MO

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MOs for Cyclobutadiene
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Energy Diagram forCyclobutadiene

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

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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.
• Nonaromatic compounds do not have a continuous
  ring of overlapping p orbitals and may be
  nonplanar. gt

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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.
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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.
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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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Cyclopentadienyl Ions

• The cation has an empty p orbital, 4 electrons,
  so antiaromatic.
• The anion has a nonbonding pair of electrons in a
  p orbital, 6 e-s, aromatic.

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Acidity of Cyclopentadiene

• pKa of cyclopentadiene is 16, much more acidic
  than other hydrocarbons.

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