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sp2

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Meta-directors deactivate all positions on the ring, but the meta position is less deactivated. ... Halogens are deactivating toward electrophilic substitution, ... – PowerPoint PPT presentation

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Title: sp2


1
Chapter 17
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sp2
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General Scheme
  • Create the electrophile (Lewis Acid needed)
  • Electrophile attacks the aromatic
  • Creating a resonance stabilized carbocation
  • Lewis Base removes the hydrogen
  • Re-establishing the stabilized aromatic

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

NO2 then forms a sigma complex with benzene,
loses H to form nitrobenzene. gt
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Sulfonation
  • Sulfur trioxide, SO3, in fuming sulfuric acid is
    the electrophile.

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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.

gt
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Energy Diagram
gt
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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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Nitration of Anisole
Ortho attack
Meta attack
Para attack
gt
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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.

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The Amino Group
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Summary of Activatorsortho para directors
gt
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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.


    gt

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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.
    gt

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Summary of DeactivatingMeta directors
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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.
    gt

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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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Multiple Substituents
Use section 17-9 as review of concepts
  • The most strongly activating substituent will
    determine the position of the next substitution.
    May have mixtures.
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