Benzene is highly prone to electrophilic substitution reactions compared to addition reactions as it loses its aromaticity during addition reaction. 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, electrophilic substitution reaction is a three step process involving:
• Generation of electrophile.
• Intermediate carbocation formation.
• Removal of proton from carbocation intermediate.
Some 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.
Mechanism for nitration of benzene:
Step 1: Nitric acid accepts a proton from sulphuric acid and then dissociates to form nitronium ion.
Step 2: The nitronium ion acts as an electrophile in the process which further reacts with benzene to form arenium ion.
Step 3: The arenium ion then loses its proton to Lewis base forming 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.
A 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.
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.
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.
For detail discussions on nitration, sulfonation and halogenation of benzene, please visit Byju’s.