According to texts in the books of organic chemistry, halogenation refers to reaction of a halogen with another substance wherein the halogen atom ends up as a part of that substance.
Halogenation can be divided into two types:
Example, addition of bromine to ethane.
Halogens react with alkanes in the influence of heat to form alkyl halides.
As you can see, the halogen atom replaces hydrogen atom into alkane hence called as substitution reaction. Aromatic compounds go through substitution reactions in the presence of Lewis acids.
Electrophilic Substitution Reaction
Benzene reacts with bromine or chlorine in an electrophilic substitution reaction only in the presence of a catalyst which is either chloride or iron.
Iron is not a catalyst because it changes permanently during the reaction. It reacts with some bromine to form iron 3 chloride, FeCl3 or iron 3 bromide, FeBr3.
Mechanism of Halogenation of Benzene
Step 1: The bromine reacts with Lewis acid to create a complex that makes bromine more electrophilic.
Step 2: The π electrons of aromatic C=C behave as a nucleophile which attacks the electrophilic Br and displaces iron tetrabromide.
Step 3: The proton is removed from sp3 C and bears the bromo group that reforms C=C and aromatic system generates HBr and regulates active catalyst.
The compounds act as a catalyst and behave similar to aluminium chloride in the following chemical reactions.
Reaction with chlorine
The reaction between chlorine and benzene gives chlorobenzene in the presence of either iron or aluminium chloride.
Reaction with bromine
The reaction between bromine and benzene gives bromobenzene in the presence of either iron or aluminium bromide. Iron is used normally because it is readily available and cheaper.
Know more about applications of halogenation of benzene along with chemical reactions involved in every application @Byju’s.