proving an unknown organic compound using reactions
The task you had being assigned is called identification by derivatization. It's one of the strongest proof for the confirmation of the structure of a molecule.
If you have an hard-to-identify compound X, you can make it react with Y and then analyze the product of the reaction, X-Y, instead.

I once synthesized a polysubstituted aromatic compound and wasn't sure whether or not a iodine atom was present on the ring. So I performed a Heck reaction on the molecule obtaining the corresponding substituted olefin, which was a known compound, present in the literature and could be identified by its NMR spectra.

Chlorobenzene is quite an anonymous compound and its derivatization does not lead to significative compounds. Just an example to answer your question, you can perform a nitration to get o-chloronitrobenzene which is a solid and can be identified by its melting point.