Benzene reacts with halogens (salt former) like chlorine and bromine and these reactions are called electrophilic substitution reaction in the presence of catalyst of Lewis acid like aluminum chloride, sulfur dichloride, ferric chloride or iron . Aluminum bromide is used when benzene reacting bromide. Iron is not a catalyst because it reacts with small amount of chlorine or bromine and form iron (III) chloride FeCl3 or iron (III) bromide FeBr3.
2Fe +3Cl2 → 2FeCl3
2Fe + 3Br2→ 2FeBr3
Reaction with Chlorine
Benzene react with chlorine in the presence of aluminum chloride or iron to prepare chlorobenzene.
C6H6 + Cl2 → C6H5Cl + HCl
Reaction with Bromine
Benzene react with bromine in the presence of aluminum bromide or iron to produce bromobenzene. Iron is used because it is readily available and cheaper.
C6H6 + Br2 → C6H5Br + HBr
Addition reaction of hot benzene undergo in the presence of ultraviolet light with chlorine or bromine. Chlorine or bromine atom adds on carbon atom and ring delocalization is permanently broken .
For example: 1,2,3,4,5,6-hexachlorocyclohexane is produced when bubble chlorine gas through hot benzene exposed to UV light for an hour.
Bromine would act similarly, the chlorine and hydrogen make numbers of isomers through sticking up and down at random position of the ring. The carbon-carbon double bond is broken and unable to rotate. Another isomer is commonly used as insecticide called as BCH, HCH, and Gammexane. Also called chlorinated hydrocarbons it is very harmful to the environment..
The halogenation of methylbenzene
Substitution reaction between methylbenzene and chlorine or bromine are take place in two ways:
- Substitution into the ring
- Substitution into the methyl group
Substitution into the ring
This reaction is carried out at room temperature in the presence of aluminum chloride or iron and in the absence of UV light. Methyl group are attached at 2 or 4 position on the ring and the new group is attach on the ring next to the methyl group. The substitution reaction with chlorine produce two products 2-chloromethylbenzene and 4- chloromethylbenzene.
C6H5CH3 + Cl2 → C6H4CH3Cl + HCl
Substitution into methyl group
Substitution reaction of chlorine or bromine in the methyl group rather than the ring with boiling methylbenzene in the presence of UV light and in the absence of catalyst.
The product formed is (chloromethyl)benzene, the name written in bracket shows that chlorine attached to methyl group and not with the ring.
Hydrogen atoms are replaced in the methyl group by a chlorine atom, all the three hydrogen atoms in the methyl group are replaced by a chlorine atom. The new products are (dichloromethyl)benzene and (trichloromethyl)benzene because hydrogen atom is replaced one at a time.
Formation of the electrophile
The delocalization of electron in the ring repel chlorine-chlorine bond, so chlorine molecule approaches the benzene ring .
Aluminum chloride help in the polarization, positive end of the chlorine molecule act as the electrophile.
Electrophilic substitution mechanism
In first stage hydrogen is removed through AlCl–4 ion, but in second stage aluminum chloride is regenerated.
Mechanism of chlorination of Benzene 
Chloronium ion formation, chlorine react with the Lewis acid to form a complex which makes the chlorine more electrophile.
Chloronium ion attack on the ring. The double bond electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic Cl, and delocalization take place.
Proton transfer regenerates the aromatic character of the ring.
- It is used in the production of pesticide DDT with the help of reaction with chloral in the presence of sulfuric acid.
- It is used as an intermediate in the preparation of commodities like herbicides, dyestuffs and rubber.
- It is also used as high boiling solvent for the preparation of adhesives, paints, paint remover, polishes, dyes and drugs.
- It is also used for the preparation of polyurethane insulation.
The person exposed to high level experience headache, numbness, sleepiness, nausea and vomiting. Chlorobenzene also effect depression of function of many parts of nervous system. In animals mild to severe exposure affect brain, liver and kidney. Also cause birth defects.