Chapter 161 through 166 Aromatic Compounds

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Chapter 16-1 through 16-6Aromatic Compounds
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Quiz

• Classify the following compound as aromatic,
  antiaromatic, or nonaromatic

Nonaromatic
The answer is
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Discovery of Benzene

• Isolated in 1825 by Michael Faraday who
  determined CH ratio to be 11.
• Other related compounds with low CH ratios had a
  pleasant smell, so they were classified as
  aromatic.
  gt

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Kekulé Structure

• Proposed in 1866 by Friedrich Kekulé, shortly
  after multiple bonds were suggested.
• Failed to explain existence of only one isomer of
  1,2-dichlorobenzene.

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Resonance Structure

• Each sp2 hybridized C in the ring has an
  unhybridized p orbital perpendicular to the ring
  which overlaps around the ring.

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Unusual Reactions

• Alkene KMnO4 ? diol (addition)Benzene KMnO4
  ? no reaction.
• Alkene Br2/CCl4 ? dibromide (addition)Benzene
  Br2/CCl4 ? no reaction.

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Unusual Stability
gt
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Annulenes

• All cyclic conjugated hydrocarbons were proposed
  to be aromatic.
• However, cyclobutadiene is so reactive that it
  dimerizes before it can be isolated.
• And cyclooctatetraene adds Br2 readily. (Acts
  like regular alkene.)
• Look at MOs to explain aromaticity.
  gt

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MO Rules for Benzene

• Six overlapping p orbitals must form six
  molecular orbitals.
• Three will be bonding, three antibonding.
• Lowest energy MO will have all bonding
  interactions, no nodes.
• As energy of MO increases, the number of nodes
  increases. gt

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MOs for Benzene
gt
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Energy Diagram for Benzene

• The six electrons fill three bonding pi orbitals.
• All bonding orbitals are filled (closed shell),
  an extremely stable arrangement.

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MOs for Cyclobutadiene
gt
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Energy Diagram forCyclobutadiene

• Following Hunds rule, two electrons are in
  separate orbitals.
• This diradical would be very reactive.
  gt

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Polygon Rule

• The energy diagram for an annulene has the same
  shape as the cyclic compound with one vertex at
  the bottom.

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Aromatic Requirements

• Structure must be cyclic with conjugated pi
  bonds.
• Each atom in the ring must have an unhybridized p
  orbital.
• The p orbitals must overlap continuously around
  the ring. (Usually planar structure)
• Compound is more stable than its open-chain
  counterpart.

Aromatic
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Anti- and Nonaromatic

• Antiaromatic compounds are cyclic, conjugated,
  with overlapping p orbitals around the ring, but
  the energy of the compound is greater than its
  open-chain counterpart (it is less stable)
• Nonaromatic compounds do not have a continuous
  ring of overlapping p orbitals and may be
  nonplanar.

Nonaromatic
Anti-aromatic
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Hückels Rule

• If the compound has a continuous ring of
  overlapping p orbitals and has 4N 2 electrons,
  it is aromatic.
• If the compound has a continuous ring of
  overlapping p orbitals and has 4N electrons, it
  is antiaromatic.
  gt

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NAnnulenes
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NAnnulenes

• 4Annulene is antiaromatic (4N e-s)
• 8Annulene would be antiaromatic, but its not
  planar, so its nonaromatic.
• 10Annulene is aromatic except for the isomers
  that are not planar.
• Larger 4N annulenes are not antiaromatic because
  they are flexible enough to become nonplanar.
  gt

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MO Derivation of Hückels Rule

• Lowest energy MO has 2 electrons.
• Each filled shell has 4 electrons.
  

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Polygon Rule

• The energy diagram for an annulene has the same
  shape as the cyclic compound with one vertex at
  the bottom.

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Describe these as aromatic, antiaromatic, or
nonaromatic.
Aromatic (6 electrons)
Nonaromatic
Aromatic (2 electrons)
Anti-aromatic (8 electrons)
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Is the cycloheptatrienyl cation planar or not?
Explain.
Yes, it is planar. Being planar allows it to be
aromatic with 6 pi electrons.
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For Next Time

• Read 16-7 to 16-11, Problems 9-11, 13, 16, 18
  a-d, f-g