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

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.

Similarly other derivatives of benzene can also undergo nitration. For example nitration of toluene.

Watch the video to better understand nitration of Benzene

Nitration of benzene conditions

The nitration of benzene is a reaction that involves the substitution of a nitro group (-NO2) for a hydrogen atom on a benzene ring. The conditions required for this reaction are:

Reagents: A mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) is used as the nitrating agent. This mixture is called a nitrating mixture or nitration mixture.

Temperature: The nitration of benzene is an exothermic reaction, which means it releases heat. To control the reaction and prevent a runaway reaction, the reaction mixture should be kept at a temperature of 50-60°C.

Catalyst: No catalyst is required for this reaction as the sulfuric acid acts as a catalyst by protonating the nitric acid, making it more reactive.

Stoichiometry: The stoichiometry of the reaction is important, and the ratio of nitric acid to benzene should be carefully controlled. Generally, a 1:1.5 to 1:2 molar ratio of nitric acid to benzene is used.

The overall reaction can be represented as:

C₆H₆ + HNO₃ + H₂SO₄ → C₆H₅NO₂ + H₂O

Where C₆H₆ is benzene, HNO₃ is nitric acid, and C₆H₅NO₂ is nitrobenzene.

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.

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.

Step 3: Loss of proton to give nitrobenzene.

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

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.

Mechanism of nitration of nitrobenzene

Overall reaction

i) Formation of electrophile

ii) Attack of electrophile

The electrophile attacks the Meta position of the benzene ring because the group which is already present on the benzene ring is an electron-withdrawing group. Electron withdrawing groups directs the incoming reagent toward the Meta position.

iii) Removal of the proton to regenerate aromaticity

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