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Chlorination of Benzene

Explanation

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 [1]. 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

Chlorination of Benzene 1

Addition Reaction

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 [2].

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.

Chlorination of Benzene 2

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..

Chlorination of Benzene 3

The halogenation of methylbenzene

Substitution Reaction

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

Chlorination of Benzene 4

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.

Chlorination of Benzene 5
Chlorination of Benzene 6

Formation of the electrophile

The delocalization of electron in the ring repel chlorine-chlorine bond, so chlorine molecule approaches the benzene ring [3].

Chlorination of Benzene 7

Aluminum chloride help in the polarization, positive end of the chlorine molecule act as the electrophile.

Electrophilic substitution mechanism

First stage

Chlorination of Benzene 8

Second stage

Chlorination of Benzene 9

In first stage hydrogen is removed through AlCl4 ion, but in second stage aluminum chloride is regenerated.

Mechanism of chlorination of Benzene [4]

First step:

Chloronium ion formation, chlorine react with the Lewis acid to form a complex which makes the chlorine more electrophile.

Chlorination of Benzene 10

Second step:

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.

Chlorination of Benzene 11

Third step:

Proton transfer regenerates the aromatic character of the ring.

Chlorination of Benzene 12

Chlorobenzene uses

  • 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.

Health issues

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.

References

  1. https://en.wikipedia.org/wiki/Electrophilic_halogenation
  2. https://www.chemguide.co.uk/organicprops/arenes/halogenation.html
  3. https://www.chemguide.co.uk/mechanisms/elsub/halogenation.html
  4. http://chemistryallclass.blogspot.com/2015/08/chlorination-of-benzene.html
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