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.
Nitration-
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).