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Chapter 12 :
Reactions of Arenes. Electrophilic Aromatic Substitution |
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Electrophilic Aromatic Substitution
Overall an electrophilic aromatic susbtitution (EArS) can be represented
as follows:
There are three fundamental components to an electrophilic substitution
reaction:
-
formation of the new s bond from a C=C
in the arene nucleophile
-
removal of the proton by breaking the C-H s
bond
-
reforming the C=C and restoring aromaticity
The mechanism is represented by the following series of events:
-
Formation of the reactive electrophile, E+
(not
shown here)
-
Slow reaction of the arene C=C with the E+
to give a resonance stabilized carbocation
(see below)
-
Loss of H+ from the carbocation
to restore the C=C and the aromatic system
The reaction of the electrophile E+
with the arene is the slow step since it results in the loss of aromaticity
even though the resulting cation is still resonance stablised. This carbocation
is also described as the cyclohexdienyl cation or arenium ion
or as a s-complex.
