Diazotisation of phenylamine

William Henry Perkin
The discoverer of azo dyes.

        Primary aromatic amines react with nitrous acid to give diazonium compounds which are very useful in a variety of synthetic routes, particularly the formation of the azo dyes. The solution of the diazonium salt is used immediately; it decomposes slowly in solution, and explodes if any attempt is made to isolate it as a solid.



Phenylamine is toxic and harmful by skin absorbtion; concentrated hydrochloric acid is corrosive.


  • Dissolve 15 cm3 of phenylamine in a mixture of 40 cm3 of concentrated hydrochloric acid and 40 cm3 of water contained in a 250 cm3 conical flask.
  • Why is phenylamine soluble in hydrochloric acid?
  • Phenylamine is a base, and reacts with acid to give a salt. This is ionic and is much more soluble than the free amine:

C6H5NH2  +  HCl  →  C6H5NH3+  +  Cl-  

  • Place a thermometer in the solution, immerse the flask in a mixture of ice and water, and cool until the temperature of the solution reaches 5oC.
  • Why must the temperature be around 5oC?
  • Nitrous acid HNO2 decomposes at room temperature.

  • Dissolve 12.5g of powdered sodium nitrite in 30 cm3 of water, and add this solution in portions of about 2 - 3 cm3 to the cold solution of phenylamine hydrochloride, keeping the latter well-stirred by means of the thermometer. A short interval should be allowed between successive additions; the temperature must not be allowed to rise above 10oC, nor below 0oC.
  • Why don't you use nitrous acid itself rather than making it in situ?
  • Nitrous acid decomposes rapidly at room temperature and is always made as it is needed. It is a pale blue solution.
  • Why is the short interval between additions needed?
  • The reaction between nitrous acid and the amine salt is quite slow at the temperature used; the reaction is exothermic, so too fast an addition might lead to thermal runaway and the temperature may rise above 10oC.

  • What happens if the temperature rises above 10oC?

  • The diazonium salt will react with the solvent water to give phenol, C6H5OH. This will then couple with unchanged diazonium compound to give an orange compound.

  • What happens if the temperature falls below 0oC?

  • The reaction becomes very slow and diazotisation does not occur.

  • The solution of benzenediazonium chloride must be used immediately in a further synthesis; it cannot be kept for more than a few hours even in a freezer.
  • Why is it not possible to keep the solution?
  • Reaction with solvent water occurs slowly even at low temperatures, and the solution turns orange for the reason given above.