Many polycyclic aromatic compounds have been synthesized by a cyclization reaction known as the Bradsher reaction or aromatic cyclodehydration. This method can be illustrated by the following synthesis of 9-methylphenanthrene:

An arenium ion is an intermediate in this reaction, and the last step involves the dehydration of an alcohol. Propose a plausible mechanism for this example of the Bradsher reaction.

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HBr acetic acid, heat 9-Methylphenanthrene