Reactions of Aromatic Compounds

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Reactions of Aromatic Compounds
Electrophilic Aromatic Substitution (EAS)
sp3 carbon
?
?
?
R.D.S.
?
Resonance-stabilized sigma (?) complex
?
?
Base-H

• Usually the first step is slow and
  rate-determining.
• There is evidence that a weaker ? complex forms
  prior to ? complex.

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?1 ?2
Ea2
E
?-complex
Ea1
Reactants
Products
Reaction Coordinate

• Halogenation of Benzene
• Electrophile Reactant X2/Lewis Acid
• X2 Lewis
  Acids
• Br2
  FeBr3 (catalyst)
• Cl2
  AlCl3 (catalyst)
• I2 HNO3 (consumed)

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Bromination of Benzene using Br2/FeBr3
? ?-
Br-Br----FeBr3
?
?
FeBr4
?
?
?
FeBr3 Br
?
Br
HBr
Bromobenzene
Chlorobenzene
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2. Nitration of Benzene HNO3/H2SO4
?
?
Nitronium Ion Electrophile
NO2?
?
?
?
?
HSO4
?
H2SO4
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3. Sulfonation of Benzene SO3
Fuming sulfuric acid 7 SO3 dissolved in conc.
H2SO4
..
..
..
..


?


..
?
..
Electrophilic Sulfur

..
..
?
?

..
?
?
H
Benzenesulfonic acid
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Sulfonation is one of the few EAS reactions that
is reversible

SO3
4. Friedal-Crafts Alkylation Electrophile R
?
?
?
Alkylbenzene
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?
Methods of generating carbocations, R

• Alkyl Halide/Lewis Acid
• RX AlX3 ? RX ----AlX3

? ?-
Highly polarized RX bond
2. Alkene/HF
?
?
HF ?
F
3. Alcohol/BF3
.. ..
..
?
?
?
?
R-O-H BF3 ? R-O---BF3 ? R
HOBF3
H
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Limitations Carbocations Rearrangements
AlCl3

ClCH2CH2CH3

How did this happen? Answer

• Friedal-Crafts Acylation
• Electrophile
  Acylium ion

?
AlCl3
Aryl ketone

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Mechanism

?
Cl3Al
?
Cl3Al-Cl


?
?
?

• Acylium ion is resonance stabilized
• Product is ketone but ketone may be reduced to
  an alkyl group if desired.

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George Olah, USC
for his contribution to carbocation chemistry
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Reactions of Monosubstituted Aromatic
Compounds I. Activating, ortho,para-directing
Groups
H2SO4
HNO3
ortho para meta 40
57 3

• Toluene undergoes ortho, para substitution in
  other EAS reactions as well.
• Toluene reacts 25 times faster than benzene in
  nitration.
• Conclusion Alkyl groups in general are
  ortho,para directing and activating
• in EAS reactions

Other groups behave similarly.
..
?
Most Reactive
Least Reactive
gt gt gt gt
gt
phenoxide anilines phenol ether
amide alkylbenzene
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Two questions 1. Why are these groups
activating compared to benzene in EAS reactions.
Answer They are electron donating and make the
ring more electron-rich and,
hence, more reactive toward E .
?
Two mechanisms for electron-donation i)
Resonance must have electron pair adjacent to
ring conjugated with ? system.
?
?
?

?

?
..
?
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ii) Induction donation of electrons through ?
bonds
sp3
sp2

• Electronegativity sp2 gt sp3 carbon
• Donates through sigma bond.

2. Why are these groups ortho, para directing?
Answer They stabilize the ?-complex that forms
from the ortho or para attack by
the electrophile, E .
?
.. .. ..
?
?
E para E meta
?
4 resonance forms
?
?
..
.. .. ..
?
3 resonance forms
?
?
?
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Conclusion Ortho, para attack by the
electrophile results in a more stable ?-complex,
therefore ortho, para substitution takes place
faster than meta substitution.
II. Deactivating, Meta-directing Groups
H2SO4
HNO3
ortho para meta 6
1 93
?
Most Reactive
Least Reactive
gt gt gt gt
gt
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Two questions 1. Why are these groups
deactivating compared to benzene in EAS
reactions?
Answer They are electron-withdrawing and make
the ring electron-poor and less reactive toward
E?.
You draw in the arrows for these resonance
forms!
?
?
?
?
?
?
?
?
?
?
?
?
?
?
2. Why are these groups meta-directing?
Answer They destabilize the ?-complex that
forms from the ortho or para
attack by the electrophile, E more than that
formed from meta attack
?
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?
E para
?
Less stable
?
?
Especially unstable since it places ?
charge adjacent to electron-withdrawing group.
?
E meta
?
?
More stable
?
Conclusion Meta attack by the electrophile
results in a more stable ?-complex, therefore
meta substitution takes place faster than ortho,
para substitution.
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Special Case Halogens Deactivating but
ortho, para directing.
1. Why deactivating? Answer
They withdraw electrons by induction, making the
aromatic ring less
nucleophilic.
2. Why ortho, para directing? Answer
They stabilize the ?-complex that forms from o,p
attack more than that
formed from meta attack. They do this by
donating a lone pair of
electrons to the ring by resonance.
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Activating Groups- more reactive than benzene
ortho, para
directing
?
Most Reactive
gt gt gt
gt gt
?
gt gt gt
gt gt gt
gt
Least Reactive
Deactivating Groups less reactive than benzene
meta
directing (except halogen)
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Predict the Products!
F-C Alkylation only works with benzene,
halobenzene and other activated compounds. Also,
the compounds tend to polyalkylate. Why? F-C
Acylation also only works with benzene,
halobenzene and other activated compounds. But,
the reactive compounds do not acylate further.
Why not?
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• What about aromatic compounds with more than one
  substituent?
• Guideline
• Activating groups are stronger directors than
  deactivating groups.
• Powerful o,p directors -OH, -OR, -NR2
• Moderate o,p directors -R, -X
• Meta directors -CN, SO3H, -COR, etc
• Order of Dominance 1 gt 2 gt 3

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Nucleophilic Aromatic Substitution (NAS)
1000C
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Mechanism NAS
?

?
Anionic intermediate stabilized by
electron- withdrawing groups in the o,p positions
Compare
No Reaction
1000C
Q How many resonance forms can you draw for the
anionic intermediate above?
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Benzyne Mechanism
?
?
HCl
3500C
Mechanism
?
OH
?
OH
?
Benzyne triple bond in ring!
H-O-H
?
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