Electrophilic aromatic substitution
Electrophilic substitution involves the attack of an electrophile to a carbon in an electron rich molecule to form a cationic intermediate, followed by the elimination of a leaving group.
When involving aromatic compounds, the intermediate that is formed loses the aromaticity and becomes a benzenium ion, a structure characterized by 4 pi electrons delocalized over 5 carbons, while the last carbon has an sp3 hybridization. When the leaving group, usually an hydrogen from the sp3 carbon, leaves the compound, the molecule regains its original aromaticity. The increase in stability given by the formation of an aromatic ring is the driving force of the reaction, which usually blocks the intermediate from reacting with the anions in the solution. The nature of the electrophile is also to be considered in these reactions. In most cases, the electrophile is added in a form that requires activation through the addition of a catalyst. For example, to achieve a chlorination we usually add the electrophile Cl2 in the presence of FeCl3.