Nitration is the process of adding a nitro group into an organic compound like aromatic compound, alcohol, glycol, glycerine, aromatic amine, and paraffin. Most compounds are nitrated by an ionic mixture of strong acids containing nitric acid and sulfuric acid. Sulfuric acid is used because nitric acid is a slow electrophile. The presence of sulfuric acid accelerates the speed of the reaction, and hence, it acts as a catalyst ^[1-4].

Nitration Examples

1. Benzene

When an aromatic compound like benzene and toluene is nitrated, the reaction occurs through electrophilic aromatic substitution. Benzene (C₆H₆) is rich in electrons. The electrophilic nitronium ion (NO₂⁺), present in the nitric acid-sulfuric acid mixture, readily attacks benzene at a temperature not exceeding 50 ˚C, producing nitrobenzene (C₆H₅NO₂). The reaction is accompanied by the loss of a water molecule ^[1-4].

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

Since sulfuric acid is the catalyst, it is not shown in the reaction. There is also a possibility of forming 1,3-dinitrobenzene.

2. Methyl Benzene

Methyl benzene or toluene (C₆H₅CH₃) reacts faster than benzene because the methyl group tends to push the electrons towards the ring. The increasing electron density makes the ring more attractive to the attacking reagent. The reaction is carried out at a lower temperature (~30 ˚C) and produces two isomers: 2-nitromethylbenzene and 4-nitromethylbenzene ^[2,3].

C₆H₅CH₃ + HNO₃ → C₆H₄CH₃NO₂ + H₂O

Just like benzene, there is a possibility of dinitrobenzene formation.

3. Methyl Benzoate

Nitration of methyl benzoate gives methyl-3-nitrobenzoate. The balanced equation is shown below.

C₆H₅COOCH₃ + HNO₃ → C₆H₄COOCH₃NO₂ + H₂O

4. Bromobenzene

Nitration of bromobenzene yield 4-nitrobromobenzene.

5. Aniline

Nitration of aniline yields meta- and para- nitration products.

6. Phenol

Nitration of phenol at 25 ˚C yields a mixture of ortho and para nitrophenols.

7. Benzaldehyde

Nitration of benzaldehyde yields meta-nitrobenzaldehydes.

8. Benzoic Acid

Nitration of benzoic acid yields 3-nitrobenzoic acid.

Nitration of Mechanism

Since most of the examples listed above are benzene derivatives, we shall discuss the mechanism of benzene nitration. The nitronium ion (NO₂⁺) present in the mixture is the active species that attacks the organic compound to form an unstable complex. The nitrated hydrocarbon is formed when a proton is removed from the complex ^[1-4].

Step 1: Generation of the nitronium ion

The reaction starts with the protonation of the hydroxy (OH^–) group in nitric acid (HNO₃) by sulfuric acid (H₂SO₄). When OH^– is protonated, a molecule of water (H₂O) is formed. Since water is a good leaving group, it is released immediately to give the highly reactive nitronium ion (NO₂⁺). The H₂SO₄ is converted to a bisulfate (HSO₄^–) ion.

Step 2: Attack of the nitronium ion

The electron-rich benzene attacks the nitronium ion to give a carbocation intermediate, forming a C-N bond and breaking the C-C and the N-O bonds.

Step 3: Deprotonation of the carbocation intermediate

It is a relatively fast step resulting in an acid-base reaction. The bisulfate ion HSO₄^– removes a proton from the carbon bearing the nitro group, breaking the C-H bond and reforming the C=C pi bond to restore the aromaticity.

Nitration Reaction Uses

There are many significant applications of nitration in industry. The most important ones are the large-scale production of nitroaromatic compounds such as nitrobenzene. Nitroaromatic compounds are of considerable use as intermediates in the synthesis of pharmaceuticals, plastics, dyes, and insecticides. Nitroaliphatics are used as solvents and synthons in organic synthesis. Nitration reactions are mainly used for the production of explosives. For example, it converts toluene to TNT (2,4,6-trinitrotoluene) ^[5].