Chapter 6
The Reactions of
Alkenes and
Alkynes
The
Stereochemistry of
Addition Reactions
Paula Yurkanis Bruice
University of California,
Santa Barbara
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An Electrophilic Addition Reaction
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Electrophilic Addition
R
R
electrophil
e
+ Y+ + Z-
R
R
R
Z
R
Y
R
R
nucleophile
Electrophilic addition to the double bond is a
two-step process:
First, a relatively slow addition of the electrophile to
the alkene to form a carbocation intermediate.
Second, the positively charged carbocation reacts
rapidly with a nucleophile to form the product.
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Addition of Hydrogen Halides
Electrophilic reagents we can use are hydrogen halides:
HF, HCl, HBr, HI
They all follow the same reaction scheme.
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Electrophilic Addition
Reactions of alkene with identical substituents
on both carbons lead to one product.
HBr
H
Br
cyclopentene
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bromocyclopentane
Electrophilic Addition
In case the alkene has different, or different
number of substituents, two products
are possible.
HBr
A
Br
H
1-bromo-1-methylcyclopentane
+
H
B
1-bromomethylcyclopentane
Is there a preference for one product?
Which is the preferred product?
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Br
Electrophilic Addition
Experiment shows that (A) is preferred over (B)
HBr
A
Br
H
+
B
H
Br
The preference for (A) is explained by the mechanism
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H
H
CH 2
CH 2
HBr
A
+
B
slow
We could obtain two structurally different
Carbocations; however, A is formed much faster.
H
CH2
Brfast
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Br
H
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 decrease the concentration of positive charge
on the carbon.
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Hyperconjugation Stabilizes
a Carbocation
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Hyperconjugation
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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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Electrophilic Addition Reactions are
Regisoselective
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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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Not Regioselective
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Regioselective reaction
HI
I
+
H
H
1-methylcycohexene
I
1-iodo-1-methylcyclohexane
2-iodo-1-methylcyclohexane
Major product
Minor product
We obtain both products; however, one is the
major product.
We have a regioselective reaction.
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Constitutional isomers
I
I
1-iodo-1-methylcyclohexane
2-iodo-1-methylcyclohexane
The molecular formula of both products is identical:
C7H13I
However, the way the atoms are connected is
different: Constitutional isomers
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Markovnikov’s Rule
The regioselectivity of this reaction was first
observed by Markovnikov.
He found that the hydrogen, which is the
electrophile, adds to the carbon that contains
the larger number of hydrogens.
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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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Acid-Catalyzed Addition of Water
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Mechanism for the Acid-Catalyzed
Addition of Water
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Regioselective vs. Stereoselective
Reactions
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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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The Product is a Racemic Mixture
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The Product is a Racemic Mixture
(racemic)
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Addition of H2 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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An Enzyme forms only one Stereoisomer;
the Reaction 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
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Therefore, Enantiomers can be separated
using an Enzyme-Catalyzed Reaction
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An Introduction to Alkynes
An alkyne is a hydrocarbon that contains a carbon–carbon
triple bond.
General formula: CnH2n–2 (acyclic)
CnH2n–4 (cyclic)
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Nomenclature of Alkynes
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Nomenclature of Alkynes
• The “yne” suffix is assigned the lowest number.
• The substituents are assigned so the lowest number is in
the name.
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The Structure of Alkynes
The triple bond is composed of a sigma bond and two π bonds.
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Alkynes (Like Alkenes) undergo
Electrophilic Addition Reactions
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The first step is
addition of an Electrophile
Addition of an electrophile
to an alkene
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Addition of an electrophile
to an alkyne
A second Addition Reaction can Occur
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Addition to a Terminal Alkyne is
Regioselective
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The second Electrophilic Addition
reaction is also Regioselective
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An Asymmetrical Internal Alkyne
forms two Products
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A Symmetrical Internal Alkyne
forms one Product
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Acid-Catalyzed Addition of
water to an Alkene
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Acid-Catalyzed Addition of
water to an Alkyne
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Keto–Enol Interconversion
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Mechanism for Acid-Catalyzed
Keto–Enol Interconversion
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Acid-Catalyzed Addition of Water
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A Terminal Alkyne is less reactive than an
Internal Alkyne
Because terminal alkynes are less reactive, the rate of the
reaction is increased with a mercuric ion catalyst.
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Addition of Hydrogen forms an Alkane
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Stopping at the Alkene
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Lindlar Catalyst
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Syn Addition
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Why Cis?
The catalyst delivers the hydrogens to one side of the triple
bond.
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A Hydrogen Bonded to an sp Hybridized
Carbon is Acidic
The electronegativity of carbon varies with the level of
hybridization.
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Acidity of alkynes
In aqueous solutions none of the protons can be removed,
pKa (H2O) = 14.
HC CH
+ H2O
HC C
However, with strong bases such as
NH2–, a proton can be removed from ethyne.
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+ H3O
Acidity
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Synthesis Using Acetylide Ions:
Alkylation
This property is used to our advantage in synthesis
when carbon–carbon bonds need to be formed.
HC CH
+ NH2
HC C
+ H3N
With the strong base , NH2–, we form a carbanion.
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Alkylation
In the second step, the carbanion displaces a bromine.
This results in carbon–carbon bond formation.
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5.15 An Introduction to Multistep Synthesis
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Multistep Synthesis
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Multistep Synthesis
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