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Nitration of Benzene

Almost all aromatic compounds contain benzene ring which is a hexagonal ring. There are six carbon atoms bonded with covalent bond with each other in cyclic manner. Each carbon atom of benzene is $sp^2$ hybridized with three $sp^2$ hybrid orbitals and one un-hybrid $p_z$ orbital. So each carbon atom forms three sigma bonds, two with neighbor carbon atoms and one with hydrogen atom. The remaining one un-hybridized $p_z$ orbital with one electron involves in pi-bond formation with neighbor carbon atom. 


Structure of Benzene

These six pi electrons in the hexagonal ring get delocalized over all the six carbon atoms of the molecule. It provides stability to the molecule and makes it different from other un-saturated compounds. 

The bond length of all the six carbon-carbon bonds are of same bond length and have intermediate value of C-C (154 pm) and C=C (133 pm) bonds, around 139 pm. The resonance or delocalization of pi-bonds in benzene molecule can be represented as given below. 

Resonance in Benzene Molecule

We know that alkenes can easily show addition reactions like hydration, hydrogenation, ozonolysis etc. This is because of presence of double bond in alkene molecules which can easily break and convert alkenes to alkane (saturated compounds). 

Since benzene molecule contains three pi-bonds, so it should give all these addition reaction under normal conditions. But this is not true for benzene. No doubt it has three pi-bonds but it readily gives electrophilic substitution reactions compare to addition reactions.

This is because of extra-stability of benzene molecule due to resonance. Because of presence of pi-bonds in benzene, the electron density is high and it readily gives electrophilic substitution reaction compare to addition reactions. Electrophilic substitution reactions are a type of substitution reactions in which coming electrophile (E+) substitute some atom of reactant to form new product. An electrophile carries positive charge so there must be more electron density or some extra electrons in reactant molecule so that an electrophile can show substitution reaction. Benzene is an electron –rich sable molecule because there are 3 pi-bonds or 6 pi-electrons in the hexagonal ring. Let’s discuss electrophilic substitution reactions of benzene molecule.

 

Nitration of Benzene Reaction

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As we discussed, benzene is an aromatic compound with six delocalized pi-electrons system. The resonance forms of benzene exhibits the stability of molecule. We know that unsaturated molecules readily give addition reaction but due to extra stability of aromatic compounds, they cannot show these reactions at normal conditions. They can easily give electrophilic substitution reactions due to high electron density (pi-electrons). Electrophile has to replace a hydrogen atom of benzene ring therefore it must be a strong enough and usually a Lewis acid is required to form a strong electrophile. Let’s discuss a general mechanism of electrophilic substitution reaction.

Electrophilic substitution reactions of benzene occur in three steps. In step-1, the electrophile reacts with benzene ring to form an intermediate. The reaction of electrophile with benzene causes lose of aromaticity of benzene ring and form a non-aromatic intermediate. Reactant molecule has to cross a high energy barrier to form this intermediate so this step is endothermic. The intermediate form during the reaction is called as sigma-complex or pi-complex or arenium ion. Next step involves lose of proton, which is an exothermic step as molecule regains its aromaticity. 

Electrophilic Substitution Reaction of Benzene

One of the electrophilic substitution reaction of benzene is nitration of benzene which introduce –NO$_2$ group in benzene molecue to form nitrobenzene. The reacion occurs in the presence of concentrated sulphric acid and concetrated nitric acid below 328 K temperature. 

Balanced Equation for Nitration of Benzene

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Nitration of benzene occurs in the presence of concentrated nitric acid and concentrated sulpuric acid. Here concentrated sulphuric acid acts as catalysts and involves in formation of electrophile that is nitronium ion.  The balance equation of nitration of benzene is as given below.
Nitration of Benzene

Nitration of Benzene Conditions

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Nitration of benzene is an electrophilic substitution reaction of benzene with concentrated nitric acid in the presence of concentrated sulphuric acid. Here sulphuric acid is more acidic compare to sulphuric acid therefore nitric acid acts as base and accept hydrogen ions to form water and nitronium ions.  The formation of nitronium ions (electrophile) can be shown as below;

$HNO_3 + H_2SO_4 \rightarrow HSO_{4-} + NO_{2+} + H_2O$.

In last step of reaction, HSO4- ions acts as base and abstract proton from intermediate to form nitrobenzene and sulphuric acid. 

Nitration of Benzene Mechanism

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Let’s discuss complete mechanism of nitration of benzene. Like other electrophilic substitution reactions of benzene, nitration also occurs in three steps:
  1. Formation of electrophile with the help of Lewis acid as we require a strong electrophile for these reactions. 
  2. Formation of arenium ion by the attack of electrophile on benzene ring. This intermediate shows resonance and gets stabilized. 
  3. Lose of proton from arenium ion forms product. This step requires some base to abstract proton from the intermediate. 
First step of nitration is the formation of electrophile in the presence of Lewis acid. Here nitronium ions are electrophile which are formed from concentrated nitric acid and concentrated sulphuric acid acts as Lewis acid. In the reaction of nitric acid and sulphuric acid, nitric acid acts as base and accepts hydrogen ions to form nitronium ions and bisulphate ions.

$HNO_3 + H_2SO_4 \rightarrow HSO_{4-} + NO_{2+} + H_2O$.

In second step, nitronium ion reacts with benzene to form arenium ion or sigma complex as intermediate which is stabilized by resonance.  

Nitration of Benzene Mechanism

In the last step, the bisulphate ion abstracts hydrogen in the form of proton to form nitrobenzene and sulphuric acid. All these three steps can be summarized as given below. 
Overall Reaction of Nitration of Benzene

The energy profile diagram of nitration of benzene shows the arenium ion as highest peak as it has maximum energy and minimum stability compare to reactant and product.

The energy level of product is less than reactants which proves that reaction is exothermic in nature. 

Energy Profile Diagram for Nitration of Benzene

Commercial Uses of Nitration of Benzene

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Nitration of benzene is one of the most common reactions which is used to predict mechanism of electrophilic substitution reactions of benzene molecule.

The product that is nitrobenzene is commercially use in manufacturing of different chemical compounds like TNT, aniline etc. The reduction of nitrobenzene with iron and dilute hydrochloric acid forms aniline or amino benzene which is mainly used to form diazonium compounds. Diazonium compounds are used in synthetic chemistry to form different chemical compounds.  Nitrobenzene is used in the production of different medications like acetaminophen, or paracetamol. It is also used in manufacturing of leather dressings, paint solvents, shoe and floor polishes. It was used as oil of mirbane which was an inexpensive perfume for soaps and cosmetics.

It acts as solvent in petroleum refining, manufacture of dinitrobenzenes and dichloroanilines, production of cellulose ethers and cellulose acetate. It dissolves anhydrous AlCl$_3$ during Friedel-Crafts reaction therefore is used to hold the catalyst in solution.
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