Compare the mechanisms for addition and substitution reactions of benzene.
An alkyl group can be added to a benzene molecule by an electrophile aromatic substitution reaction called the Friedel‐Crafts alkylation reaction. One example is the addition of a methyl group to a benzene ring.The electrophile attacks the π electron system of the benzene ring to form a nonaromatic carbocation.
Sulfonation of Benzene
Sulfonation is a reversible reaction that produces benzenesulfonic acid by adding sulfur trioxide and fuming sulfuric acid. The reaction is reversed by adding hot aqueous acid to benzenesulfonic acid to produce benzene.
Chlorination of benzene
As a chlorine molecule approaches the benzene ring, the delocalized electrons in the ring repel electrons in the chlorine-chlorine bond. It is the slightly positive end of the chlorine molecule which acts as the electrophile. The presence of the aluminum chloride helps this polarization.
Hydrogenation is an addition reaction in which hydrogen atoms are added all the way around the benzene ring. These reactions destroy the electron delocalisation in the original benzene ring, because those electrons are being used to form bonds with the new hydrogen atoms.