Primary amines can react with nitrous acid to produce diazonium ions.  Alkyl diazonium ions are unstable at room temperature and immediately decompose to reactive carbocations which can produce a variety of products (the procedure has little synthetic utility).  However, arylamines (anilines) produce aryl diazonium ions which are stable for a few hours at ice-water temperatures (0 C), and can undergo useful displacement reactions toward formation of an array of substituted aryl products. These include fluoro, chloro, bromo, iodo, cyano (nitrile), and hydroxy derivatives (phenols).  Additionally, the diazonium ion can be replaced with "H".

The methods for displacement of the aryl diazonium ion (releasing N₂) are outlined as follows:

Note formation of aryl iodides and aryl fluorides which cannot be made by directed electrophilic halogenation.  The aryl nitrile can be further hydrolyzed to the aryl carboxylic acid; the phenol can be converted into aryl alkyl ethers.