What is the product of para-sulfonyltoluene with #"Br"_2# catalyzed by #"FeBr"_3#, followed by treatment with dilute sulfuric acid?
Our reaction is:
- The sulfonic acid group is, like the carboxylic acid group, a meta director.
- The methyl group, an electron-donating group, is an ortho-para director, but the para position is blocked.
Therefore, the overall directing is towards the position ortho to the methyl group.
Treatment with acid and "super-heated steam", i.e. heating with dilute acid, takes advantage of the reversibility of sulfonation and desulfonates the first product to generate the final product, o-bromotoluene.
The mechanism for desulfonation is the microscopic reverse of the forward sulfonation mechanism.
(Note that the reverse reaction depicted above won't necessarily occur to produce the first product in that way; it is only provided for context.)