Benzene Reactions - Sulfonation, Nitration and Halogenation

Benzene is highly prone to electrophilic substitution reactions compared to addition reactions as it loses its aromaticity during addition reactions. As benzene contains delocalized electrons spanning over carbon atoms in the ring, it is highly attractive to electrophiles and is also highly stable to electrophilic substitutions. Generally, the electrophilic substitution reaction of benzene is a three-step process involving:

  • Generation of the electrophile.
  • Intermediate carbocation formation.
  • Removal of a proton from carbocation intermediate.

Common benzene reactions are

Nitration of Benzene

Benzene reacts with concentrated nitric acid at 323-333K in the presence of concentrated sulphuric acid to form nitrobenzene. This reaction is known as nitration of benzene.

Nitration of nitrobenzene

Nitration of nitrobenzene

The mechanism for nitration of benzene:

Step 1: Nitric acid accepts a proton from sulphuric acid and then dissociates to form nitronium ion.

Nitric acid accepts a proton from sulphuric acid

Nitronium ion

Step 2: The nitronium ion acts as an electrophile in the process which further reacts with benzene to form an arenium ion.

Arenium ion

Step 3: The arenium ion then loses its proton to Lewis base forming nitrobenzene.

Formation of nitrobenzene

Sulfonation of Benzene

Sulfonation of benzene is a process of heating benzene with fuming sulphuric acid (H2SO4 +SO3) to produce benzenesulfonic acid. The reaction is reversible in nature.

Sulfonation of benzene

The mechanism for Sulfonation of benzene

Due to higher electronegativity, oxygen present in sulphuric acid pulls an electron towards itself, generating an electrophile. This attacks the benzene ring, leading to the formation of benzenesulfonic acid.

Formation of benzenesulfonic acid

Halogenation of Benzene

Benzene reacts with halogens in the presence of Lewis acid like FeCl3, FeBr3 to form aryl halides. This reaction is termed as halogenation of benzene.

Halogenation of benzene

Halogenation of benzene

The mechanism for halogenation of benzene:

Step 1: Being a Lewis acid, FeBr3 helps in the generation of electrophile bromine ion by combining with the attacking reagent.

Generation of bromine ion

Generation of bromine ion

Step 2: The bromine ion acts as an electrophile in the process which further reacts with benzene to form arenium ion which finally converts to bromobenzene.

Formation of bromobenzene

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