16. Chemistry of Benzene: Electrophilic Aromatic Substitution Part 1

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16. Chemistry of Benzene Electrophilic Aromatic
Substitution Part 1

• Based on
• McMurrys Organic Chemistry, 6th edition, Chapter
  16

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Substitution Rxns of Benzene Derivatives

• Benzene is aromatic a cyclic conjugated compound
  with 6 ? electrons.
• Reactions of benzene retain the aromatic core.
• Electrophilic aromatic substitution replaces a
  proton on benzene with another electrophile.

Electrophile (E) is looking for electron density.
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Bromination of Double Bonds

• Alkenes react readily with Br2 to give an
  addition product

• Under these conditions, benzene Br2 give no
  reaction

Using a Lewis acid catalyst results in a
substitution reaction
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Bromination of Aromatic Rings

• In reactions with Lewis acids, Benzenes ?
  electrons participate as a Lewis base (or as a
  nucleophile).
• FeBr3 is added as a catalyst to polarize the
  bromine reagent and make Br2 more electrophilic
  forms Br

Role of catalyst
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Mechanism of Bromination 2 Steps

• STEP ONEThe pi electrons act as a nucleophile,
  attack Br (complexed with FeBr4-).The
  cationic intermediate formed (not aromatic) is
  high in energy.

Step 1
E
FeBr3
Benzene - acts as nucleophile
Delocalized cationic intermediate3
resonance structures, all equal energy
(Know how to draw!!)
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Formation of Product from Intermediate

• STEP TWOThe cationic addition intermediate
  transfers a proton to FeBr4- (from Br- and
  FeBr3).
• This restores aromaticity and gives a
  substitution product (in contrast with addition
  seen in alkenes).

Step 1
Step 2
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Reaction with other Halogens

• Cl2 Like the bromination reaction, chlorine
  will react with benzene using FeCl3 as
  catalyzt to form chlorobenzene.
• F2 Fluorine is too reactive and cannot be used
  in standard EAS halogenation reactions.
• I2 Iodine has very low reactivity, requiring
  special reagents (Cu or peroxide). We will
  not cover this reaction.

skip
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Aromatic Nitration

• Use nitric acid (HNO3) plus catalytic sulfuric
  acid to add a nitro group to the aromatic ring.

Overall Reaction
With isopropyl, nitro group adds at the para
position.