Chap 15 Electrophilic aromatic substitution (???????)

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Chap 15 Electrophilic aromatic substitution
(???????)

• 15.1 Electrophilic aromatic substitution
  reactions
• These reactions are of the general type shown
  below

E-------Electrophilic reagent ????
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Fig 15.1 Electrophilic aromatic substitution
reactions.
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15.2 A general mechanism for electrophilic
aromatic substitution(???????????) Arenium
ions(????)
p

• A considerable body of experimental evidence
  indicates that electrophiles attack the psystem
  of benzene to form a delocalized nonaromatic
  carbocation known as an arenium ion (or sometimes
  as a scomplex)

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We wish to show the mechanism using the modern
formula for benzene.
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Fig 15.2 The potential energy diagram for an
electrophilic aromatic substitution reaction.
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15.3 Halogenation of benzeneBenzene does not
react with bromine or chlorine unless a Lewis
acid is present in the mixture. When Lewis acids
are present, however, benzene reacts readily with
bromine or, chlorine, and the reactions give
bromobenzene and chlorobenzenein good yields.
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Fluorine (F2) reacts so rapidly with benzene that
aromatic fluorination requires special conditions
and special types of apparatus.

• Iodine (I2), on the other hand, is so unreactive
  that a special technique has to be used to effect
  direct iodination the reaction has to be carried
  out in the presence of an oxidizing agent such as
  nitric acid

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The mechanism for aromatic bromination is as
follows
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15.4 Nitration of benzene

• Benzene reacts slowly with hot concentrated
  nitric acid to yield nitrobenzene. The reaction
  is much faster if it is carried out by heating
  benzene with a mixture of concentrated nitric
  acid and concentrated sulfuric acid

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Concentrated sulfuric acid increases the rate of
the reaction by increasing the concentration of
the electrophile-the nitronium ion (NO2)
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The nitronium ion reacts with benzene by
attacking the pcloud and forming an arenium ion
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15.5 Sulfonation of benzene ????

• Benzene reacts with fuming sulfuric acid at room
  temperature to produce benzene sulfonic acid.
  Fuming sulfuric acid is sulfuric acid that
  contains added sulfur trioxide (SO3)

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The mechanism of sulfonation
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We may remove the sulfonic acid group by
desulfonation
We shall see later that sulfonation and
desulfonation reactions are often used in
synthetic work.
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15.6 Friedel-Crafts alkylation????-???????

• A general equation for a Friedel-Crafts
  alkylation reaction is the following

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The mechanism for this reaction
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More Friedel-Crafts alkylation
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15.7 Friedel-Crafts Acylation????-???????
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Friedel-Crafts acylations can also be carried out
using carboxylic acid anhydrides(??).
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The mechanism for Acylation of Friedel-Crafts
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15.8 Limitations(??) of Friedel-Crafts reactions

• 1. Form the more stable carbocation.

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2. Friedel-Crafts reactions do not occur when
powerful electron-withdrawing groups are present
on the aromatic ring or when the ring bears an
NH2, -NHR, or NR2 group
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The amino groups, -NH2, -NHR, and NR2, are
changed into powerful electron-withdrawing groups
by the Lewis acids used to catalyze
Friedel-Crafts reactions
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3 Aryl and vinylic halides can not be used as the
halide component because they do not form
carbocations readily.
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Polyalkylations often occur (????????)

• Alkyl groups are electron-releasing groups, and
  once one is introduced into the benzene ring it
  activates the ring toward further substitution.

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15.9 Synthetic applications of Friedel-Crafts
acylations The Clemmensen reduction (??????)
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Target molecule
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15.10 Effect of substitutions on reactivity and
orientation???????????????
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We can divide substituent groups into two classes
according to their influence on the reactivity of
the ring.

• Those that cause the ring to be more reactive
  than benzene itself we call activating
  groups(???). Those that cause the ring to be less
  reactive than benzene we call deactivating
  groups(???).

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15.10A Activating groups Ortho-para directors

• The methyl group is an activating group and an
  ortho-para director. Toluene reacts considerably
  faster than benzene in all electrophilic
  substitutions.

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When we nitrate toluene with nitric and sulfuric
acids, we get mononitrotoluenes in the following
relative proportions.
Therefore, methyl group is ortho-para director
group.
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All alkyl groups are activating groups, and they
are all also ortho-para directors. The methoxy
group, CH3O- and acetamido group, CH3CONH-, are
strong activating groups and both are ortho-para
directors.
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The hydroxyl group and the amino group are very
powerful activating groups and are also powerful
ortho-para directors.
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15.10B Deactivating groups Meta-Directors
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The nitro group(-NO2), the carboxyl group
(-COOH), the sulfo group (-SO3H), and the
trifluoromethyl group (-CF3), are also meta
directors.
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15.10C Halo substituents Deactivating
ortho-para directors

• The chloro and bromo groups are weak deactivating
  groups. But they are ortho-para directors.

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Electrophilic substitutions of Chlorobenzene
Therfore, chloro and bromo are ortho-para
directors.
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15.10D Classification of substituents
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15.11 Theory of substituent effects on
electrophilic aromatic substitution

• 15.11A Reactivity The effect of
  electron-releasing and electron-withdrawing
  groups
• (??????????)

We find that the relative rates of the reactions
depend on whether S(???) withdraws or release
electrons. If S is an electron-releasing group,
the reaction occurs faster than the corresponding
reaction of benzene. If S is an
electron-withdrawing group, the reaction is
slower than that of benzene
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15.11B Inductive and resonance effects Theory
of orientation?????????---????
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Inductive effect explanation??????
Ortho-Para directors ??????
??????????,?????????,?????????????????,??????(E)?
?????
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15.11C Meta-Directing groups ?????

• ??????????,?????????,?????????????????,??????(E)?
  ????

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Resonance effect explanation??????
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The trifluoromethyl group is a powerful meta
director.
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15.11D Ortho-Para-Directing groups (??????)
Do you know why E attacks ortho and para
position ?
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• Therfore, chloro group is ortho-para-Director.

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15.11E Ortho-Para Direction and reactivity of
alkylbenzenes
Therfore, methyl or R- group is
ortho-para-Director.
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15.12 Reactions of the side chain of alkylbenzenes

• Synthesis of alkylbenzenes (??????)

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15.12A Halogenation of the side chain benzylic
radicals
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Side-Chain chlorination of toluene
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15.13C Oxidation of the side chain
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15.14 Synthetic applications
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The reaction of toluene
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The reaction of toluene
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The reaction of aniline ?????
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The synthesis of o-nitroacetanilide
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15.14A Orientation in disubstituted benzenes

• ???????????---??????

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????????????---???????????,?????????????????,?????
?????
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Homework15.23, 15.24, 15.25, 15.27, 15.28,
15.31