1 tropylium ion the cycloheptatrienyl cation has 6 p electrons and an empty p orbital. aromatic:...
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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
>=
H OH
H+ , H2O
H
+
2
Dianion of [8]Annulene
Cyclooctatetraene easily forms a -2 ion.Ten electrons, continuous overlapping p
orbitals, so it is aromatic .
>=
+ 2 K + 2 K+
3
PyridineHeterocyclic aromatic compound.
Nonbonding pair of electrons in sp2 orbital, so weak base, pKb = 8.8.
=>
4
PyrroleAlso aromatic, but lone pair of electrons is delocalized, so much weaker base.
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5
Basic or Nonbasic?
NNPyrimidine has two basicnitrogens.
N N H Imidazole has one basicnitrogen and one nonbasic.
N
N
N
N
H
Purine?=>
6
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 PAH’s will add Br2.
H Br
H BrH Br
Br
H
(mixture of cis and trans isomers) =>
9
Fused Heterocyclic Compounds
Common in nature, synthesized for drugs.
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10
Allotropes of CarbonAmorphous: small particles of graphite; charcoal, soot, coal, carbon black.
Diamond: a lattice of tetrahedral C’s.
Graphite: layers of fused aromatic rings.
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