Chapter 9
Substitution and
Elimination
Reactions
Paula Yurkanis Bruice
University of California,
Santa Barbara
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The Families of Group II
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The Compounds in Group II
Are Electrophiles
All of the compounds in Group II
have an electron-withdrawing atom or group
that is attached to an sp3 carbon.
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Because Group II Compounds Are Electrophiles,
They React with Nucleophiles
substitution reaction—the electronegative group is replaced
by another group.
elimination reaction—the electronegative group is eliminated
along with a hydrogen.
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Alkyl Halides
the first of the families
in Group II
Alkyl halides have relatively good leaving groups.
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A Substitution Reaction
more precisely called a nucleophilic substitution reaction
because the atom replacing the halogen is a nucleophile
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What is the Mechanism of the Reaction?
The kinetics of a reaction—the factors that affect the rate of
the reaction—help determine the mechanism.
The Rate Law
The rate law tells us what molecules are involved in
the transition state of the rate-limiting step.
an SN2 reaction
substitution nucleophilic bimolecular
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Relative Rates of an SN2 Reaction
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Inverted Configuration
If the halogen is bonded to an asymmetric center,
the product will have the inverted configuration.
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Summary of the Experimental Evidence
for the Mechanism of an SN2 Reaction
1. Both the alkyl halide and the nucleophile are in
the transition state of the rate-limiting step.
2. The relative rate:
primary alkyl halide > secondary alkyl halide > tertiary alkyl halide
3. The configuration of the product is inverted
compared to the configuration of the reacting
chiral alkyl halide.
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The Mechanism
back-side attack
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Why Back-Side Attack?
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Why Bimolecular?
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Why Do Methyl Halides React the Fastest
and Tertiary the Slowest?
steric hindrance
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Steric Hindrance Decreases the Rate
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Both are Primary Alkyl Halides,
but They React at Different Rates
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Although it is Primary, it Reacts Very Slowly
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Why the Configuration of the Product
is Inverted
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The Weakest Base is the Best Leaving Group
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The Rate of an SN2 Reaction
is Affected by the Leaving Group
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Bases and Nucleophiles
A base shares its lone pair with a proton.
A nucleophile shares its lone pair with an atom other than a proton.
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Base Strength and Nucleophile Strength
A negatively charged atom is a stronger base and a
better nucleophile than the same atom that is neutral.
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Base Strength and Nucleophile Strength
If atoms are in the same row,
the strongest base is the best nucleophile.
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Polarizability
The larger the atom, the more polarizable it is
(can move more freely toward a positive charge).
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Polarizability versus Nucleophilicity
Does the greater polarizability of the larger atoms make up for
their decreased basicity that makes them poorer nucleophiles?
No, if they are in an aprotic polar solvent.
I− is still the poorest nucleophile.
Yes, if they are in a protic polar solvent.
I− is now the best nucleophile.
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Solvents
nonpolar solvents: hexane, benzene
Negatively charged species cannot dissolve in nonpolar solvents.
protic polar solvents: have a hydrogen attached to an O
(water, alcohols)
aprotic polar solvents: do not have a hydrogen attached to an O
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Base Strength and Nucleophile Strength
The strongest base is the best nucleophile unless they
differ in size and they are in a protic polar solvent.
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Why Do Protic Polar Solvents Make the Strongest
Bases the Poorest Nucleophiles?
F− is the best nucleophile in an aprotic polar solvent.
I− is the best nucleophile in a protic polar solvent.
Strong bases form strong ion–dipole interactions.
The ion–dipole interactions must be broken
before the nucleophile can react.
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Aprotic Polar Solvents
They can solvate a cation (+)
better than they can solvate an anion (−).
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Steric Hindrance Decreases Nucleophilicity
tert-Butoxide ion is a stronger base than ethoxide ion,
but it is a poorer nucleophile.
Its large size makes it difficult for it
to approach the back side of the carbon.
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SN2 Reactions Can Be Used to Make
a Variety of Compounds
The reactions are irreversible
because a weak base cannot
displace a strong base.
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Synthesizing an Amine
K2CO3 makes the solution basic
so that the amine will exist in its basic form.
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A Substitution Reaction
a tertiary alkyl halide and a poor nucleophile
The reaction is surprisingly fast,
so it must be taking place by a different mechanism.
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The Rate Depends Only on the
Concentration of the Alkyl Halide
The Rate Law
Only the alkyl halide is in the transition state of the rate-limiting step.
an SN1 reaction
substitution nucleophilic unimolecular
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Solvolysis Reaction
Most SN1 reactions are solvolysis reactions;
the solvent is the nucleophile.
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The Mechanism
The leaving group departs before the nucleophile approaches.
The slow step is carbocation formation.
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The Reaction Coordinate Diagram
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Substitution Reactions of Alkyl Halides
Primary alkyl halides undergo only SN2 reactions.
Secondary alkyl halides undergo only SN2 reactions.
Tertiary alkyl halides undergo only SN1 reactions.
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The Product is a Pair of Enantiomers
If the halogen is bonded to an asymmetric center,
the product will be a pair of enantiomers.
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Why a Pair of Enantiomers?
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Most SN1 Reactions Lead to
Partial Racemization
Generally more inverted product is formed,
because the front side is partially blocked.
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Summary of the Experimental Evidence
for the Mechanism of an SN1 Reaction
1. The rate of the reaction depends only on the
concentration of the alkyl halide.
2. Tertiary alkyl halides, but not primary or
secondary alkyl halides, undergo SN1 reactions.
3. If the halogen is attached to an asymmetric
center, the product will be a pair of enantiomers.
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The Weakest Base is the Best Leaving Group
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Comparing SN2 and SN1 Reactions
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