The Reactions of Alkenes
The Stereochemistry of Addition Reactions
Assoc. Prof. Ts. Dr. Wan Nor Roslam Wan Isahak
Universiti Kebangsaan Malaysia
(Bruice Textbook)
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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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Hyperconjugation Stabilizes a Carbocation
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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 postulate says that the transition state is
more similar in structure to the species to which it is more
similar in energy.
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Why the Difference in Rate?
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 reaction forms more of one constitutional isomer than another.
• Completely regioselective
• Highly regioselective
• Moderately 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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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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More Reactions
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Formation of an Epoxide
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An Oxygen–Oxygen Single Bond
is a Weak Bond
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MCPBA is a Commonly Used Peroxyacid
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Mechanism for Epoxidation
the mechanism is similar to that for the addition of Br2
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Nomenclature of Epoxides
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Ozonolysis
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Mechanism for Ozonide Formation
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The Ozonide is Converted to
Ketones and/or Aldehydes
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Ketones and Aldehydes
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Ozonolysis
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Ozonolysis
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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 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 center
forms 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 as
products 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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Only Syn Addition
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Syn Addition to a Cis Isomer
Forms Only the Cis Stereoisomers
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Syn Addition to a Trans Isomer
Forms Only the Trans Stereoisomers
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If the Substituents are the Same,
the Cis Stereoisomer is a Meso Compound
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Only Syn Addition
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The H and OH Add
to the Same Side of the Ring
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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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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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Compounds That Can Be Formed from an Alkene
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