Aromatic compounds all contain benzene or aromatic rings.
Aromatic compounds are susceptible to
attack by electrophiles and undergo electrophilic substitution reactions. Recall that
electrophiles are electron deficient species. Benzene rings since they contain clouds
of delocalised
electrons above and below the plane of the carbon atoms (see image opposite) are prone to attack by
electrophiles. The
clouds of delocalised electrons are sterically unhindered and
easily accessible by electrophile.
The benzene ring acts as a source of electron, that is it is a
nucleophile. In an electrophilic substitution
reaction:
The delocalised electrons in an
aromatic molecule
will attack an electrophile. The electrophile
will then replace or substitute for
one of the hydrogen atoms in the benzene ring to generate a positively charged intermediate cation (ion with a positive charge).
This intermediate ion is resonance stabilised.
The final part of the reaction involves the attack of a nucleophile on
the intermediate cation to generate the final product.
Electrophilic substitution is a very versatile reaction and it is possible to add a variety of substituents onto a benzene ring using this reaction, for example:
The mechanism for a typical electrophilic substitution reaction is outlined below. It can be seen that this electrophilic substitution reaction simply consists of two separate steps:
The diagram below shows the mechanism for electrophilic substitution using both the Kekulé representation and the circle representation for the benzene ring. It is up to you as to which one you prefer to use, personally I prefer the Kekulé representation simply because I feel that it enables you keep track of how the electrons are moving throughout a mechanism, but its up to you as to which representation you prefer!
The intermediate cation formed in step 2 is as we have mentioned is resonance stabilised.
Addition of the electrophile
to the benzene ring is likely to be the slow step in the above reaction since it will remove
or destroy the aromatic stabilisation
that results from the delocalisation of the pi(π) electrons in the
ring. Resonance is simply where the nuclei of the atoms
stay in the same place and while the electrons move. In the diagram below it is possible to draw 3 resonance hybrid structures
for the intermediate cation. Resonance helps to stabilise the ion and generally the more
resonance structures you
can draw the more stable the ion is likely to be.
It is important to mention that the double head arrow used to indicate
resonance does NOT imply that all
the structures actually exist, rather it is suggesting that the structure of the intermediate ion is a combination of
all the resonance structures.
Practice questions