Nitration of Arenes

Nitration of Arenes:

in this reaction, a nitro group is introduced to the benzene ring by using the nitrating mixture. The reagents can be represented as

1. Concentrated HNO₃ + Concentrated H₂SO₄ 
2. Concentrated HNO₃ + Concentrated H₂SO₄ + SO₃
3. Concentrated HNO₃ +Oleum

A Nitrating mixture is formed by mixing concentrated nitric acid and concentrated sulphuric acid. Sometimes oleum is used instead of concentrated acid to increase the rate of reaction.

The general representation of reaction is as below

the mechanism of Nitration of Arenes, the reaction starts with the formation of nitronium ion. Sulphuric acid is a stronger acid because the conjugate base formed by the dissociation of sulphuric acid has greater resonance stabilization as a result of which sulphuric acid readily dissociates into H⁺ ion and Sulphate ion. The H⁺ ion formed reacts with nitric acid to produce oxonium ion which then stabilizes to form nitronium ion as mentioned below

In the Nitration of Arenes, the nitronium ion formed reacts with an aromatic ring to produce resonating forms of the arene. This positive charge on the aromatic ring a stabilized by resonating forms. In a resonance hybrid structure, the positive charges are distributed over five carbon atoms so the relative positive charge density on each carbon atom is very less and thus it is stabilized.

The structure three formed in the resonating forms further stabilizes to give mono substituted nitro derivative.

The H+ ion release in this reaction joins with the conjugate base to form a byproduct here it is termed as spent acid.

H⁺  + HSO₄ ^-1 → H₂SO₄
The monosubstituted derivative of arenes uses a nitrating mixture in different concentrations of sulphuric acid and nitric acid but the Concentration of sulphuric acid is always kept higher as it is a hygroscopic material and it absorbs water formed in the first step as a result of which more forward reaction takes place and concentration often nitronium ion increases the reaction mixture thus rate of reaction increases and formation of product is higher

 

 

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