So, electrophilic aromatic substitution. This
is the general default model for this particular
reaction and this is one of the most common
ones. Because, of course, the benzene ring
is electron rich even though the bonds are
a bit stronger, you tend to only really see
these types of reactions unless you have somehow
activated your benzene ring with electron
What happens in this case is that two of the
electrons from one of the idealised alkene
bonds in our aromatic compound attacks; remember,
electrons move, protons do not move, neither
do positively charged species. Electrons attack
on to, in this case, idealised electrophile,
here E+. What this does is that it affectively
breaks open or destroys the aromaticity of
that ring temporarily. We note we only have
two double bonds now. We don’t have 6 pi
And so, what then happens in order to reform
the stability associated with the aromatic
compound is one or two things must happen.
Either E+ is lost again, just so that we can
regenerate aromaticity, or we lose the single
hydrogen that is attached to the carbon on
which the electrophile becomes attacked.
In this case, we have shown a successful electrophilic
aromatic substitution where we lose H+ and
a new substituent in its place is E, our electrophile.
E+ is usually an electrophile which is generated
in situ via a preliminary reaction.
And these are the different reaction types.
Each of these mechanisms is almost identical
to the one I have shown you earlier.
Halogenation is achieved via Cl+/Br+ which
is usually generated in situ, for example,
bromine in the presence of iron bromide. Nitration,
which is generated in situ via the reaction
of sulphuric acid and nitric acid to give
the NO2+ electrophile. Sulfonation, usually
it is sulphur trioxide in the presence of
acid. This gives rise to a sulphonation electrophile
SO3H+. Alkylation is one of the more desirable
reactions that you want to do on an aromatic
compound and that cannot directly be achieved
by reacting it with, for example, an alkyl
halide. You cannot just add that over a double
bond. Friedel-Crafts Alkylation takes place
in the presence of a Lewis acid catalyst.
Usually, for example, in the presence of a
chloroalkane, you would use something like
aluminium trichloride to facilitate that reaction.
And the same applies with Friedel-Crafts Acylation.
This is particularly important when you want
to introduce an acyl group. This effectively
will resemble a ketone group being formed
on your benzene ring.
Usually, as we will see, we use something
like an acid chloride again in the presence
of an aluminium trichloride catalyst. Electrophiles
are formed in situ in a preliminary step.
So, here we have an example of such an electrophilic
aromatic substitution taking place. We have
substituted E+ for Cl+ and in the first step,
we can see the electrons from the benzene
ring, taking it from an idealised double bond,
attack our electrophile leaving electrons
on to that and forming a sigma bond between
the two. What we then see is this intermediate,
this positively charged species which has
had the aromaticity of the ring disrupted.
The second step involves the elimination of
the H+ and the real formation of the benzene
ring and is this reformation of the benzene
ring which is a driving force. And this is
important to bear in mind, where there is
the possibility for a structure to adopt an
aromatic state, this is usually good energetic
reason why it would take place.