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Halogenation of Benzene

Simple arenes generally consist of a functional group or a carbon chain attached to a benzene ring and yet despite unsaturated nature of benzene it does not readily undergo addition reaction. Most functional groups and carbon chains are introduced into the benzene molecule by means of substitution reaction.

The unsaturated benzene molecule does not readily undergo electrophilic reactions which are very common in alkenes as the saturated carbon – carbon bonds destroy the delocalized pi ($\pi$ ) cloud.

Substitution reactions allows the delocalized pi ($\pi $) cloud of electrons to remain intact as this delocalized structure is particularly stable.


Halogenation of Benzene Ring

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Halogen molecules are not strong electrophiles and except fluorine, they do not react with benzene. However, in presence of a Lewis acid, the reaction occurs readily.

The main function of the used catalyst is to take away lone pair of electrons from the halo molecule, which in turn becomes electron deficient at one end. A lewis acid accepts electron pair and base donates an electron pair.

The actual electrophile is probably the complex formed from the halogen and the catalyst. Benzene undergoes electrophilic substitution reaction in which the delocalized pie ($\pi $) cloud of electrons is preserved. The product of such substitution reaction with a halogen is halogenoarene such as chlorobenzene. In order to substitute a halogen for hydrogen in benzene, a catalyst called a halogen carrier is needed as well as the halogen itself as the pie ($\pi $) cloud of electrons is more stable than the simple pie ($\pi $) bond in an alkene.

The six membered carbon skeleton in benzene molecule is sandwiched between the two halves of the delocalized pi ($\pi $) cloud of electrons and this is a region of high electron density.

Benzene is therefore attacked by electrophiles, nucleophiles are eventually repelled. The halogen carriers make the halogen strongly electrophilic. These results in heterolytic fission of the Cl – Cl bond in the chlorine molecule or polarize it to form $Cl^{+\delta } – Cl^{-\delta }$.
The electrohilic Cl+ ion then attacks the ring. The typical halogen carrier catalysts are lewis acids like $AlCl_{3}$ or $FeCl_{3}$ help in this type of reactions. The electrophilic substitution is the characteristic reaction of all arenes. Bromination of arenes is one of the most common halogenation process.

In this type the halogen carrying catalysts are $FeBr_{3}$ and $AlBr_{3}$ which help in forming bromoarenes. 

Halogenation of Benzene Ring

Halogenation of Benzene Mechanism

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The mechanism of electrophilic substitution starts with attack by the electrophile Cl+ ion on benzene. As the electrostatic potential work out the high electron density is both above and below the plane which evidently provides the direction of the attack.

An intermediate called a wheland intermediate is formed, in which the positive charge is shared by the five other carbon atoms in the ring. This intermediate loses a proton to form chlorobenzene.

$C_{6}H_{6} + Cl^{+} \rightarrow C_{6}H_{5}Cl + H^{+}$

The reaction of $Cl_{2}$ with the Lewis acid $AlCl_{3}$ which acts as an electron acceptor results in a complex that is significant and more reactive than molecular chlorine. The terminal chlorine is this complex is a very reactive electrophile because the Cl – Cl bond is strongly polarised toward the bridging positively charged chlorine.

The formation of the Cl – Al bond in the complex weakens the Cl – Cl bond that must be broken and to some extent the charge separation in the ion pair consisting of arenium ion and counter ion is already partially developed in the complex.

Loss of proton from the carbon that has bonded to electrophile returns a pair of electrons to the pie (π) system and restore aromaticity.
All of these electrophilic aromatic substitution reactions are carried out by two step mechanism.

The first step involves the benzene reacting with an electrophile $(Y^{+})$ which forms a carbocation intermediate. The structure of the carbocation intermediate can be estimated by three resonance contributors.

The second step of the mechanism involves a base in the reaction mixture which pulls of a proton from carbocation intermediate and the electrons that held the proton move into the ring to re-establish the aromaticity. The proton is always removed from the carbon that has formed the new bond with the electrophile. 

Halogenation of Benzene Mechanism

Side Chain Halogenation of Benzene

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In the absence of a lewis acid, halogenation of toluene at its boiling point with bromine or chlorine and under the ultra violet irradiation occurs in the side chain. The reaction proceeds by a free radical mechanism that is initiated by the photolytic dissociation of a chlorine molecule while the benzyl radical is stabilised by resonance.

The chlorination of side chain can also be achieved using sulfuryl chloride in dark but in presence of a radical initiator such as benzoyl peroxide as well as t – butyl hypochlorite through which free radicals are generated. 

It is also possible to replace all three hydrogen atoms of a methyl group of toluene in sequence by chlorine which leads to form chloro methyl benzene, di chloro methyl benzene and tri chloro methyl benzene. Introduction of the first chlorine atom proceeds at a much faster rate than the second and so it is possible to prepare chlor methyl benzene selectively.

In order to achieve the required degree of chlorination, chlorine gas is passed over the reaction mixture until all the mass gain and reach appropriate level of substitution. The higher homologue of toluene such as ethyl benzene are not usually halogenated selectively and mixtures are often produced.

Free Radical Halogenation of Benzene

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An allylic carbon is the carbon next to C = C and the hydrogens on such carbons are termed as allylic hydrogens. Similarly the carbon next to a benzene ring is termed as benzylic carbon and the hydrogen on a benzylic carbon are called benzylic hydrogens.

These allylic and benzylic positions are more reactive than other positions in free radical halogenations. 

Allylic Carbon Benzylic Carbon
The chlorination of 2o benzylic position is favoured by more than 2: 1 over chlorination of other 2o position. A small amount of chlorination occurs at the methyl. 

Chlorination of Substituted Bezene
Bromination is much more selective than chlorination and hence only benzylic bromination is observed.

The radicals formed in benzylic and allylic halogenation are resonance stabilized which lowers the energies of intermediates and hence products derived from these are considered to be major products. 

Bromination of Substituted Bezene

Conditions for Halogenation of Benzene

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For benzene halogenation conditions are as follows:

Substitution reactions with chlorine and bromine :

Reagents: Cl2 or Br2
Catalysts are halogen carriers like FeCl3 or FeBr3 or AlCl3 at room temperature.
The products of these reactions are either chloro benzene or bromo benzene.
Benzene reacts with Cl2 in presence of AlCl3 to give chloro benzene and steamy fumes of HCl. 

Chlorination of Bezene
Bromine reacts in the same manner and the reddish brown colour of bromine fades away the acidic fumes of HBr. The bromine water is avoided as in presence of water AlCl3 hydrolyses readily and is no longer available or function as halogen carrier. 

Bromination of Bezene
Since methyl benzene contains a benzene ring and a methyl side chain, this also undergoes substitution of benzene and other typical reactions of alkanes. In methyl benzene, substitution reactions are quicker with methyl benzene than benzene. The substitution takes place only at position 2 and 4 on the benzene ring. With methyl benzene, halogenation can occur in the aromatic nucleus or side chain depending on reaction conditions. The methyl benzene undergoes substitution in aromatic ring and a mixture of 2 chloro and 4 chloro methyl benzene are produced along with steamy fumes of HCl. 

Chlorination of Toluene
The reaction of methyl benzene with $Cl_2$ in presence of ultra violet light or sunlight shows substitution in the side chain. These side chain substitution results in mono, di, or tri, substituted products but these depends upon the amount of halogen present. Along with these halogenated products steamy fumes of HCl are also produced.
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