Electrophilic aromatic substitution

Electrophilic aromatic substitution

source : chem.libretexts.org

Electrophilic aromatic substitution-

Electrophilic aromatic substitution is a reaction in which an atom of aromatic system (generally hydrogen) is replaced by electrophilic reagent or electrophile.

Ex-  Nitration , Sulphonation , Halogenation, Acylation, Alkylation of aromatic ring.

General mechanism –

Halogenation –

The halogenation is catalysed by lewis acid like FeCl3 or FeBr3 , AlCl3 & ZnCl2 .

(i) FeCl3 polarises the Cl2 molecule into oppositely charged ions & electrophilic attacking species Cl+ is formed.

(ii) The electrophilic species attacks the benzene ring to produce an intermediate carbonium ion which is resonance        stabilised .

(iii) Carbonium ion loses a proton & restore the stable benzene ring system.


In nitration , the attacking electrophilic species is nitronium ion  NO2+. It is produced by the action of sulphuric acid on nitric acid.

(i) Conc. H2SO4 ionises HNO3.

2H2SO4 + HNO3  ——–>  H3O +(hydronium ion)  +  2 HSO4       + NO2 +     (nitronium ion )

(ii) Nitronium ion (NO2+) attacks the benzene ring & carbonium ion is formed as intermediate . carbonium ion is

highly reactive.

(iii) Carbonium ion stabilises to give a proton to form nitrobenzene.

Sulphonation –

 In sulphonation , SO3 is attacking electrophilic species.

(i) Sulphur trioxide is formed by the dissociation of sulphuric acid.

2 H2SO4  ———> H3O+ + HSO4 (hydrogen sulphate ion) + SO3 (sulphur trioxide as electrophile)

(ii) Electrophilic species SO3 attacks the benzene ring to form an intermediate carbonium ion which is stabilised by resonance.

(iii) Carbonium ion gives a proton.

(iv) The negative charge of SO3 is removed by the reaction of H3O+ (hydronium ion).