Primary aromatic amines yield diazonium ions in a solution of sodium nitrite. Upon heating this solution with copper(I) chloride, the diazonium group is replaced by -Cl. This is a comparatively easy method to make aryl halides as the gaseous product can be separated easily from aryl halide.
When an iodide is to be made, copper chloride is not needed. Addition of potassium iodide with gentle shaking produces the haloalkane.