Question

The electrophile in sulphonation of benzene using fuming sulphuric acid is:

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

Answer 1

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

Sulphonation of benzene is the substitution of the hydrogen atom of benzene molecule by $-S{O}_{3}H$ (sulphonic acid) group.
Sulphur trioxide $S{O}_3$ is active species in sulphonation of benzene. $S{O}_3$ acts as an electrophile and accepts a pair of pi electrons from benzene.
Generation of electrophile
$H_{2}S{O}_4+H_{2}S{O}_4\rightleftharpoons H_3{O}^{+}+HS{O}_4^{-}+S{O}_3$