Electrophillic Aromatic Substitution
Chapter 18

As we look at the aromatic ring we see that it is full of double bonds; it is electron rich and will
probably be nucleophillic. it should react with electrophiles. Review reactions of alkenes
Aromatic rings are especially stable. It will be difficult to remove the aromaticity.
They will react with only especially strong electrophiles. Will not react the same way as alkenes.


Electrophillic Aromatic Substitution follows the same general mechanism although there are many
different reactions.
General Electrophillic Aromatic Substitution Mechanism
E
Y-
E
Y
E
H
+
slow
H
Note the following things:
1. The E is a general electrophile and will be substituted for a H in the aromatic ring. We will learn a
number of specific electrophiles next.
2. The first step is a slow step or the rate determining step.
3. The carbocation is a reactive intermediate
4. The re-aromatization is the fast step.
Nitrating the ring:
1. The reagents
HNO3
H2SO4
2.
Formation of the “hot electrophile’
H
O
H2SO4
N+
H
N
+
O
+
N
+
O
O -
O -
3.
O
O
H
O
+ HSO4-
+H2O + HSO4-
The mechanism:
O
NO 2
N+
H
O
slow
+
H
H2O
E
Sulfonation
1.
SO3
The reagents
H2SO4
2. Formation of the “hot electrophile’ : (SO3 is already hot)
3.
O
SO3-
+ S
SO3-
H
O
-O
HSO4-
+
slow
H
H2SO4
1.
2.
Bromination(and chlorination)
F too fast, I too slow (I2/Cu2+ works for I)
The reagents
or Br2/Fe
Br
2
FeBr3
3.
Formation of the “hot electrophile”
Br
4.
+
Br
FeBr3
mechanism
Br
+
Br
FeBr3
Br
-
BrFe Br 3
Br
H
slow
+
HBr
H
FeBr3
1.
2.
3.
4.
5.
Freidal Crafts Alkylation
Similar to bromination
No electron withdrawing groups allowed on ring
Difficult to stop at mono alkylation
rearrangements of alkyl group can occur so be careful with primary alkyl halides
Reagents: alkyl only
(CH3)CCl
AlCl 3
6. Formation of the “hot electrophile”
(CH3)CCl
AlCl 3
(CH3)C+
+ AlCl 4-
proton
Xfer
Freidal Crafts Acylation
1.
2.
3.
No rearragnements
Stops at 1 reaction.
Reagents:
O
Cl
4.
AlCl 3
Formation of “Hot electrophile”
O
O
O
AlCl 3
+
Cl
5.
Cl
-
+
AlCl 3
Reaction
O
COCH 3
+
AlCl 4-
O
H
slow
+
H
Substituent effects.
Substituents can be electron donating/Electron Withdrawing
There are two kinds of effects, pi(resonance effect) and sigma (inductive effect) networks.
HCl
Activating groups/ o,p directors!!
-NH2, -OH, -OR, -NR2
These are all groups with lone pairs next to the ring. They can stabilize the reactive intermediate with an
extra resonance structure. They can only do this when E is being added in the o or p position. If it is in the
m position the + charge will not line up.
OH
OH
H
H
E
OH
E
O
H
E
H
E
-CH3 and other alkyl groups.
Donate electron density thru sigma bonds or through the sigma network.
Less effective than the resonance activators. No extra resonance structures
CH3
H
E
H
Benzene.
The halogens.
Deactivate because the withdraw electron density through the sigma network.
Are o,p directors because they can resonance stabilize.
E
Y-
E
Y
H
+
slow
Cl
Cl
less likely to occur
Cl
H
Cl
E
H
E
Deactivators and metadirectors
The whole ring is deactivated by the electron withdrawing nature of the substituent.
The o, and p positions are deactivated even worse by the positive charge next to the ring so if a reaction
occurs it will occur in the meta position.
O
O
O
N
H
O
O
E
H
O
N
N
E
E
H
Others in this group are –CN, -CHO and other carbonyls such as acids and esters,
The worst of this group of meta deactivators is –NR3+ which is different mostly in the fact that it has
a complete + charge and it is not on a p orbital so there are no resonance effects.
Adding a third substituent:
1. If the first two support each other than it is all good but..
2. If the two are not in support of each other than the more activating group overcomes. (usually)
3. Sterics can play a role
Other reactions
Br2, hv
Br
or
NBS, ROOR
O
1. KMnO4, (OH-)
OH
2. H3O+
O
Zn(Hg), HCl
or
H2NNH2, KOH
H2O, heat
H2, Pd/C
NO2
or
Sn, HCl
or
Fe, HCl
NH2