This page looks at the ways of making the aryl halides, chlorobenzene, bromobenzene and iodobenzene.
Benzene reacts with chlorine in the presence of a catalyst, replacing one of the hydrogen atoms on the ring by a chlorine atom. The reaction happens at room temperature. The catalyst is either aluminium chloride or iron. Strictly speaking iron is not a catalyst, because it gets permanently changed during the reaction. It reacts with some of the chlorine to form iron(III) chloride, FeCl3.
\[2Fe + 3Cl_2 \rightarrow 2FeCl_3\]
This compound acts as the catalyst and behaves exactly like aluminium chloride, \(AlCl_3\), in this reaction. The reaction between benzene and chlorine in the presence of either aluminium chloride or iron gives chlorobenzene.
or, written more compactly:
\[ C_6H_6 + Cl_2 \rightarrow C_6H_5Cl + HCl\]
The reaction between benzene and bromine in the presence of either aluminum bromide (rather than aluminum chloride) or iron gives bromobenzene. Iron is usually used because it is cheaper and more readily available. If you use iron, it is first converted into iron(III) bromide by the reaction between the iron and bromine.
\[ C_6H_6 + Br_2 \rightarrow C_6H_5Br + HBr\]
Iodobenzene can be made from the reaction of benzene with iodine if they are heated under reflux in the presence of concentrated nitric acid, but it is normally made from benzenediazonium chloride solution. If cold potassium iodide solution is added to ice-cold benzenediazonium chloride solution, nitrogen gas is released and oily droplets of iodobenzene are formed.
There is a simple reaction between the diazonium ions present in the benzenediazonium chloride solution and the iodide ions from the potassium iodide solution.
Jim Clark (Chemguide.co.uk)