Benzene and the Concept of Aromaticity

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Benzene and the Concept of Aromaticity
two equivalent resonance forms
other representations
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(Un)Reactivity (stability) of Benzene

• Highly unsaturated (r db 4), yet Br2 or HBr
  does not add across multiple bonds as with
  alkenes.
• Reacts with Br2 in presence of FeBr3 catalyst by
  SUBSTITUTION rather than by addition, which is
  the way alkenes react with Br2.

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Stability of Benzene
Hypothetical cyclohexatriene
Actual Benzene
(same product)
4
Pi Bonding in Benzene
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Delocalization in Benzene
Note complete delocalization of p electrons!
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Hückel Definition of Aromaticity

• For a system to be aromatic, it must have
• 4n 2 p electrons (for n any integer 0, 1, 2,
  etc.)
• in the periphery
• of a monocyclic
• planar
• delocalized (conjugated) system
• (Hückels n 2, 6, 10, 14, 18,
  etc.)

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Examples of Aromatic Systems

• p e 6 2
  6 6

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Examples of Aromatic Systems
1 more res. form
3 more res. forms
5 more res. forms
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Some Non-Aromatic Systems

• p e 4 8 4
  10
  (not planar!)

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Some Heteroaromatic Systems

• pyridine pyrrole
  imidazole
• (has a lp in sp2) (lp is in p orbital)
  (has a lp in sp2 hybrid
  orbital a lp in p orbital)

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Some Heteroaromatic Systems
4
4
3
4
3
5
3
5
5
2
2
6
2
1
1
1

• pyridine pyrrole
  imidazole
• (has a lp in sp2) (lp is in p orbital)
  (has a lp in sp2 hybrid
  orbital a lp in p orbital)

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Bonding in Pyrrole Imidazole
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Polycyclic Aromatic Compounds

• 10 p e- 14 p e- 14 p e-
• naphthalene anthracene
  phenanthrene
• All three are found in coal. Note that in
  this Kekule
• resonance form they obey Hückels rule try
  others!

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Spectra of Aromatic Compounds

• IR Ar C-H 3030 cm-1
• Ar CC 1600, 1500, (1450) cm-1
• (2 or 3 sharp bands)
• 1H-NMR Ar H 6.5-8.0 d
• 13C-NMR Ar C 110-160 d
• UV n - p 205 nm (strong)
• p - p 255-275 nm (weak)