Chapter 16 Aromatic Compounds

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Chapter 16Aromatic Compounds
Organic Chemistry, 5th EditionL. G. Wade, Jr.

• Jo Blackburn
• Richland College, Dallas, TX
• Dallas County Community College District
• ã 2003, Prentice Hall

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Discovery of Benzene

• Isolated in 1825 by Michael Faraday who
  determined CH ratio to be 11.
• Synthesized in 1834 by Eilhard Mitscherlich who
  determined molecular formula to be C6H6.
• Other related compounds with low CH ratios had a
  pleasant smell, so they were classified as
  aromatic.

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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.

Cl2

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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 ?
• Benzene KMnO4
• Alkene Br2/CCl4 ? Benzene Br2/CCl4 ?
• Benzene FeCl3/Br2 ?

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Unusual Stability

• Hydrogenation of just one double bond in benzene
  is endothermic!

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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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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.

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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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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.

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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.

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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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Arromaticity of NAnnulenes

• 4Annulene?
• 8Annulene?
• 10Annulene?
• Larger annulenes?

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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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Tropylium Ion

• The cycloheptatrienyl cation has 6 p electrons
  and an empty p orbital.
• Aromatic more stable than open chain ion

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Dianion of 8Annulene

• Cyclooctatetraene easily forms a -2 ion.
• Ten electrons, continuous overlapping p orbitals,
  so it is aromatic.

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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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Basic or Nonbasic?
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Other Heterocyclics
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Fused Ring Hydrocarbons

• Naphthalene

• 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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Physical Properties

• Melting points
• Boiling points
• Density
• Solubility

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IR and NMR Spectroscopy

• CC stretch
• sp2 C-H stretch
• 1H NMR
• 13C NMR

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Mass Spectrometry
gt
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UV Spectroscopy
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End of Chapter 16