Reactions of Aromatic Compounds

Sulfonation of Benzene
Benzene reacts with fuming sulfuric acid (concentrated sulfuric acid
plus added SO3, the actual electrophile) to give benzenesulfonic acid.
Fuming H2SO4 25 o O SO3H S=O
O
C
. . .
.
+
Benzenesulfonic acid
In concentrated sulfuric acid alone, an equilibrium-limited supply of
SO 3 effects slow sulfonation.
(1) Generation of the Electrophile
:O:
H-O-S-O-H
: :
= = :O:
: :
=
: :
:O:
+ H-O-S-O-H
: :
= = :O:
:O:
+
H-O-S-O-H
H
:
=
= = :O:
: :
: :
Sulfur trioxide
: :
:O:
- +
:O:
H-O-S-O: + H-O-S-O-H
H
: :
= = :O:
: :
:
H3O + O
+ S=O
O
.. . =
: :
=
: :
= = :O:
Sulfur trioxide
: :