Which of the following electrophilic substitutions of benzene is reversible?

ANitration
BSulfonation
CChlorination with $Cl X_{2}$ / $FeCl X_{3}$
DFriedel-Crafts acylation

Answer: B

The sulfonic acid group can be removed by heating with dilute sulfuric acid or steam, so sulfonation is an equilibrium. The other three reactions are effectively irreversible under normal conditions.

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