Carey Chapter 8 Nucleophilic Sub

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8.6Unimolecular Nucleophilic Substitution SN1
2
Tertiary alkyl halides are very unreactive in
substitutions that proceed by the SN2
mechanism.

• But they do undergo nucleophilic substitution.
• 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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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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unimolecular slow

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bimolecular fast
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carbocation formation
carbocation capture
R
proton transfer
RX
ROH
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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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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 substitutionby 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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8.8Stereochemistry of SN1 Reactions
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• 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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Figure 8.8


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8.11Carbocation Rearrangements in 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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8.10Solvent Effects
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• 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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• SN2 Reaction Rates Increase inPolar Aprotic
  Solvents

An aprotic solvent is one that doesnot have an
OH group. So it does not solvate anion
well allowing it to be effective nucleophile
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Table 8.7SN2 Reactivity versus Type of Solvent
Table 8.7 SN2 Reactivity vs 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