Reactions of Aromatic Compounds

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Reactions of Aromatic Compounds
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Electrophilic Aromatic Substitution

• Electrophile substitutes for a hydrogen on the
  benzene ring.

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Mechanism
Animation
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Bromination of Benzene
Br2, FeBr3

• Requires a stronger electrophile than Br2.
• Use a strong Lewis acid catalyst, FeBr3.

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Energy Diagramfor Bromination
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Chlorination and Iodination

• Chlorination is similar to bromination. Use
  AlCl3 as the Lewis acid catalyst.
• Iodination requires an acidic oxidizing agent,
  like nitric acid, which oxidizes the iodine to an
  iodonium ion.

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Nitration of Benzene

• Use sulfuric acid with nitric acid to form the
  nitronium ion electrophile.

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Sulfonation

• Sulfur trioxide, SO3, in fuming sulfuric acid is
  the electrophile.

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Desulfonation

• All steps are reversible, so sulfonic acid group
  can be removed by heating in dilute sulfuric
  acid.
• This process is used to place deuterium in place
  of hydrogen on benzene ring.

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Nitration of Toluene

• Toluene reacts 25 times faster than benzene. The
  methyl group is an activator.
• The product mix contains mostly ortho and para
  substituted molecules.

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Sigma Complex

• Intermediate is more stable if nitration occurs
  at the ortho or para position.

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Energy Diagram
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Activating, O-, P-Directing Substituents

• Alkyl groups stabilize the sigma complex by
  induction, donating electron density through the
  sigma bond.
• Substituents with a lone pair of electrons
  stabilize the sigma complex by resonance.

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Animation
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The Amino Group

• Aniline reacts with bromine water (without a
  catalyst) to yield the tribromide. Sodium
  bicarbonate is added to neutralize the HBr thats
  also formed.

Animation
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Summary ofActivators
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Deactivating Meta-Directing Substituents

• Electrophilic substitution reactions for
  nitrobenzene are 100,000 times slower than for
  benzene.
• The product mix contains mostly the meta isomer,
  only small amounts of the ortho and para isomers.
• Meta-directors deactivate all positions on the
  ring, but the meta position is less deactivated.
  

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Ortho Substitutionon Nitrobenzene
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Para Substitution on Nitrobenzene
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Meta Substitutionon Nitrobenzene
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Energy Diagram
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Structure of Meta-Directing Deactivators

• The atom attached to the aromatic ring will have
  a partial positive charge.
• Electron density is withdrawn inductively along
  the sigma bond, so the ring is less electron-rich
  than benzene.

Animation
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Summary of Deactivators
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More Deactivators
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Halobenzenes

• Halogens are deactivating toward electrophilic
  substitution, but are ortho, para-directing!
• Since halogens are very electronegative, they
  withdraw electron density from the ring
  inductively along the sigma bond.
• But halogens have lone pairs of electrons that
  can stabilize the sigma complex by resonance.
  

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Sigma Complexfor Bromobenzene
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Energy Diagram
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Summary of Directing Effects
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2 or more directing groups
When they reinforce each other, pretty easy to
predict the product.
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2 or more directing groups
The directing groups conflict each other.
Mixture of products results. If activating and
deactivating groups conflict, activating groups
usually win.