Chapter 16 & 17 Discussion
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Transcript of Chapter 16 & 17 Discussion
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Chapter 16 & 17 Discussion
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Endo and exo are meaningless without substitutents to provide frame of reference
Meso = identical
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1) Recognize that this is a Diels Alder reaction by the diene and dienophile (on the right)
2) Draw a cyclohexene ring
3) Build models of the possible isomers that could result from the cycloaddition
4) Use resonance to help predict regioisomer
5) Temperature for endo/exo preference.
diene dienophile
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3) Build models of the possible isomers that could result from the cycloaddition4) Use resonance to help predict regioisomer
5) Temperature for endo/exo preference.
endo
exo
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Endo and exo
+ enantiomer + enantiomer
Exo (thermodynamic product)
Endo (kinetic product)
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?
1) Recognize that this is a Diels Alder reaction by the diene and dienophile (on the right)
2) Draw a cyclohexene ring
3) Build models of the possible isomers that could result from the cycloaddition
4) Use resonance to help predict regioisomer
5) Temperature for endo/exo preference.
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Four possible regioisomers and diastereomers (+ enantiomers)
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Resonance contributors show which regioisomers will form
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Regiochemistry is established. How about endo or exo?
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?
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No meta.
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Problem:Diels-Alder Reactants?
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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. Antiaromatic
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