Question

what is the purpose of a sandmeyer reaction (to get 4-chlorotoluene )

Answer 1

That will actually depends of what is your starting product. In general, The Sandmeyer reaction is a chemical reaction used to synthesize aryl halides from aryl diazonium salts, The reaction is a method for substitution of an aromatic amino group via preparation of its diazonium salt followed by its displacement with a nucleophile, often catalyzed by copper(I) salts. The nucleophile can include halideanions, cyanide, thiols, water, and others.

The Sandmeyer Reaction is a very important transformation in aromatic chemistry, because it can result in some substitution patterns that are not achievable by direct substitution.

So, if your starting reactant is the aniline, and you need the p-chlorotoluene, the amine group is activating the molecule but unfortunately if you try to do chlorination there, or alkyls, it will react in all the molecules, for that we use sand meyer to reduce the amine, to a diazonium salt and then, a sustitution in there to get the halide.

For example: