The two most common methods of preparing aryl halides are by direct halogenation of benzene and via diazonium salt reactions. Halogenation of benzene The halogenation of benzene to synthesize aryl halides is the oldest method known. Sandmeyer reaction A second method for preparing aryl halides is the Sandmeyer reaction. During a Sandmeyer reaction, a diazonium salt reacts with copper (I) bromide, copper (I) chloride, or potassium iodide to form the respective aryl halide. The diazonium salt is prepared from aniline by reaction with nitrous acid at cold temperatures.