Radical reactions Forming of a radical

1
Radical reactionsForming of a radical
2
Radical reactionsForming of a radical
Homolysis
3
Radical reactionsForming of a radical
Homolytic cleavage by Photolysis
4
Radical reactionsForming of a radical
Reactions of molecules with other radicals
5
Radical reactionsStability
6
Radical substitution reactionsHalogenation
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Radical substitution reactionsHalogenation
Chlorine rather unselective !!! (number of
hydrogens versus radical position)
Bromine less reactive- More selective !!!
8
Radical substitution reactionsHalogenation
Stereochemistry
9
Radical substitution reactionsHalogenation
Allylic and benzylic substitution
10
Radical substitution reactionsHalogenation
Allylic radical substitution
If polar solvent -gt electrophilic addition
(stabilization of carbocation)

• If apolar solvent -gt radical substitution
• If low Br2 concentration -gt allylic radical
  substitution
• If high Br2 concentration -gt radical addition to
  double bond

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Radical substitution reactionsHalogenation
Allylic radical substitution
If substrate not chosen carefully -gt 3 products
12
Radical substitution reactionsHalogenation
Allylic radical substitution -gt NBS is better
reagent if selective bromination wanted (in
apolar solvent)
NBS keeps concentration of Br2 low -gt just
allylic radical substitution (no radical addition
to double bomd)
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Radical substitution reactionsHalogenation
Benzylic radical substitution -gt NBS is better
reagent if effective bromination wanted
Cl2 is more reactive than Br2 -gt chlorination can
occur at different positions
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Radical addition reactionsAddition of HBr to
alkenes
15
Radical addition reactionsAddition of HBr to
alkenes
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Radical addition reactionsAddition of HBr to
alkenes
Addition to conjugated dienes
17
Radical polymerization reactionspolymerization
of alkenes
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Radical polymerization reactionspolymerization
of alkenes
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Radical addition reactionAddition of oxygen -gt
autooxidation
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Radical addition reactionAddition of oxygen -gt
autooxidation
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Radical addition reactionAddition of oxygen -gt
autooxidation
Autooxidation of unsaturated fats and oils -gt
origins of rancidity (unpleasant smell and taste
of badly stored fats
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Radical addition reactionAddition of oxygen -gt
autooxidation
Autooxidation of unsaturated fats and oils -gt
origins of rancidity (unpleasant smell and taste
of badly stored fats
-gt Oxidation (acid) and chain shortening
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Radical addition reactionAddition of oxygen -gt
autooxidation
Antioxidants and health
Vitamin E
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Radical reactionsPhenolic oxidative coupling
Many natural products are produced that way -gt
Enzymes (oxidases, peroxidases,) catalyze
reactions
25
Radical reactionsPhenolic oxidative coupling
Many natural products are produced that way -gt
Enzymes (oxidases, peroxidases,) catalyze
reactions
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Electrophilic ReactionsAromatic substitution
Electrophilic reactions with aromatic substrates
-gt substitution
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Electrophilic ReactionsAromatic substitution
Halogenation
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Electrophilic ReactionsAromatic substitution
Nitration
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Electrophilic ReactionsAromatic substitution
Sulfonation
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Electrophilic ReactionsAromatic substitution
Electrophilic alkylations -gt Friedel-Craft
alkylation
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Electrophilic ReactionsAromatic substitution
Electrophilic alkylations -gt Friedel-Craft
alkylation -gt Reaction mechanism dependent on
structure of alkyl
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Electrophilic ReactionsAromatic substitution
Electrophilic alkylations -gt Friedel-Craft
alkylation -gt alkylation of aromatic ring can be
achieved by any system that generates carbocation
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Electrophilic ReactionsAromatic substitution
Electrophilic alkylations -gt Friedel-Craft
alkylation -gt Limitation of alkylation by
rearrangement reaction of carbocation
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Electrophilic ReactionsAromatic substitution
Electrophilic acylation -gt Friedel-Craft Acylation
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Electrophilic ReactionsAromatic substitution
Electrophilic acylation -gt Friedel-Craft Acylation
36
Electrophilic ReactionsAromatic substitution
Effect of substituents -gt influence rate and
position of substitution
Electron withdrawing substituents -gt deactivate
and hinder further substitution (-gt destabilize
ortho and para -gt substitution preferable in meta
position) Electron donating substituents -gt
activate for further substitution (-gt preferable
in ortho and para)
Substituent has maximum effect on reaction if it
is in ortho or para position -gt electron
releasing substituents (ortho and para) -gt
electron withdrawing substituents (destabilize
arenium cation -gtmeta)
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Electrophilic ReactionsAromatic substitution
Effect of substituents -gt influence rate and
position of substitution
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Electrophilic ReactionsAromatic substitution
Effect of substituents -gt Inductive effect
Nitration of Toluene -gt E NO2
Nitration of Toluene -gt 59 ortho, 37 para, 4
meta Nitration of Nitrobenzene -gt 93 meta, 6
ortho, 1 para
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Electrophilic ReactionsAromatic substitution
Effect of substituents -gt Resonance Effect -gt
substitutents with strong electron releasers
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Electrophilic ReactionsAromatic substitution
Effect of substituents
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Electrophilic ReactionsAromatic substitution
Effect of substituents -gt reaction is
regioselective
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Electrophilic ReactionsAromatic substitution
Converting substituents (introduced by
electrophilic substitution) -gt into other
functional groups
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Electrophilic Reactionssubstitution on
polycyclic aromatic compounds
Monosubstitution -gt predominant at position 1
Effect of substituents on further substitution
Electron releasing substituent
Electron withdrawing substituent