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CHAPTER 9: ARENES - Coggle Diagram
CHAPTER 9:
ARENES
Reactivity of Arenes
3.1. Electrophilic Aromatic Substitution
Two steps, 1st step is the RDS
More stable sigma complex → faster reaction
Acid Lewis catalyst: FeX3 or AlX3
Reaction with F2 is very strong and hard to control.
Reaction with I2 is very slow
3.1.1. Halogenation
3.1.2. Sulfonation
The -SO3H group can be removed by using dilute H2SO4
3.1.3. Nitration
The -NO2 group can be reduced to -NH2 by using Sn/Fe/Zn in acid
The -NH2 group can be removed by using converting into diazonium salt, then treating with H3PO2
3.1.4. Friedel-Crafts Alkylation
For primary alkyl, carbocation rearrangement may happen.
C sp2 or sp → No reaction
Alkylation can also occur with alkene and protic acids
For 1° or 2° alkyl, carbocation rearrangement may happen.
3.1.5. Friedel-Crafts Acylation
For primary alkyl, to avoid carbocation rearrangement, we can do Friedel-Crafts acylation, then perform Clemmensen or Wolff-Kishner reduction
Formylation (-CHO: formyl group)
Gattermann-Koch reaction
Vilsmeier–Haack reaction
3.1.6. Effect of Substituents
Electron Donation Group (EDG)
Halogens
For halogen: -I > +R
Halogens deactivate the aromatic ring (-I)
But they direct the electrophile to the ortho and para positions (+R)
More Considerations
If EWG is more deactivating than halogen → cannot undergo Friedel-Crafts alkylation/acylation
Lone pair of nitrogen can poison the Lewis acid catalyst → Aniline & N-substituted anilines do not undergo Friedel- Crafts alkylation/acylation
Aniline & N-substituted anilines also do not undergo nitration, because amines can be oxidized under nitration reaction condition
Multiple Substituents
EDG vs. EWG 🡪 EDG determines the location of electrophile
EDG vs. EDG 🡪 Stronger EDG determines the location of electrophile
Steric Effect
Synthesis Strategy
3.2. Nucleophilic Aromatic Substitution
Very harsh reaction
EWGs must be positioned ortho/para to the leaving group
More EWGs → easier to carry out the reaction
3.3. Reactions at Benzylic Position
3.3.1. Free Radical Substitution (SR)
Mechanism involves 3 steps
Initiation
Propagation
Termination (similar to SR reaction of alkanes)
3.3.2. Oxidation
With strong oxidant (MnO4-, Cr2O72-), if the benzylic position has at least 1 H → oxidized to benzoic acid
3.4. Reduction
3.4.1. Fully reduce benzene to cyclohexane
3.4.2. Birch reduction
3.5. Oxidation
Ozonolysis: Similar to other hydrocarbons, arenes can also undergo combustion
Aromaticity
Criteria for aromaticity: Huckel’s rule
un-interrupted cyclic π cloud above & below the plane of the molecule
The π cloud must contain (4n + 2) π electrons (n = 0, 1, 2…)
Aromatic (4n + 2)
Not aromatic (not 4n + 2)
Not aromatic (interrupted π system)
Preparation of Arenes
Other arenes can be synthesize from benzene or its derivatives
Synthesis of benzene from acetylene (trimerization)