Resonance Structure

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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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MOs for Benzene
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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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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.
• Look at MOs to explain aromaticity.
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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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Cyclobutadiene

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

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Unstable di-radical
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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)
• Aromatic the compound is more stable than its
  open-chain counterpart
• Antiaromatic 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

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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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MO Derivation of Hückels Rule

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

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Pyridine

• Heterocyclic aromatic compound.
• Nonbonding pair of electrons in sp2 orbital, so
  weak base, pKb 8.8.

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Pyrrole

• Also aromatic, but lone pair of electrons is
  delocalized, so much weaker base.

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Other Heterocyclics
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Fused Ring Hydrocarbons

• Naphthalene- 10es not 12es in two separate rings

• Anthracene

• Phenanthrene

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Reactivity of Polynuclear Hydrocarbons

• As the number of aromatic rings increases, the
  resonance energy per ring decreases, so larger
  PAHs will add Br2.

(mixture of cis and trans isomers)
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Fused Heterocyclic Compounds

• Common in nature, synthesized for drugs.

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Allotropes of Carbon

• Amorphous small particles of graphite charcoal,
  soot, coal, carbon black.
• Diamond a lattice of tetrahedral Cs.
• Graphite layers of fused aromatic rings.

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Some New Allotropes

• Fullerenes 5- and 6-membered rings arranged to
  form a soccer ball structure.
• Nanotubes half of a C60 sphere fused to a
  cylinder of fused aromatic rings.

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Common Names of Benzene Derivatives
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Disubstituted Benzenes
The prefixes ortho-, meta-, and para-
are commonly used for the 1,2-, 1,3-, and
1,4- positions, respectively.
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3 or More Substituents
Use the smallest possible numbers, but the carbon
with a functional group is 1.
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Common Names forDisubstituted Benzenes
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Phenyl and Benzyl
Phenyl indicates the benzene ring attachment.
The benzyl group has an additional carbon.
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IR and NMR Spectroscopy

• CC stretch absorption at 1600 cm-1.
• sp2 C-H stretch just above 3000 cm-1.
• 1H NMR at ?7-?8 for Hs on aromatic ring.
• 13C NMR at ?120-?150, similar to alkene carbons.
  
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