14.0 Sulphonation

  Alkyl Halides and Aryl Halides

When aryl halides are heated with concentrated sulphuric acid, sulphonation takes place. For example, when chlorobenzene is heated with concentrated sulphuric acid $4$-chlorobenzene sulphonic acid is obtained as the major product and $2$-chlorobenzene sulphonic acid is the minor product.

Mechanism:

Step 1: Generation of sulfurtrioxide (electrophile)

$$2{H_2}S{O_4} \rightleftharpoons S{O_3} + HS{O_4}^ - + {H_3}{O^ + }$$

Step 2: Attack of nitronium ion and formation of a carbocation

Step 3: Loss of proton, formation of $2$-Chlorobenzenesulfonic acid and $4$-Chlorobenzenesulfonic acid, regeneration of catalyst.