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How do I predict the outcome of this reaction?

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I expected this to be an acid base reaction leading to the formation of the weak acid para-chlorobenzenesulfonic acid. However, the answer given in the worksheet was surprising $\ce{-SO3H}$ was eliminated and the product stated was $\ce{C6H5Br}$. I would like to be acquainted with the mechanism of this unusual reaction. I couldn't find it in Clayden or Solomon Frhyle.

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    $\begingroup$ Electrophilic sulfonation of aromatic rings is reversible. The firt paragraph here may be helpful. $\endgroup$
    – ron
    Mar 20, 2018 at 17:00

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I think @ron has hit the bull's eye in the comments! I would like to elaborate upon that:

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Since each step in the sulfonation of benzene is an equilbirum, sulfonation is a reversible reaction. According to Le-Chatlier's principle addition of $\ce{H+}$ ions in the solution would shift the equilibrium to the left in the last step of sulfonation where arenium is deprotonated, hence addition of $\ce{HCl(aq)}$ accompanied with heating for kinetic requirements results in the production of 4-bromobenzenesulfonic acid in the given reaction. This process is called protodesulphonylation.

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    $\begingroup$ Well said! I'll add one more point, since aromatic sulfonation is reversible it can be synthetically useful. You can use the group to block and direct (see here for an example) and then remove the sulfonic acid group afterwards. $\endgroup$
    – ron
    Mar 20, 2018 at 20:42

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