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A chemical reaction in which a hydrogen atom on a benzene ring is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution is known as sulphonation.

The SO₃H group can be used to block a position on the ring. This is a common procedure to make ortho isomers when the para position is more reactive. To get an ortho product, the para position is blocked using SO₃H, substituting the ortho position, and then removing the blocking group.

Here, the sulphonation of the benzene ring forms a para-substituted product. The para-substituted product undergoes chlorination at ortho of methyl (+I effect) and meta of sulpho group (-M effect).

The hydrolysis eliminates the sulpho group and the final oxidation step forms ortho-chlorobenzoic acid.

Here, the oxidation of the methyl group forms benzoic acid.

The chlorination of benzoic acid adds the chlorine group at meta-position to the carboxylic group (-COOH). This is because -COOH group (-M effect) is extracting electron density from ortho and para position.

Here, firstly the chlorination goes at the para position as it is not occupied.

Later, the compound is oxidized to form para-chlorobenzoic acid.