Nitration of Benzene- introduction, mechanism and solved examples

What is nitration of benzene?

The addition of nitro (NO2) group into benzene is called nitration of benzene.

Equation

image showing over all reaction equation of nitration of benzene

Explanation

Nitration of benzene is an example of electrophilic aromatic substitution reaction. Benzene reacts with concentrated nitric acid in the presence of sulfuric acid to give nitrobenzene.

In this reaction sulfuric acid acts as a catalyst. Catalyst is a substance which enhances the rate of chemical reaction but does not take part in chemical reaction.

Watch the video to better understand nitration of Benzene

Video lecture on Nitration of Benzene

Mechanism of nitration of benzene

The mechanism of nitration of benzene involves three steps which are given below.

STEP 1: Formation of nitronium ion (NO2+) from Nitric acid

The nitronium ion is formed by the reaction of nitric acid with sulfuric acid. Sulfuric acid will protonate nitric acid leading to a loss of water molecule and formation of nitronium ion. The nitronium ion is a powerful electrophile and is attacked quickly by the aromatic ring.

Remember that during this step temperature must not exceed 50 °C.

image showing mechanism for formation of nitronium ion

Important point in Nitration of Benzene

In this reaction both reactants are acids while proton is only denoted by sulfuric acid . First reason is that sulfuric acid is a strong acid. Second reason is that lone pair of oxygen atom that is directly bonded to sulfur atom in sulfuric acid is involved in resonance. This resonance increase the stability. On the other hand lone pair of oxygen atom directly bonded to nitrogen atom in nitric acid is available for attack on proton. This is the reason that nitric acid does not provide its electron in this reaction.

Step 2: Attack of electrophile to give arenium ion.

In this step electrophile (nitronium ion) attacks on the benzene ring which gives the formation of arenium ion. It is also known as sigma complex.

image showing step of formation of sigma complex or arenium ion during nitration of benzene

Step 3: Loss of proton to give nitrobenzene.

In this step sulphonate ion accepts proton from arenium ion which gives the nitrobenzene.

image showing final step and recovering of sulfuric acid in nitration of benzene

From above image sulfuric acid is also recovered that was used as a catalyst at the start of the reaction. Finally our product nitrobenzene is produced at the end of reaction.