Learning Outcome : Predict the products of addition reactions
of alkenes and alkynes and use addition reactions to complete
multistep synthesis problems.
Addition Reactions of Alkenes
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 Alkenes are electron rich.
 The electron density of the  bond is concentrated
above and below the plane of the molecule.
 Alkenes are nucleophiles; therefore, they react with
electrophiles.
Hydrohalogenation—Electrophilic Addition
of HX
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 Two bonds are broken and two new  bonds are
formed.
 The H—X bond is polarized.
 This structural feature makes HX a good electrophile.
Mechanism – Electrophilic Addition of HX to an Alkene
Regiochemistry: Markovnikov’s Rule and
the Addition of HX to an Alkene
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 The addition of HX to an unsymmetrical alkene
follows Markovnikov’s rule.
 The rule states that in the addition of HX to an
unsymmetrical alkene, the H atom bonds to the
carbon that has the greater number of hydrogens
(“The rich get richer.”)
The Physical Basis for Markovnikov’s Rule
Stereochemistry of Addition Reactions
Hydrohalogenation—Reaction Stereochemistry
A racemic mixture 
Hydrohalogenation—Reaction Stereochemistry
• A and B are enantiomers. Since attack from either direction
occurs with equal probability, a racemic mixture of A and B is
formed.
EP2: Predict the products of the following
reactions:
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 Cyclopentene + HI
 1,2-dimethylcyclohexene + HBr
EP4: Draw the products formed when each
alkene is treated with HBr:
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 1-methylcyclohexene
 2-methylpropene
 2-methyl-1-hexene
EP5: Draw the product of the following reaction:
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 1,5,5-trimethylcyclohexene + HBr
Carbocation Rearrangement: A 1,2-hydride Shift
© 2014 Pearson Education, Inc.
Carbocation Rearrangement: A 1,2-methyl Shift
© 2014 Pearson Education, Inc.
The carbocation does not rearrange when there is no
improvement in the stability of the carbocation.
© 2014 Pearson Education, Inc.
EP6: Predict the product(s) for the following
addition reactions:
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 HBr to 3-methyl-1-pentene
 ICl to 3-methylcyclohexene
EP7: Provide mechanistic explanations for the
following observations:
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 The addition of hydrogen chloride to 3methyl-1-butene produces two products: 2chloro-3-methylbutane and 2-chloro-2methylbutane.
 The addition of hydrogen chloride to 3,3dimethyl-1-butene produces two products: 3chloro-2,2-dimethylbutane and 2-chloro-2,3dimethylbutane.
Addition Reactions of Alkenes
Hydration—Electrophilic Addition of Water
• Hydration is the addition of water to an alkene to form an
alcohol.
Mechanism - Electrophilic Addition of Water
Hydration: Electrophilic Addition of Water
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 Three consequences to the formation of
carbocation intermediates:
 Markovnikov’s rule applies
 Addition of H and OH occurs in both syn and anti
fashion
 Carbocation rearrangements can occur
EP8: What two alkenes give rise to each alcohol as the
major product by acid-catalyzed hydration?
 2-methyl-2-pentanol
 1-methylcyclohexanol
 2-butanol
Hydration: Oxymercuration - Demercuration
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 Reaction is initiated by electrophilic addition of Hg2+
ion to the alkene
 Regiochemistry of the reaction corresponds to
Markovnikov addition of H2O
 Anti Addition
Hydroboration—Oxidation
The net result is the addition of H2O to an alkene.
Hydroboration—Oxidation
With unsymmetrical alkenes, the boron atom bonds to the
less substituted carbon atom.
The larger boron atom bonds to the less sterically
hindered, more accessible carbon atom.
Hydroboration—Oxidation
The overall result of this two-step sequence is a syn
anti-Markovnikov addition of the elements of H and OH
to a double bond.
EP5: Draw the products formed when each alkene is
treated with BH3 followed by H2O2 and OH-.
 1-butene
 1-ethylcyclopentene
 1,4-dimethylcyclohexene
EP6: Acid-Catalyzed Hydration versus HydroborationOxidation
 Draw the product formed in each reaction
 Methylenecyclopentane + H2O/H2SO4
 Methylenecyclopentane + (1) BH3 + (2) H2O2, OH-
Learning Outcome: Predict the regio- and stereochemistry of
addition reactions of alkenes.
Halogenation—Addition of Halogen
• Halogenation is the addition of X2 (X = Cl or Br) to an
alkene to form a vicinal dihalide.
Halogenation—Addition of Halogen
Mechanism for the Addition of X2 to an
Alkene
 There are two observations that suggest that
the addition of X2 follows a mechanism that is
different from that of the addition of HX and
H2O:
There are no rearrangements of the carbon
skeleton.
Only anti addition occurs.
Mechanism for the halogenation reaction of alkenes:
Halogenation—Reaction Stereochemistry
Halogenation – Reaction Stereochemistry
EP1: Provide a step-by-step mechanism for the
halogenation reaction involving cyclobutene with Br2.
EP2: Draw the products of each reaction, including
stereochemistry.
 4-methylcyclopentene + Br2
 1-methylcyclohexene + Cl2
Halohydrin Formation – Addition of X and OH
Halohydrin Formation
Even through X¯ is formed in step [1] of the mechanism,
its concentration is small compared to H2O.
Halohydrin Formation
Backside attack of H2O leads to X and OH being added in an anti
fashion and the formation of trans products.
With unsymmetrical alkenes, the major product is the one in which
the electrophile X+ is bonded to the less substituted carbon, and
the nucleophile (H2O) binds to the more substituted carbon.
Halohydrin Formation
Nucleophilic attack occurs at the more substituted carbon
because that carbon is better able to accommodate the
partial positive charge in the transition state.
EP3: Draw the products of each reaction, including
stereoisomers:
 Cyclobutene + Cl2 in H2O
 methylenecyclopentane + Br2 in H2O