Nitration of Benzene

The nitration of benzene is a fundamental organic reaction that introduces a nitro group (-NO₂) into the benzene ring. The nitronium ion (NO₂⁺) acts as an electrophile and it is substituted to the benzene ring through an electrophilic aromatic substitution (EAS) mechanism.

Since benzene is a less reactive compound, it requires a catalyst and high temperature to occur in this reaction. So, this reaction is carried out in a medium of concentrated sulfuric acid and concentrated nitric acid.

Nitrobenzene is a pale yellow toxic with the odor of bitter almonds. Nitrobenzene is primarily used as a raw material in the industry for the production of aniline, dyes, pigments, pharmaceuticals, etc.

When benzene is heated up to 55 °C in the presence of concentrated sulfuric acid (H₂SO₄) and concentrated nitric acid (HNO₃), a -NO₂ group is substituted onto the benzene and yields nitrobenzene as the product. If the temperature is higher than 90 °C, two NO₂ groups are substituted and form 1,3-dinitrobenzene.

The reaction mechanism of the nitration of benzene

When compared to the acidity of HNO₃, H₂SO₄ is more acidic. That means H₂SO₄ can easily dissociate and give H⁺ ions to the medium. Since this is a concentrated situation, sulfuric acid will dissociate and give only one H⁺ ion to the medium. The nitric acid will not dissociate in these conditions.

In nitric acid, HNO₃, there are three types of N-O bonds. There is an N=O double bond, an N->O dative bond, and an N-O-H sigma bond. The oxygen atom that has bonded to both nitrogen and hydrogen has two lone pairs of electrons.

These electrons will attack the H⁺ ions in the medium and form an O-H dative bond. This results in a positive charge on the oxygen atom. Oxygen is a highly electronegative atom. So, it is difficult for oxygen to bear a positive charge on it. Therefore, it takes electrons from the N-O bond and eliminates them as an H₂O (water) molecule.

Since the electrons have been taken from the N-O bond, it forms a positive charge on the Nitrogen atom. Thus, the NO₂⁺ electrophile is formed. In this process, a water molecule is eliminated from the nitric acid. That means the nitric acid has been dehydrated by sulfuric acid.

The generated NO₂⁺ ion (the electrophile) is attacked by the pi electrons of the benzene when it comes closer to the benzene. Then, it forms an intermediate "Arenium ion (σ-complex)". The positive charge is delocalized in the benzene ring, and then it recreates the pi electron cloud by removing a hydrogen atom as an H⁺ ion.

This H⁺ ion and the HSO₄^- ion in the medium regenerate the sulfuric acid. Sulfuric acid has been used in the reaction and regenerated at the end. Therefore, it acts as a homogeneous catalyst here.