MAKING DIAZONIUM SALTS FROM PHENYLAMINE

This page looks at the reaction between phenylamine (also known as aniline and  aminobenzene) and nitrous acid - particularly its reaction at temperatures of  less than 5°C to produce diazonium salts. If you want to know about the reactions of the diazonium ions formed, you will find a link at the bottom of the page. The reactions of phenylamine with nitrous acid Nitrous acid (also known as nitric(III) acid) has the formula HNO2. It is  sometimes written as HONO to show the way it is joined up. Nitrous acid decomposes very readily and is always made in situ. In the case of  its reaction with phenylamine, the phenylamine is first dissolved in  hydrochloric acid, and then a solution of sodium or potassium nitrite is added.  The reaction between the hydrochloric acid and the nitrite ions produces the  nitrous acid. You get the reaction: Because nitrous acid is a weak acid, the position of equilibrium lies well the  right. Phenylamine reacts with nitrous acid differently depending on the temperature. The reaction on warming If the mixture is warmed, you get a black oily product which contains phenol  (amongst other things), and nitrogen gas is given off.
The reaction at low temperatures The solution of phenylamine in hydrochloric acid (phenylammonium chloride solution) is  stood in a beaker of ice. The sodium or potassium nitrite solution is also cooled in the ice. The solution of the nitrite is then added very slowly to the phenylammonium chloride  solution - so that the temperature never goes above 5°C. You end up with a solution containing benzenediazonium chloride: The positive ion, containing the -N2+ group, is known as a diazonium ion. The "azo"  bit of the name refers to nitrogen.