the major product(s) for the reactions below
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9-46
Klein, Organic Chemistry 2e
9.7 Catalytic Hydrogenation
• The addition of H2 across a C=C double bond
• If a chirality center is formed, syn addition is observed
• Draw the stereoisomers
that are produced
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9-47
Klein, Organic Chemistry 2e
9.7 Catalytic Hydrogenation
• Analyze the energy diagram below
• Why is a catalyst
necessary?
• Does the catalyst affect
the spontaneity of the
process?
• Typical catalysts include
Pt, Pd, and Ni
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9-48
Klein, Organic Chemistry 2e
9.7 Catalytic Hydrogenation
• The metal catalyst is believed to both adsorb the H
atoms and coordinate the alkene
• The H atoms add to the same side of the alkene pi
system
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9-49
Klein, Organic Chemistry 2e
9.7 Catalytic Hydrogenation
• Draw product(s) for the reaction below. Pay close
attention to stereochemistry
• How many chirality centers are there in the alkene
reactant above?
• How does the term, mesocompound, describe the
product(s) of the reaction?
• Practice with SkillBuilder 9.5
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9-50
Klein, Organic Chemistry 2e
9.7 Catalytic Hydrogenation
• If catalysis takes place on the surface of a solid
surrounded by solution, the catalyst is heterogeneous.
WHY?
• Homogeneous catalysts also exist
• What advantage might a homogeneous catalyst have?
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9-51
Klein, Organic Chemistry 2e
9.7 Asymmetric Hydrogenation
• In 1968, Knowles modified Wilkinson’s
catalyst by using a chiral phosphine ligand
• A chiral catalyst can produce one desired enantiomer
over another. HOW?
• Why would someone want to synthesize one
enantiomer rather than a racemic mixture?
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9-52
Klein, Organic Chemistry 2e
9.7 Asymmetric Hydrogenation
• A chiral catalyst allows
one enantiomer to be
formed more frequently
in the reaction mixture
• Some chiral catalysts
give better
enantioselectivity than
others. WHY?
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9-53
Klein, Organic Chemistry 2e
9.7 Asymmetric Hydrogenation
• BINAP is a chiral ligand that gives pronounced
enantioselectivity
• For any reaction, stereoselectivity can only be achieved
if at least one reagent (reactant or catalyst) is chiral
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9-54
Klein, Organic Chemistry 2e
9.7 Asymmetric Hydrogenation
• Predict the major product(s) for the reactions below
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9-55
Klein, Organic Chemistry 2e
9.8 Halogenation
• Halogenation involves adding two halogen atoms across
a C=C double bond
• Halogenation is a key step in the production of PVC
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9-56
Klein, Organic Chemistry 2e
9.8 Halogenation
• Halogenation with Cl2 and Br2 is generally effective, but
halogenation with I2 is too slow and halogenation with
F2 is too violent
• Halogenation occurs with anti addition
• Given the stereospecificity, is it likely to be a concerted
or a multi-step process?
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9-57
Klein, Organic Chemistry 2e
9.8 Halogenation
• Let’s look at the reactivity of Br2. Cl2 is similar
• It is nonpolar, but it is polarizable. WHY?
• What type of
attraction exists
between the
Nuc:1- and Br2?
• Does the Br2
molecule have a
good leaving
group attached
to it?
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9-58
Klein, Organic Chemistry 2e
9.8 Halogenation
• We know alkenes can act as nucleophiles
• Imagine an alkene attacking Br2. You might imagine the
formation of a carbocation
• However, this mechanism DOES
NOT match the stereospecificity
of the reaction. HOW? WHY?
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9-59
Klein, Organic Chemistry 2e
9.8 Halogenation
• Mechanism continued on next slide
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9-60
Klein, Organic Chemistry 2e
9.8 Halogenation
• Only anti addition is observed. WHY?
• Prove to yourself that the products are enantiomers
rather than identical
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9-61
Klein, Organic Chemistry 2e
9.8 Halogenation
• Only anti addition is observed
• Can you design a synthesis for
?
• Practice with conceptual checkpoint 9.26
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9-62
Klein, Organic Chemistry 2e
9.8 Halogenation
• Predict the major product(s) for the reactions below
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9-63
Klein, Organic Chemistry 2e
9.8 Halohydrin Formation
• Halohydrins are formed when halogens (Cl2 or Br2) are
added to an alkene with WATER as the solvent
• The bromonium ion forms from Br2 + alkene, and then it
is attacked by water
• Why is the bromonium attacked by water rather than a
Br1- ion? Is water a better nucleophile?
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9-64
Klein, Organic Chemistry 2e
9.8 Halohydrin Formation
• A proton transfer completes the mechanism producing a
neutral halohydrin product
• The net reaction is the addition of –X and –OH across a
C=C double bond
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9-65
Klein, Organic Chemistry 2e
9.8 Halohydrin Regioselectivity
• The –OH group adds to the more substituted carbon
• The key step that determines regioselectivity is the
attack of water on the bromonium ion
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9-66
Klein, Organic Chemistry 2e
9.8 Halohydrin Regioselectivity
• When water attacks the bromonium ion, it will attack
the side that goes through the lower energy transition
state
Transition state
• Water is a small molecule that can easily access the
more sterically hindered site
• Practice with SkillBuilder 9.6
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9-67
Klein, Organic Chemistry 2e
9.8 Halohydrin Regioselectivity
• Predict the major product(s) for the reactions below
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9-68
Klein, Organic Chemistry 2e
9.9 Anti Dihydroxylation
• Dihydroxylation occurs when two –OH groups are added
across a C=C double bond
• Anti dihydroxylation is achieved through a multi-step
process
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9-69
Klein, Organic Chemistry 2e
9.9 Anti Dihydroxylation
• First, an epoxide is formed
• Replacing the relatively unstable O-O single bond is the
thermodynamic driving force for this process
• Is there anything unstable about an epoxide?
• Is an epoxide likely to react as a nucleophile (Lewis base)
or as an electrophile (Lewis acid)?
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9-70
Klein, Organic Chemistry 2e
9.9 Anti Dihydroxylation
• Water is a
poor
nucleophile,
so the
epoxide is
activated
with an acid
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9-71
Klein, Organic Chemistry 2e
9.9 Anti Dihydroxylation
• Note the similarities between three key intermediates
• Ring strain and a +1 formal charge makes these
structures GREAT electrophiles
• They also each yield anti products, because the
nucleophile must attack from the side opposite the
leaving group
• Practice with SkillBuilder 9.7
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9-72
Klein, Organic Chemistry 2e
9.10 Syn Dihydroxylation
• Like other syn additions, syn dihydroxylation adds across
the C=C double bond in ONE step
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9-73
Klein, Organic Chemistry 2e
9.10 Syn Dihydroxylation
• Because OsO4 is expensive and toxic, conditions have
been developed where the OsO4 is regenerated after
reacting, so only catalytic amounts are needed
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9-74
Klein, Organic Chemistry 2e
9.10 Syn Dihydroxylation
• MnO41- is similar to OsO4 but more reactive
• Syn dihydroxylation can be achieved with KMnO4 but
only under mild conditions (cold temperatures)
• Diols are often further oxidized by MnO41-, and MnO41- is
reactive toward many other functional groups as well
• The synthetic utility of MnO41- is limited
• Practice with conceptual checkpoint 9.33
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9-75
Klein, Organic Chemistry 2e
9.11 Oxidative Cleavage with O3
• C=C double bonds are also reactive toward oxidative
cleavage
• Ozonolysis is one such process
• Ozone exists as a resonance hybrid of two contributors
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9-76
Klein, Organic Chemistry 2e
9.11 Oxidative Cleavage with O3
• Common reducing agents include dimethyl sulfide and
Zn/H2O. Practice with SkillBuilder 9.8
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9-77
Klein, Organic Chemistry 2e
9.11 Oxidative Cleavage with O3
• Predict the major product(s) for the reaction below
• Predict a bicyclic reactant used to form the product
below
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9-78
Klein, Organic Chemistry 2e
9.12 Predicting Addition Products
1. Analyze the reagents used to determine what groups
will be added across the C=C double bond
2. Determine the regioselectivity (Markovnikov or antiMarkovnikov)
3. Determine the stereospecificity (syn or anti addition)
• Each step can be achieved with minor reagent
memorization and a firm grasp of the mechanistic
rational
• The more familiar you are with the mechanisms, the
easier predicting products will be
• Practice with SkillBuilder 9.9
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9-79
Klein, Organic Chemistry 2e
9.12 Predicting Addition Products
• Predict the major product(s) for the reaction below
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9-80
Klein, Organic Chemistry 2e
9.13 One Step Syntheses
•
•
To set up a synthesis, assess the reactants and
products to see what changes need to be made
Label each of the processes below
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9-81
Klein, Organic Chemistry 2e
9.13 One Step Syntheses
•
•
To set up a synthesis, assess the reactants and
products to see what changes need to be made
Give reagents and conditions for the following
•
Practice with SkillBuilder 9.10
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9-82
Klein, Organic Chemistry 2e
9.13 Multi-Step Syntheses
•
Multistep syntheses are more challenging, but the
same strategy applies
•
This is not a simple substitution, addition or
elimination, so two processes must be combined
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9-83
Klein, Organic Chemistry 2e
9.13 Multi-Step Syntheses
•
•
For the strategy to work, the regioselectivty must be
correct
A smaller base
should be used
to produce the
more stable
Zaitsev
product
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9-84
Klein, Organic Chemistry 2e
9.13 Multi-Step Syntheses
•
•
For the strategy to work, the regioselectivty must be
correct
Will the
regioselectivity for
the HBr reaction
give the desired
product?
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9-85
Klein, Organic Chemistry 2e
9.13 Multi-Step Syntheses
•
Multistep syntheses are more challenging, but the
same strategy applies
•
This is not a simple substitution, addition or
elimination, so two processes must be combined
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9-86
Klein, Organic Chemistry 2e
9.13 Multi-Step Syntheses
•
How can the alcohol be eliminated to give the less
stable Hoffmann product?
•
•
H3O+ will give the Zaitsev product
OH- is too poor of a leaving group to use the bulky
base, t-BuOK
The OH must first be converted to a better leaving
group, and then t-BuOK can be used
•
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9-87
Klein, Organic Chemistry 2e
9.13 Multi-Step Syntheses
•
In the last step, –H and –OH must be added across the
C=C double bond
•
Is the desired addition Markovnikov or antiMarkovnikov?
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9-88
Klein, Organic Chemistry 2e
9.13 Multi-Step Syntheses
•
Use reagents that give anti-Markovnikov products
•
Is stereochemistry an issue in this specific reaction?
•
Practice with SkillBuilder 9.11
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9-89
Klein, Organic Chemistry 2e
9.13 Multi-Step Syntheses
•
Solve the multistep syntheses below
•
Again, two processes must be combined
•
•
What reagents should be used?
Practice with SkillBuilder 9.12
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9-90
Klein, Organic Chemistry 2e
Additional Practice Problems
•
If you want to favor addition rather than elimination,
do you generally want a high or low temperature, and
why?
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9-91
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Predict the major product for the addition reaction
below. Be aware of possible rearrangements and
stereochemical concerns.
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9-92
Klein, Organic Chemistry 2e
Additional Practice Problems
•
How and why will the concentration of acid affect
whether an acid catalyzed hydration will favor products
or reactants at equilibrium?
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9-93
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Give an example reaction for Markovnikov hydration
without the possibility of rearrangement.
•
Give an example reaction for syn antiMarkovnikov
hydration.
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9-94
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Should a halogenation reaction be overall first or
second order kinetics? Also, Explain why it gives anti
addition rather than syn.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
9-95
Klein, Organic Chemistry 2e
Additional Practice Problems
•
What reagents are necessary to achieve the following
synthesis?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
9-96
Klein, Organic Chemistry 2e