Certain activated benzene rings can be chlorinated by hypochlorous acid, HOCL, and this reaction is catalyzed by \(\mathrm{H}^{+}\). Can you suggest a possible function of \(\mathrm{H}^{+}\) ?

First, let's get familiar with the main components of the reaction. An activated benzene ring is a benzene molecule that has one or more activating groups, which makes it more reactive than benzene itself. Hypochlorous acid (HOCl) is a weak, unstable acid and a good source of chlorine atoms. The presence of a proton (\(\mathrm{H}^{+}\)) is said to catalyze this reaction.

A catalyst is a substance that speeds up the rate of a reaction without being consumed in the process. In this case, the proton (\(\mathrm{H}^{+}\)) is acting as a catalyst. The possible role of \(\mathrm{H}^{+}\) might be to stabilize the reaction intermediates, making it easier for the reaction to proceed.

One possible function of the proton is to protonate the oxygen atom in the hypochlorous acid (HOCl), making it a better electrophile (an electron-seeking species): \[ \mathrm{H}^{+} + \mathrm{HOCl} \rightarrow \mathrm{HOClH}^{+} \]

The protonated hypochlorous acid, \(\mathrm{HOClH}^{+}\), is a better electrophile due to its increased positive charge and, as a result, the electron-rich activated benzene ring is more attracted to it. This leads to an electrophilic substitution reaction, where the activated benzene ring donates its electrons to form a bond with the chlorine atom of hypochlorous acid, while the oxygen-hydrogen moiety leaves (with the proton returning to the medium): \[ \text{Activated benzene ring} + \mathrm{HOClH}^{+} \rightarrow \text{Chlorinated benzene} + \mathrm{H}_{2}\mathrm{O} + \mathrm{H}^{+} \] This catalytic role of the proton helps facilitate this electrophilic substitution reaction and thus increases the rate at which the benzene undergoes chlorination.