Chlorination of Nitrobenzene

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Chlorination of Nitrobenzene
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The chlorination of nitrobenzene is an electrophilic substitution reaction, which involves the replacement of a hydrogen atom on the benzene ring with a chlorine atom. The reaction is typically carried out in the presence of a catalyst, such as ferric chloride (FeCl₃), and a chlorinating agent, such as chlorine gas (Cl₂).

Introduction to Electrophilic Substitution

Chlorination of Nitrobenzene is electrophilic di substitution, where an electrophile (an electron-deficient species) replaces a leaving group on arene molecule. In the case of aromatic compounds, the carbon atom is having higher electron density due to delocalization of π-electrons in aromatic ring.

The electrophilic substitution reaction involves three steps

Formation of Electrophile
Formation of Sigma complex.
Stabilization of Sigma complex

Formation of Electrophile

In the chlorination of nitrobenzene, the electrophile is the chloronium ion (Cl⁺), which is generated in situ from chlorine gas (Cl₂) and a Lewis acid catalyst, such as ferric chloride (FeCl₃) or Anhydrous AlCl₃.

Formation of Sigma complex

The Chloronium ion formed as a reactive intermediate in the above step will join with the arene (here Nitrobenzene). Now when benzene is already having a substituent present on the ring the directive effect is applicable on the new incoming group (in this case chloronium ion as incoming group). The purpose of directive effect is to achieve the most stable end product where both already present group and incoming group are not inducing instability to the final structure.

Here nitro group is – I-effect (negative inductive effect) group and chlorine will join only on those positions where stability of nitro group is not decreased. The resonating forms for the substitution reaction are as below

In above resonating forms Structure (I) and structure (II) are exceptionally unstable structures. As positive charge is present on the carbon atom attached to an electron withdrawing group. Thus, the relative stability of meta-attack is higher than corresponding ortho and para-attacks.

Thus, incoming group will join at meta position on nitrobenzene.

Reestablishment of aromatic character

The sigma complex formed loses a proton and aromaticity of arene is reestablished .