Aromatic Compounds
4.0 Sulphonation
4.0 Sulphonation
Sulphonation is another synthetically important reaction. It is often accomplished with concentrated sulphuric acid or fuming sulphuric acid containing excess of $S{O_3}$ or chlorosulphonic acid, $ClS{O_2}OH$.
It is believed that the electrophile varies with the reagent, though in all cases $S{O_3}$ is involved either free or along with a carrier, like in ${H_3}SO_4^ + (S{O_3} + {H_3}{O^ + })$ or ${H_2}{S_2}{O_7}$. Sulphur trioxide is generated from sulphuric acid as follows $2{H_2}S{O_4} \rightleftharpoons HS{O^ - }_4 + {H_3}{O^ + } + S{O_3}$ . The mechanism of sulphonation of benzene is given below:
Sulphonation is different from other aromatic electrophilic substitution reactions. Firstly, it is reversible and secondly, it shows some amount of isotope effect which is totally absent in other cases.
