A2-Level Benzene, Aromatic Chemistry
Electrophiles (electron deficient species) are attracted to the ring of delocalised electrons in benzene, nucleophiles are repelled by the delocalised electrons.
Benzene reacts with strong electrophiles
Electrophiles react with benzene in electrophilic substitution reactions, forming a bond with a carbon atom in the ring and forcing the hydrogen atom already bonded to the carbon atom to be lost.
Benzene has a ring of delocalised electrons that represent a region of high electron density. Electrons are negatively charged, so electron deficient species (electrophiles) are attracted to the high electron density.
In (very) simple terms, positively charged species are attracted to the ring in the middle of benzene as it contains lots of electrons.
Species that want to donate electrons (nucleophiles) or have a negative charge are repelled by the delocalised electrons. This is why benzene does not react with nucleophiles, only electrophiles.
If you want to add something to a benzene ring, it needs to accept electrons. This is the basic principle behind all benzene reactions.
An electrophile accepts electrons from the ring of delocalised electrons, forming a new bond with a carbon atom.
This removes one electron from the delocalised ring and makes it positively charged. The delocalised ring needs another electron to become stable again. To get this electron, the carbon-hydrogen bond breaks and the electrons go to the ring of delocalised electrons.
A positive hydrogen ion is released (proton) and the original electrophile is now bonded onto the benzene ring.
The hydrogen has been substituted for an electrophile, making the reaction an electrophilic substitution.
This basic mechanism should be fully understood – it forms the basis of the benzene based reactions that you will come across at A-level chemistry. The only differences will be the electrophiles that substitute onto the benzene ring and how they are formed.