© 2014 Pearson Education, Inc. The Reactions of Alkenes The Stereochemistry of Addition Reactions Paula Yurkanis Bruice University of California, Santa

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Slide 1© 2014 Pearson Education, Inc.
An Electrophilic Addition Reaction
© 2014 Pearson Education, Inc.
Addition of Hydrogen Halides
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© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Relative Stabilities of Carbocations
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Look Like?
It Looks Like What it is Closer To
The Hammond postulate says that the transition state is more similar in structure to the species to which it is more similar in energy.
© 2014 Pearson Education, Inc.
The more stable carbocation is formed more rapidly.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens
A regioselective reaction forms more of one constitutional isomer than another.
Highly regioselective
Completely regioselective
Moderately regioselective
More Highly Regioselective?
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© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
BH3 is an Electrophile
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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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Follow the Same Rule
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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.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
The intermediate is a cyclic bromonium ion.
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A Cyclic Bromonium Ion
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© 2014 Pearson Education, Inc.
bonded to the most hydrogens.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Smallest Heat of Hydrogenation
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Relative Stabilities of Alkenes
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© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
relative stabilities of dialkyl-substituted alkenes
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© 2014 Pearson Education, Inc.
When a reactant that does not have an asymmetric center
forms a product that has one asymmetric center,
the product is a racemic mixture.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
When a reactant that has an asymmetric center
forms a product that has new asymmetric center,
the product is a pair of diastereomers.
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© 2014 Pearson Education, Inc.
Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate
syn and anti addition
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Forms Only the Erythro Stereoisomers
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the Erythro Stereoisomer is a Meso Compound
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Forms Only the Threo Stereoisomers
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Forms Only the Cis Stereoisomers
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the Cis Stereoisomer is a Meso Compound
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Cyclic Alkenes are Cis,
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© 2014 Pearson Education, Inc.
Forms Only the Threo Stereoisomers
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the Cis Stereoisomer is a Meso Compound
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the Erythro Stereoisomer is a Meso Compound
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Forms Only the Trans Stereoisomers
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TRANS-ANTI-(ERYTHRO or CIS) OK
CIS-ANTI-(THREO or TRANS) OK
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© 2014 Pearson Education, Inc.
It is Completely Stereoselective
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© 2014 Pearson Education, Inc.
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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© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.