Question

In the sulphonation of benzene, the active electrophilic species is:

• A. $S{O}_2$
• B. $S{O}_3$
• C. $S{O}_4^{3-}$
• D. $HS{O}_4^{-}$

Answer 1

The correct option is B $S{O}_3$

Sulphonation of benzene is an aromatic electrophilic reaction. It occurs by the replacement of H-atom by sulphonic acid group in the benzene ring.
In the sulphonation of benzene, the electrophilic species is $S{O}_3$.

In step 1, two moles of sulphuric acid will reacts where one will act as acid and one will act as base to form conjugate acid and base of sulphuric acid.

In step 2, $H_{2}O$ molecule gets removed from the conjugate acid of sulphuric acid to form $\stackrel{+}{S}{O}_{3}H$.

In step 3, $H^{+}$ gets removed from $\stackrel{+}{S}{O}_{3}H$ to form $H_3{O}^{+}$ and $S{O}_3$.