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The Reactions of Alkenes
The Stereochemistry of Addition Reactions
Paula Yurkanis BruiceUniversity of California,
Santa Barbara
Chapter 6
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The Mechanism of the Reaction
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An Electrophilic Addition Reaction
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Addition of Hydrogen Halides
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Which sp2 Carbon Gets the H+?
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The Mechanism
Carbocation formation is the rate-limiting step.
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Relative Stabilities of Carbocations
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Carbocation Stability
Alkyl groups decreasethe concentration of positive charge on the carbon.
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Hyperconjugation Stabilizes a Carbocation
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Stabilization by Hyperconjugation
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Hyperconjugation
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What Does the Transition State Look Like?
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It Looks Like What it is Closer To
The Hammond postulateThe Hammond postulate says that the transition state is says that the transition state is
more more similar in structure similar in structure to the species to which it is more to the species to which it is more
similar in energy.similar in energy.
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Why the Difference in Rate?
The more stable carbocation is formed more rapidly.The more stable carbocation is formed more rapidly.
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The Difference in Carbocation Stability Determines the Products
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What Product Will Be Formed?
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The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens
A regioselective reactionA regioselective reaction forms more of one constitutional isomer than another. forms more of one constitutional isomer than another.
• Highly regioselectiveHighly regioselective
• Completely regioselectiveCompletely regioselective
• Moderately regioselectiveModerately regioselective
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Why is the First Reaction More Highly Regioselective?
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Not Regioselective
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Acid-Catalyzed Addition of Water
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Mechanism for the Acid-Catalyzed Addition of Water
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Acid-Catalyzed Addition of an Alcohol
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Mechanism for the Acid-Catalyzed Addition of an Alcohol
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The Major Product is a Surprise
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The Major Product is a Surprise
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Carbocation Rearrangement(a 1,2-hydride shift)
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Carbocation Rearrangement(a 1,2-methyl shift)
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The Carbocation Does Not Rearrange(No Improvement in Carbocation Stability)
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BH3 is an Electrophile
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Hydroboration–Oxidation
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The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens
The reagents are numbered because the second set of reagents is not added until the first reaction is over.
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Mechanism for Hydroboration
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Addition of BH3 and Addition of HBr Follow the Same Rule
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BH3 Has Three Potential Hydride Ions So a Dialkylborane and a Trialkylborane Can Be Formed
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R2BH Allows Only Monoalkylation
Because of its bulky R groups, it has a stronger preference for the less substituted sp2 carbon.
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The Mechanism is the Same
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OH Replaces BR2
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Mechanism for the Oxidation Reaction
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No Carbocation Rearrangements
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Addition of Br2 or Cl2
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The Product is a Vicinal Dihalide
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The Mechanism for the Addition of a Halogen
The intermediate is a cyclic bromonium ion.
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A Cyclic Bromonium Ion
The carbons are the most electrophilic atoms.
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No Carbocation Rearrangements
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Alkenes Do Not Add I2
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Formation of Halohydrins
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Mechanism for Halohydrin Formation
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Why Does it Follow the Same Rule?
The electrophile adds to the sp2 carbon bonded to the most hydrogens.
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What Alkene Gave the Ozonolysis Products?
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Addition of Hydrogen
catalytic hydrogenation a reduction reaction
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Mechanism for Hydrogen Addition
catalytic hydrogenation
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Relative Stabilities of Alkenes
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The Most Stable Alkene Has the Smallest Heat of Hydrogenation
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Relative Stabilities of Alkenes
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Trans is More Stable Than Cis
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The Cis Isomer Has Steric Strain
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The Relative Stabilities of Disubstituted Alkenes
relative stabilities of dialkyl-substituted alkenes
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A Regioselective Reaction
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A Stereoselective Reaction
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A Stereospecific Reaction
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The Product Does Not Have Stereoisomers
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The Product is a Racemic Mixture
When a reactant that does not have an asymmetric center forms a product that has one asymmetric center,
the product is a racemic mixture.
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Formation of a Racemic Mixture
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Each Product Will Be a Racemic Mixture
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Formation of a Second Asymmetric Center
When a reactant that has an asymmetric centerforms a product that has new asymmetric center,
the product is a pair of diastereomers.
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Reactions that Form Two Asymmetric Centers
the stereoisomers obtained asproducts depend on the mechanism
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Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate
syn and anti addition
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Only Syn Addition
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Syn Addition to a Cis Isomer Forms Only the Erythro Stereoisomers
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If the Substituents are the Same, the Erythro Stereoisomer is a Meso Compound
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Syn Addition to a Trans Isomer Forms Only the Threo Stereoisomers
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Syn Addition to a Cis Isomer Forms Only the Cis Stereoisomers
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If the Substituents are the Same,the Cis Stereoisomer is a Meso Compound
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Cyclic Alkenes are Cis, Unless They Have at Least Eight Ring Atoms
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Addition of a Halogen is an Anti Addition
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Anti Addition to a Cis Isomer Forms Only the Threo Stereoisomers
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If the Substituents are the Same,the Cis Stereoisomer is a Meso Compound
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If the Substituents are the Same,the Erythro Stereoisomer is a Meso Compound
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Anti Addition to a Cis Isomer Forms Only the Trans Stereoisomers
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CIS-SYN-(ERYTHRO or CIS)
Can change two terms but not one
TRANS-ANTI-(ERYTHRO or CIS) OK
CIS-ANTI-(THREO or TRANS) OK
CIS-ANTI-(ERYTHRO or CIS) not OK
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An Enzyme Forms One Stereoisomer;It is Completely Stereoselective
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An Enzyme Can Block One Side of the Reactant
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An Enzyme Reacts with Only One Stereoisomer;It is Completely Stereospecific
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The Stereospecificity of an Enzyme-Catalyzed Reaction Allows Enantiomers to Be Separated
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Enzymes and Receptors are Chiral
A chiral molecule binds only one of a pair of enantiomers.
A right-handed glove fits only a right hand.
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A Receptor Binds One Enantiomer
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When A Reacts, B is Synthesized
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Compounds That Can Be Formed from an Alkene
