Nitration is an electrophilic substitution reaction of aromatic hydrocarbons in which a nitro group (–NO2) is introduced into the aromatic ring by replacing a hydrogen atom. It is carried out using a mixture of concentrated nitric acid and sulphuric acid, which generates the active electrophile. Due to the stability of the aromatic ring, the reaction proceeds by substitution rather than addition, thereby preserving aromaticity.
Nitrating Mixture (Reagents)
Nitration is carried out using a mixture of concentrated nitric acid (HNO3) and concentrated sulphuric acid (H₂SO₄), commonly known as the nitrating mixture. Both acids play specific roles in the reaction.
- Nitric acid acts as the source of the nitro group (–NO2).
- Sulphuric acid, being a stronger acid, protonates nitric acid and facilitates the removal of a water molecule, leading to the formation of the active electrophile, the nitronium ion (NO2+).
- Thus, the main role of the nitrating mixture is to generate a strong electrophile required for the electrophilic substitution reaction in aromatic hydrocarbons.
Electrophile Formation
In the nitration reaction, the active electrophile is the nitronium ion (NO2⁺). It is generated from the nitrating mixture of concentrated nitric acid and concentrated sulphuric acid.
- Sulphuric acid protonates nitric acid, which then loses water to form NO2⁺.
HNO3 + H2SO4 → NO2+ + HSO4 -+ H2O
- NO2⁺ is electron-deficient and acts as a strong electrophile.
- This makes it capable of attacking the electron-rich benzene ring in the next step of the reaction.
Nitration Mechanism of Benzene
Nitration of aromatic hydrocarbons follows the electrophilic substitution mechanism, in which the nitronium ion (NO2+) attacks the benzene ring.
Step 1: Electrophilic Attack
The nitronium ion (NO2+), which is a strong electrophile, attacks the electron-rich benzene ring. This results in the formation of an unstable intermediate called the arenium ion (σ-complex). In this step, the aromaticity of benzene is temporarily lost.
Step 2: Resonance Stabilization
The intermediate formed is resonance-stabilised, where the positive charge is delocalised over the ring. However, it is still less stable than benzene.
Step 3: Deprotonation
A base (HSO4⁻) removes a hydrogen ion (H⁺) from the intermediate. This restores the aromaticity of the ring and forms the final product, nitrobenzene.
Reaction of Nitration
In nitration, benzene reacts with concentrated nitric acid (HNO3) in the presence of concentrated sulphuric acid (H2SO4) to form nitrobenzene.
C_6H_6 + HNO_3 \xrightarrow{conc.\ H_2SO_4,\ 50-60^\circ C} C_6H_5NO_2 + H_2O
- In this reaction, a hydrogen atom of the benzene ring is replaced by a nitro group (–NO2).
- Sulphuric acid acts as a catalyst and helps in the formation of the electrophile (NO2⁺).
- The reaction is usually carried out at a temperature of about 50–60°C.
- The product formed, nitrobenzene, is an important compound used in the preparation of dyes, drugs, and other chemicals.