Chapter 17 & 18 Discussion
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Transcript of Chapter 17 & 18 Discussion
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Chapter 17 & 18 Discussion
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Flat, cyclic, conjugated pi system and 4n + 2 pi electrons
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Aromatic if 2, 6, 10, 14, 18 pi electrons…..Anti-aromatic if 4, 8, 12, 16, ….. pi electrons
If neither then not aromatic or anti-aromatic.
Anti-aromatic more stable than expected.
Anti-aromatic – less stable and likely will not form at all.
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4 pi electrons. Anti-aromatic
Cyclopropenyl anion
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Aromatic?
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6 pi electrons, aromatic
Cyclopentadienyl anion
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Aromatic?
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10 pi electrons aromatic
Lactarius azulene
chamomileYarrow
Azulene
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aromatic : cyclic, 10 electrons, flat, conjugated
Naphthalene
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Why is naphthalene nonpolar and azulene polar?
Azulenenaphthalene
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Aromatic, anti-aromatic or neither?
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Aromatic, anti-aromatic or neither?
Depends on conformation. If flat and fully conjugated, antiaromatic!!!!
Lower energy conformation (than anti-aromatic) is not flat
8 pi electrons
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NMR
• Proton deshielded by ring current from aromatic pi system to 6-8 ppm
• Carbon peaks between 115-140 ppm
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H NMR of aromatics
Singlet 7.26 ppm
Singlet (3H) 2.34 ppm &
Second order spin coupling
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• Electrophilic substitution
• Diazonium chemistry
• Nucleophilic substitution
• Misc. modification of substituents
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Electrophilic Substitution Reactions of Benzene and Its Derivatives
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Halogens are ortho para directors, but are still deactivating compared with benzene
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Meta selective: electron withdrawing groups
•electron withdrawing group inductively destabilizes adjacent carbocations in ortho and para cases. • No adjacent carbocation in meta regiochemistry resonance structures
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Summary of activation versus deactivation and ortho & para versus meta direction
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With multiple substitutents there can be antagonistic or reinforcing effects
Antagonistic or Non-Cooperative
D = Electron Donating Group (ortho/para-directing)W = Electron Withdrawing Group (meta-directing)
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Multiple Substituents in antagonistic systems
The most strongly activating substituent will determine the position of the next substitution. May have mixtures.
OCH3
O2N
SO3
H2SO4
OCH3
O2N
SO3H
OCH3
O2N
SO3H
+
=>
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Weak step here
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