Chapter 5. Alkenes and Alkynes II: Reactions. Elimination Reactions.
Chapter 9 Addition Reactions of Alkenes '13 BW(1)
Transcript of Chapter 9 Addition Reactions of Alkenes '13 BW(1)
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Chapter 9. Addition Reactions of Alkenes
Junha Jeon
Department of Chemistry
University of Texas at Arlington
Arlington, Texas 76019Chem 2321, Fall 123
9.1 Addition Reactions: A Pi Bond is Converted to a Sigma Bond
Various Addition Reactions Various Addition Reactions
9.2 Addition vs. Elimination: Thermodynamics
in equilibrium (thermodynamics)
Addition vs. Elimination: A Thermodynamic Perspective
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Addition vs. Elimination: A Thermodynamic Perspective Addition vs. Elimination: A Thermodynamic Perspective
Addition vs. Elimination: A Thermodynamic Perspective Addition vs. Elimination: A Thermodynamic Perspective
Addition vs. Elimination: A Thermodynamic Perspective 9.3 Hydrohalogenation
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Hydrohalogenation
Regiochemistry!!
9.3 Hydrohalogenation
Markovnikov Addition
9.3 Hydrohalogenation
Markovnikov Addition
Hydrogen tends to add to the carbon already bearing more H atoms.
9.3 Hydrohalogenation
Markovnikov Addition
Hydrogen tends to add to the carbon already bearing more H atoms.
Hydrohalogenation
Markovnikov Addition
Halogen is generally placed at the more substituted position.
Hydrohalogenation
Regioselective!
Markovnikov Addition
Halogen is generally placed at the more substituted position.
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Hydrohalogenation
Of course,
Anti-Markovnikov Addition
in Chapter 11
Hydrohalogenation
Mechanism of Hydrohalogenation
Mechanism of Hydrohalogenation
Two Mechanistic Pathways Two Mechanistic Pathways
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Two Mechanistic Pathways Mechanistic Pathways
Which transition state (TS) is
more stable? And why?
Mechanistic Pathways
Which transition state (TS) ismore stable? And why?Hammond Postulate
Recall the Hammond Postulate
The structure of a transition state resembles the structure of the nearest stablespecies. Transition states for endergonic steps structurally resemble products,and transition states for exergonic steps structurally resemble reactants.
Mechanistic Pathways
Which transition state (TS) ismore stable? And why?Hammond Postulate
Mechanistic Pathways
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Ionic Stepwise Reaction: Markovnikov Addition (Regioselectivity)
Ea
carbocationcharacters
Ionic Stepwise Reaction: Markovnikov Addition (Regioselectivity)
carbocationcharacters
The take home message is that the regioselectivity of an ionic addition
reaction is determined by the preference for the reaction to proceed
through the more stable carbocation intermediate: Markovnikov addition
Ionic Stepwise Reaction: Markovnikov Addition (Regioselectivity)
carbocationcharacters
The take home message is that the regioselectivity of an ionic addition
reaction is determined by the preference for the reaction to proceed
through the more stable carbocation intermediate: Markovnikov addition
Markovnikov Addition:
Hydrogen tend to add to the carbon already bearing more H atoms.
Halogen is generally placed at the more substituted position.
Stereochemistry of Hydrohalogenation
Stereochemistry of Hydrohalogenation
Two enantiomers as a racemic mixture are produced
Stereochemistry: Carbocation Intermediate
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Acid-Catalyzed Hydration: Markovnikov Addition Mechanism and Source of Regioselectivity
Recall: Addition vs. Elimination: A Thermodynamic Perspective Acid-Catalyzed Hydration
Le Chateliers Principle:
If a chemical system at equilibrium experiences a change in concentration,
temperature, volume, or partial pressure, thenthe equilibrium shifts to
counteract the imposed change and a new equilibrium is established.
Acid-Catalyzed Hydration: Stereochemistry Observation Recall Stereochemistry: Carbocation Intermediate
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Oxymercuration Demercuration
OxymercurationDemercuration:
1. Markovnikov Addition
2. Not readily undergo carbocation rearrangements
9.6 HydroborationOxidation
1. Regioselectivity: Anti-Markovnikov Addition
2. Stereospecificity: Syn Addition
9.6 HydroborationOxidation
1. Regioselectivity: Anti-Markovnikov Addition
2. Stereospecificity: Syn Addition
HydroborationOxidation: Boron Isoelectronic to Carbocation
CH3+
BH3
The BH3molecule is similar to a carbocation but not as reactive,
because it does not carry a formal charge.
Diborane
The BH3molecule is similar to a carbocation but not as reactive, because it does
not carry a formal charge. But,
because of their broken octet, BH3molecules is still reactive and undergo
intermolecular resonance to help fulfill their octets.
Diborane
The BH3molecule is similar to a carbocation but not as reactive, because it does
not carry a formal charge. But,
because of their broken octet, BH3molecules is still reactive and undergo
intermolecular resonance to help fulfill their octets.
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Diborane: Three-center, Two-electron Bonds
Three-center, Two-electron Bonds
Stabilizing Borane: Diborane via Three-center, Two-electron Bonds
B
H
HHB
H
HH +
H
B
H
B
H
HH
H
Stabilizing Borane: Ate Complex HydroborationOxidation: Mechanism
1. Regioselectivity: Anti-Markovnikov Addition
2. Stereospecificity: Syn Addition
HydroborationOxidation: Mechanism
1. Regioselectivity: Anti-Markovnikov Addition
2. Stereospecificity: Syn Addition
HydroborationOxidation: Mechanism Electronic Considerations
1.Regioselectivity: Anti-Markovnikov Addition
2. Stereospecificity: Syn Addition
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HydroborationOxidation: Mechanism Steric Considerations
1. Regioselectivity: Anti-Markovnikov Addition
2. Stereospecificity: Syn Addition
Oxidation Mechanism
Oxidation Mechanism Oxidation Mechanism
HydroborationOxidation: Stereochemistry HydroborationOxidation: Stereochemistry
to the board
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9.7 Catalytic Hydrogenation Catalytic Hydrogenation: Stereospecificity
# The addition of H2across a C=C double bond:
# If a chirality center is formed, SYN addition is observed.
Catalytic Hydrogenation: The Role of the Catalyst Catalytic Hydrogenation: Catalysts Pt, Pd, Ni
Catalytic Hydrogenation: Catalysts Pt, Pd, Ni Yes, IRONMAN!! Pt, Pd, NiPalladium Arc Reactor
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Pt, Pd, NiPalladium Arc ReactorAre you kidding? Molecular Hydrogen Bound to Catalyst and Breaking HH bonds
Alkene Bound to Catalyst One H Atom Tranfer: Partially Reduced Intermediate
A Second H Transfer: Fully Reduced Alkene SynAddition Homogeneous Catalysts
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Homogeneous Catalysts Homogeneous Catalysts
Skip this section: A Chiral Phosphine Ligand Skip this section: Asymmetric Hydrogenation
NH2
OH
O
NH2
OH
O
OH
HO HO
OH
L-dopafor
Parkinson's disease
(S)catalyst
H2
Skip this section: Asymmetric Hydrogenation 9.8 Halogenation and Halohydrin Formation
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Halogenation
# Halogenation with Cl2and Br2 is generally effective, but halogenation
with I2is too slow, and halogenation with F2is too violent.
# Halogenation occurs with ANTI addition# Given the stereospecificity, is it likely to be a concerted or a multi-step
process?
Halogenation
# Halogenation with Cl2and Br2 is generally effective, but halogenation
with I2is too slow, and halogenation with F 2is too violent.
# Halogenation occurs with ANTI addition# Given the stereospecificity, is it likely to be a concerted or a multi-step
process?
Halogenation
# Halogenation with Cl2and Br2 is generally effective, but halogenation
with I2is too slow, and halogenation with F2is too violent.
# Halogenation occurs with ANTI addition.# Given the stereospecificity, is it likely to be a concerted or a multi-step
process?
Halogenation
# Halogenation with Cl2and Br2 is generally effective, but halogenation
with I2is too slow, and halogenation with F 2is too violent.
# Halogenation occurs with ANTI addition.# Given the stereospecificity, is it likely to be a concerted or a multi-step
process?
Halogenation: London Dispersion Forces (Induced Dipole Moment) Halogenation: London Dispersion Forces (Induced Dipole Moment)
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Halogenation Should be Anti Addition??? Halogenation: Formation of Bromonium Ion
Halogenation: AntiAddition Halogenation: AntiAddition
Stereochemistry: E and Z alkenes Stereochemistry: E and Z alkenes
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Halohydrin: Intercepting the Bromonium Intermediate
# Halohydrins are formed when halogens (Cl2or 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.
Halohydrin: Intercepting the Bromonium Intermediate
Halohydrin Halohydrin: Regioselectivity
Halohydrin: Regioselectivity Halohydrin: Regioselectivity
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Halohydrin: Regioselectivity Stable Transition State: Partial C+ Halohydrin: Regioselectivity Stable Transition State: Steric?
Water is a small
molecule that can
easily access the
more sterically
hindered site.
9.9 AntiDihydroxylation AntiDihydroxylation
AntiDihydroxylation Formation of an Epoxide
#
Replacing the relatively unstable OO single bond is the thermodynamicdriving force for this process.
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Peroxy Acid (RCO3H)
# Replacing the relatively unstable OO single bond is the thermodynamic
driving force for this process.
Formation of an Epoxide
Water is apoornucleophile,
so theepoxide isactivated
with an acid.
Key Intermediates 9.10 SynDihydroxylaton: Concerted Process
SynDihydroxylaton SynDihydroxylaton# Because OsO4is expensive and toxic, conditions have been developed where
the OsO4is regenerated after reacting, so only catalytic amounts are needed.
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SynDihydroxylaton
# MnO41-is similar to OsO4but more reactive.
# SYN dihydroxylation can be achieved with KMnO4but only under mild conditions
(cold temperatures).# Diols are often further oxidized by MnO4
1-, and MnO41- is reactive toward many
other functional groups as well.
# The synthetic utility of MnO41- is limited.
9.11 Oxidative Cleavage of Alkenes by Ozonolysis
Ozone exists as a resonance hybrid of two contributors Ozone exists as a resonance hybrid of two contributors
Common reducing agents include dimethyl sulfide (Me2S) and Zn.
Ozone exists as a resonance hybrid of two contributors
Common reducing agents include dimethyl sulfide (Me2S) and Zn.
Example of Ozonolysis
1) O3
2) DMS (Me2S)
H
O
O
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Example of Ozonolysis
1) O3
2) DMS (Me2S)
H
O
O
Example of Ozonolysis
1) O3
2) DMS (Me2S)
H
O
O
Predicting the Products of an Addition Reaction
1.# What are the identities of the groups being added the double bond?
2.# What is the expected regioselectivity (Markovnikovor anti-Markovnikovaddition)?
3.# What is the expected stereospecificity (syn or antiaddition)?
Predicting the Products of an Addition Reaction
1.# What are the identities of the groups being added the double bond?
2.# What is the expected regioselectivity (Markovnikovor anti-Markovnikovaddition)?
3.# What is the expected stereospecificity (syn or antiaddition)?
Predicting the Products of an Addition Reaction
1.# What are the identities of the groups being added the double bond?
2.# What is the expected regioselectivity (Markovnikovor anti-Markovnikovaddition)?
3.# What is the expected stereospecificity (syn or antiaddition)?
9.13 Synthesis Strategies: One-Step Syntheses
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Synthesis Strategies: One-Step Syntheses Synthesis Strategies: One-Step Syntheses
Synthesis Strategies: Multi-Step Syntheses
!#Changing the position of a leaving group
What are our weapons?
1. Substitution reactions
2. Elimination reactions
3. Addition reactions
Synthesis Strategies: Multi-Step Syntheses
!#Changing the position of a leaving group
What are our weapons?
1. Substitution reactions
2. Elimination reactions
3. Addition reactions
Synthesis Strategies: Multi-Step Syntheses
!#Changing the position of a leaving group
What are our weapons?
1. Substitution reactions
2. Elimination reactions
3. Addition reactions
Synthesis Strategies: Multi-Step Syntheses
!#Changing the position of a leaving group
What are our weapons?
1. Substitution reactions
2. Elimination reactions
3. Addition reactions
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A Choice of Reagents
1. Substitution reactions
2. Elimination reactions
3. Addition reactions
Hydrohalogenation
!#Changing the position of a leaving group
What are our weapons?
1. Substitution reactions
2. Elimination reactions
3. Addition reactions
Synthesis Strategies: Multi-Step Syntheses Example Synthesis Strategies: Multi-Step Syntheses Example
Synthesis Strategies: Multi-Step Syntheses
!#Changing the position of a $bond