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Explanation

When the benzene ring contians `(-OH) or (_NH_(2))` group, then the ring is cleaved by oxidising agents such as alkaline `KMnO_(4)`. Hence, before `(-CH_(3))` is not be changed into `(-COOH)` group, `(-OH)` group should be protected by reaction with `TsCl` or by accetylation.

(Alkaline solution of `PbO_(2)` can also be used in such cases without protecting the `(-OH)` group.)