Honors Chemistry II
AROMATIC REACTIONS
Aromatic Reactions
1. Oxidation Reactions

Oxidation only occurs if a substituent off of the ring has a “benzyl” hydrogen. No
consistent mechanism is known for this reaction.
benzyl hydrogens
H
H
H
H
H
H
KMnO4
or
or
H3O+
Electrophilic Aromatic Substitution Reactions

Lewis acids are electron pair acceptors (called _______________, or E+), so they
attack the pi electrons of the benzene ring.

Instead of attacking and breaking apart the ring itself, the electrophile
_______________ a hydrogen atom.

General Equation for Electrophilic Substitution Reactions:
H
H
H
H
H
E+
H

If you were to attempt to react Br2 with benzene, nothing would happen!
Special reaction conditions are required for aromatic reactions.
2. Halogenation
To add a halogen, it must be done in the presence of an iron compound that
contains the halogen itself. The iron compound activates the electrophile.
Br2
FeBr3
Mechanism:
Generate the Electrophile
3. Nitration
To add a nitro group, it must be done in the presence of H2SO4. The sulfuric acid
activates the electrophile.
HNO3
H2SO4
Mechanism:
Generate the Electrophile
4. Friedel-Crafts Alkylation
This method adds alkyl groups to the benzene ring by reacting benzene with a
chlorinated alkane in the presence of aluminum chloride. The aluminum chloride creates
a carbocation that bonds to the benzene ring.
Cl
AlCl3
Mechanism:
Generate the Electrophile
5. Friedel-Crafts Acylation
This method adds a ketone side-chain (acyl group) to the benzene ring.
O
Cl
AlCl3
Mechanism:
Generate the Electrophile
Substitution on an Already-Substituted Ring

What options do we have for the following general reaction?
Group
+ E+ 

Where will the electrophile go?
o It depends on the group already on the ring. The group on the ring “directs”
where the electrophile substitutes. There are two types of “directors.”
Ortho-Para directors: Directs incoming electrophiles to ortho or para positions.
Examples:

How do these ortho-para directors work? Take methoxybenzene for example.
Draw its resonance structures.

Each carbon on which a negative charge can exist is where the electrophile can
attack.
Meta directors: Directs incoming electrophiles to meta positions.

A common characteristic of meta directors is that the atom directly attached to the
benzene ring is multiply bonded to an electronegative atom. How do these meta
directors work? Draw its resonance structures.

The only positions that do not have a positive charge on them are the meta
positions. This is where the electrophile will attack.