Benzene Reactions
Addition causes a loss of delocalisation. Substitution can occur as the electron density is spread over a larger area.

There are two steps to substitution: 1. Addition, 2. Elimination.

Addition: The electrophile is attracted to the delocalised electron cloud. This forms a coordinate bond between one of the carbons and the electrophile. An unstable positively charged intermediate is formed.
Substitution: The C-H bond breaks. Its pair of electrons restores the unstable delocalised electron structure and a hydrogen ion is eliminated.

Nitration of Benzene

Nitro group
NO2 is substituted into benzene in place of hydrogens.
Benzene becomes oil yellow liquid nitrobenzene.
Smells of bitter almonds.
Overall equation: C6H6 + HNO3 ---> C6H5NO2 + H2O
HNO3 isn't the electrophile
To make the electrophile NO2, react concentrated sulphuric acid and concentrated nitric acid. This produces a nitronium ion, water and HSO4-.
H2SO4 is produced at the end of the reaction by reacting H+ and HSO4-. This is why H2SO4 is the catalyst for the reaction.
The reaction has to occur at 50 degrees otherwise more nitro groups can be added meaning nitrobenzene will not be produced.

Halogenation

Alkylation

Add an alkyl group
Reacted with a haloalkane
Halogen carrier catalyst. AlCl3 ---> Generates an electrophile
Process increases number of carbon atoms. It is important for synthesis reactions.
Can be called Friedel-Crafts Alkylation.
AlCl3 + C2H5Cl ---> AlCl4- + +C2H5.
+C2H5 is the electrophile
AlCl4- + H+ ---> AlCl3 +HCl

Acylation

Requires a COCl group.
Add acyl chloride which forms an aromatic keystone. Creates a C-C bond which is useful in synthesis.