Sulfonation results in the substitution of a sulfonyl hydroxide (SO3H) group with a H atom.

The electrophile is thought to be either sulfur trioxide (SO3) or protonated SO3.

The mechanism proceeds as follows.


Benzene sulfonic aid can be hydrolyzed back to benzene by treatment with dilute H2SO4.  As we will see sulfonation can be used as a method of blocking or protecting a position on a ring to prevent reactions there.  In some respect, it can be considered a protecting group.

For example you could synthesize ortho-bromocumene as follows.

If you didn't block/unblock the para position the following would occur producing mostly the para-bromocumene isomer.