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Bromination of Benzene

Organic compounds can be mainly classified as aromatic and aliphatic compounds. Aliphatic compounds are open chain carbon compounds which contain long parent chain with carbon atoms. Here carbon atoms can be bonded with single, double or triple covalent bonds. On the basis of type of covalent bond, they can be further classified as alkane, alkene and alkyne. Alkane contains all single bonded carbon atoms therefore they are saturated hydrocarbons. Alkenes and alkynes have double and triple bonded carbon atoms in the parent chain therefore they are un-saturated hydrocarbons.

Unlike aliphatic hydrocarbons, aromatic hydrocarbons are cyclic compounds with conjugated pi-electron system. The conjugation in pi-electrons stabilized the molecule with resonance. This phenomenon is called as aromaticity. Because of aromaticity, benzene does not show normal addition reactions like other unsaturated hydrocarbons like alkenes and alkynes. The presence of pi-electron cloud in aromatic ring makes it reactive towards electrophile (E+) therefore aromatic compounds can show electrophilic substitution reaction easily. Electrophilic substitution reactions involve mainly three steps during the mechanism. 
  1. Formation of electrophile 
  2. Formation of arenium ion
  3. Formation of product
Since aromatic compounds are stable because of resonance therefore they require strong electrophile to react with aromatic compound. Therefore for the production of electrophile, we use a Lewis acid which reacts with reagent to form electrophile. 

Energy Profile Diagram

The intermediate forms during the reaction carries positive charge which remains delocalized over benzene ring and due to this delocalization of charge; it is more stable compare to transition states. Remember, arenium ion is not an aromatic species but stable compare to transition state. 

For example for nitration reaction, concentrated sulfuric acid is used to produce nitronium ion which acts as electrophile for this reaction. Nitronium ion attacks on benzene ring to form an intermediate which is called as arenium ion or sigma complex. This sigma complex gets stabilized by resonance. In the presence of base, arenium ion loses proton to form desired product. 

 

Bromination of Benzene Reaction

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The halogenations reaction of benzene which is an example of electrophilic substitution reaction. Halogenation of benzene can be carried out in the presence of halogen like $Cl_2$, $Br_2$ and catalyst $FeCl_3$ or $FeBr_3$. 
Bromination of Benzene
Overall it involves the transformation of aromatic compound; $Ar-H$ to $Ar-X$ in the presence of halogen and appropriate Lewis acid catalyst. Here $FeX_3$ is used as catalyst which is formed by the reaction of $Fe$ with $X_2$. The electrophile form due to reaction of catalyst with halogen is called as halonium ion (X+). The halogenation of benzene is restricted to $Cl_2$ and $Br_2$ only.

Bromination of Benzene Conditions

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An electrophilic substitution reaction of benzene with chlorine or bromine in the presence of a catalyst either aluminium chloride or aluminium bromide iron bromide is called as halogenation reaction. Usually iron halides are used as catalyst which can be formed by the reaction of metal with halogen.

$2Fe + 3 Cl_2 \rightarrow 2FeCl_3$

$2Fe + 3 Br_2 \rightarrow 2 FeBr_3$

Reaction occurs at high temperature and in the presence of catalyst which helps in the production of electrophile. 

Electrophilic Bromination of Benzene

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The bromination of benzene is an electrophilic substitution reaction in which bromonium ion bonded with benzene ring by the substitution of hydrogen atom of molecule. The electrophilic substitution reaction involves the substitution of one of the bonded atom with electrophile to form products. 

Bromination of benzene occurs in the presence of bromine and ferric bromide at high temperature. First step involves the formation of bromonium ion with the reaction of bromine with catalyst. The reaction between Br2 and $FeBr_3$ forms $Br^+$ ion and $FeBr^{4-}$ ions.

Here $Br^+$ acts as electrophile and $FeBr^{4-}$ ion acts as base which involves in elimination of proton from the intermediate formed during reaction. Next step is reaction of bromonium ion with ferric bromide which forms an intermediate that is stabilized by resonance. The intermediate reacts in step-3 with base ($FeBr^{4-}$ ion) to form ferric bromide and bromobenzene with hydrogen bromide. 

Bromination of Benzene Mechanism

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Like other electrophilic substitution reactions, bromination of benzene also involves three steps. 

Bromination of Benzene Mechanism

Step-1 is formation of bromonium ion that is electrophile for this reaction. The electrophile reacts with aromatic ring to form an arenium ion which is an intermediate. The energy profile diagram proves that intermediate is a high energy species which readily converts into product. 

Energy Profile Diagram for Bromination of Benzene
The bromination of benzene is an exothermic reaction which releases around 10.8 kcal /mol of energy during reaction. The intermediate is a stable form compare to transition state of reaction. This intermediate is stabilized by resonance. It readily loses the proton in the presence of base to form bromobenzene and $HBr$. For this reaction, $FeBr^{4-}$ ion acts as base and converts itself in $FeBr_3$ that is catalyst for this reaction. 

Radical Bromination of Benzene

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There are two types of halogenations reactions aromatic compounds. First halogenation follows electrophilic substation mechanism and occurs in the presence of some catalyst which promotes the formation of electrophile. Here electrophile attacks on aromatic ring to form a positively charged intermediate which is called as arenium ion. For example, bromination of benzene occurs in the presence of $FeBr_3$ and $Br_2$ to form bromobenzene. Similarly bromination of toluene results the formation of ortho and para-bromobenzene as products. Here ortho and para substitution is because of the presence of methyl group which is ortho, para directing group in the molecule.  

Another type of halogenations reaction is side chain halogenation which occurs in the presence of heat or light. No other catalyst is required for this reaction. This is an example of free radical substitution which cannot occur at aromatic ring but substitute the hydrogen atom of side chain directly bonded with aromatic ring. For example, bromination of toluene in the presence of light forms benzyl bromide. 

Bromination of Toluene
Let’s discuss the mechanism of radical bromination of benzene and toluene (a benzene derivative). The radical halogenations of benzene forms complete halogenated products such as chlorination of benzene in the presence of light results the formation of benzene hexachloride which is also known as BHC. It is mainly used as pesticide. 

The bromination of toluene or other benzene derivatives also exhibit free radical mechanism and halogenation occurs at side chain to form substituted products. 

The step-1 for mechanism is the formation of radical in the presence of light. It breaks the Br-Br bond ad forms two bromine radicals. 

Initation
Step-2 is chain propagation step which continue the chain reaction by formation of new radicals. Broime radical reacts with side chain ( methyl group) or tolune and form $HBr$ with another radical. This newly formed radical again reacts with $Br_2$ molecule to form new bromine radical and benzyl.
Propagation
Last step of bromination would be chain termination step in which two radicals combine to form a molecule. 

Termination
More topics in Bromination of Benzene
Allylic Bromination
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