8.8 Unimolecular Nucleophilic Substitution SN1

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8.8Unimolecular Nucleophilic SubstitutionSN1
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A question...
Tertiary alkyl halides are very unreactive in
substitutions that proceed by the SN2
mechanism.Do they undergo nucleophilic
substitution at all?

• Yes. But by a mechanism different from SN2.
  The most common examples are seen in solvolysis
  reactions.

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Example of a solvolysis. Hydrolysis of
tert-butyl bromide.


Br
H
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Example of a solvolysis. Hydrolysis of
tert-butyl bromide.



O
C


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Example of a solvolysis. Hydrolysis of
tert-butyl bromide.
..



C
Br

O
C
..

This is the nucleophilic substitutionstage of
the reaction the one withwhich we are
concerned.
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Example of a solvolysis. Hydrolysis of
tert-butyl bromide.
..



C
Br

O
C
..

The reaction rate is independentof the
concentration of the nucleophileand follows a
first-order rate law. rate k(CH3)3CBr
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Example of a solvolysis. Hydrolysis of
tert-butyl bromide.


The mechanism of this step isnot SN2. It is
called SN1 and begins with ionization of
(CH3)3CBr.
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Kinetics and Mechanism
rate kalkyl halide First-order kinetics
implies a unimolecularrate-determining step.

• Proposed mechanism is called SN1, which stands
  forsubstitution nucleophilic unimolecular

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Mechanism
unimolecular slow

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Mechanism
bimolecular fast
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carbocation formation
carbocation capture
proton transfer
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Characteristics of the SN1 mechanism

• first order kinetics rate kRX
• unimolecular rate-determining step
• carbocation intermediate
• rate follows carbocation stability
• rearrangements sometimes observed
• reaction is not stereospecific
• much racemization in reactions of optically
  active alkyl halides

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8.9Carbocation Stability and SN1 Reaction Rates
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Electronic Effects Govern SN1 Rates
The rate of nucleophilic substitutionby the SN1
mechanism is governedby electronic
effects. Carbocation formation is
rate-determining.The more stable the
carbocation, the fasterits rate of formation,
and the greater the rate of unimolecular
nucleophilic substitution.
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Table 8.5 Reactivity toward substitution by the
SN1 mechanism
RBr solvolysis in aqueous formic acid

• Alkyl bromide Class Relative rate
• CH3Br Methyl 1
• CH3CH2Br Primary 2
• (CH3)2CHBr Secondary 43
• (CH3)3CBr Tertiary 100,000,000

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Decreasing SN1 Reactivity
(CH3)3CBr
(CH3)2CHBr
CH3CH2Br
CH3Br
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8.10Stereochemistry of SN1 Reactions
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Generalization

• Nucleophilic substitutions that
  exhibitfirst-order kinetic behavior are not
  stereospecific.

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Stereochemistry of an SN1 Reaction
R-()-2-Bromooctane
H
CH3
H2O
C
HO
(CH2)5CH3
(R)-()-2-Octanol (17)
(S)-()-2-Octanol (83)
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Figure 8.8
Ionization stepgives carbocation threebonds to
stereogeniccenter become coplanar

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Figure 8.8

Leaving group shieldsone face of
carbocationnucleophile attacks faster at
opposite face.
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8.11Carbocation Rearrangementsin SN1 Reactions
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Because...

• carbocations are intermediatesin SN1 reactions,
  rearrangementsare possible.

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Example
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Example
CH3
CH3
C
CHCH3
C
CH2CH3
CH3
CH3
(93)
H
Br
OH
H2O
CH3
CH3
C
CHCH3
C
CHCH3
CH3
CH3


H
H
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8.12Solvent Effects
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In general...

• SN1 Reaction Rates Increase in Polar Solvents

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Table 8.6SN1 Reactivity versus Solvent Polarity
Solvent Dielectric Relative constant rate ace
tic acid 6 1 methanol 33 4 formic
acid 58 5,000 water 78 150,000
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transition state stabilized by polar solvent
R
energy of RX not much affected by polarity of
solvent
RX
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transition state stabilized by polar solvent
activation energy decreases rate increases
R
energy of RX not much affected by polarity of
solvent
RX
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In general...

• SN2 Reaction Rates Increase inPolar Aprotic
  Solvents

An aprotic solvent is one that doesnot have an
OH group.
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Table 8.7SN2 Reactivity versus Type of Solvent
CH3CH2CH2CH2Br N3

• Solvent Type Relative rate
• CH3OH polar protic 1
• H2O polar protic 7
• DMSO polar aprotic 1300
• DMF polar aprotic 2800
• Acetonitrile polar aprotic 5000

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• Mechanism SummarySN1 and SN2

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When...

• primary alkyl halides undergo nucleophilic
  substitution, they always react by the SN2
  mechanism
• tertiary alkyl halides undergo nucleophilic
  substitution, they always react by the SN1
  mechanism
• secondary alkyl halides undergo nucleophilic
  substitution, they react by the
• SN1 mechanism in the presence of a weak
  nucleophile (solvolysis)
• SN2 mechanism in the presence of a good
  nucleophile