In my lab, acetanilide was added to bromine in aqueous acetic acid, and discolouration of the reaction mixture was obtained, indicating that bromine was consumed.
After some discussion, it was concluded that an electrophilic aromatic substitution might be taking place. This was backed up by a procedure we found online. What was left to do was to confirm the presence of the product, 4-bromoacetanilide.
The challenge is that the reaction mixture contains unreacted acetanilide as well as 4-bromoacetanilide, acetic acid, and possibly some polyhalogenated byproducts. Any conditions that could result in amide hydrolysis, e.g. heating with base, should also be avoided. No residue was observed in the reaction mixture.
So, what chemical property can be used to determine the presence of 4-bromoacetanilide in the reaction mixture?