8.7 Nucleophiles and Nucleophilicity

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8.7Nucleophiles and Nucleophilicity
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Nucleophiles
The nucleophiles described in Sections
8.1-8.6have been anions.
etc.
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Nucleophiles
The nucleophiles described in Sections
8.1-8.6have been anions.
etc.
Not all nucleophiles are anions. Many are
neutral.
..

NH3
for example
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Nucleophiles
The nucleophiles described in Sections
8.1-8.6have been anions.
etc.
Not all nucleophiles are anions. Many are
neutral.
..

NH3
for example
All nucleophiles, however, are Lewis bases.
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Nucleophiles
Many of the solvents in which nucleophilic
substitutions are carried out are
themselvesnucleophiles.
..
for example
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Solvolysis
The term solvolysis refers to a
nucleophilic substitution in which the
nucleophile is the solvent.
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Solvolysis
substitution by an anionic nucleophile
RX Nu
RNu X
solvolysis

RNuH X
RX NuH
step in which nucleophilicsubstitution occurs
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Solvolysis
substitution by an anionic nucleophile
RX Nu
RNu X
solvolysis

RNuH X
RX NuH
products of overall reaction
RNu HX
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Example Methanolysis
Methanolysis is a nucleophilic substitution in
which methanol acts as both the solvent andthe
nucleophile.
H

RX
The product is a methyl ether.
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Typical solvents in solvolysis
solvent product from RX water
(HOH) ROH methanol (CH3OH) ROCH3 ethanol
(CH3CH2OH) ROCH2CH3 formic acid
(HCOH) acetic acid (CH3COH)
ROCH
ROCCH3
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Nucleophilicity is a measure of the reactivity of
a nucleophile

• Table 8.4 compares the relative rates of
  nucleophilic substitution of a variety of
  nucleophiles toward methyl iodide as the
  substrate. The standard of comparison is
  methanol, which is assigned a relativerate of
  1.0.

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Table 8.4 Nucleophilicity

• Rank Nucleophile Relative rate
• strong I-, HS-, RS- gt105
• good Br-, HO-, 104
• RO-, CN-, N3-
• fair NH3, Cl-, F-, RCO2- 103
• weak H2O, ROH 1
• very weak RCO2H 10-2

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Major factors that control nucleophilicity

• basicity
• solvation
• small negative ions are highly solvated in
  protic solvents
• large negative ions are less solvated
• polarizability

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Table 8.4 Nucleophilicity

• Rank Nucleophile Relative rate
• good HO, RO 104
• fair RCO2 103
• weak H2O, ROH 1

When the attacking atom is the same (oxygenin
this case), nucleophilicity increases with
increasing basicity.
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Major factors that control nucleophilicity

• basicity
• solvation
• small negative ions are highly solvated in
  protic solvents
• large negative ions are less solvated
• polarizability

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Figure 8.4
Solvation of a chloride ion by ion-dipole
attractiveforces with water. The negatively
charged chlorideion interacts with the
positively polarized hydrogensof water.
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Table 8.4 Nucleophilicity

• Rank Nucleophile Relative rate
• strong I- gt105
• good Br- 104
• fair Cl-, F- 103

A tight solvent shell around an ion makes itless
reactive. Larger ions are less solvated
thansmaller ones and are more nucleophilic.
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Major factors that control nucleophilicity

• basicity
• solvation
• small negative ions are highly solvated in
  protic solvents
• large negative ions are less solvated
• polarizability

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Table 8.4 Nucleophilicity

• Rank Nucleophile Relative reactivity
• strong I- gt105
• good Br- 104
• fair Cl-, F- 103

More polarizable ions are more nucleophilic
thanless polarizable ones. Polarizability
increases with increasing ionic size.