5.11.6 Diazotization

A primary amine reacts with nitrous acid to give a diazonium salt. This process is called diazotization. Both aliphatic and aromatic diazonium salts can be made but aromatic diazonium salts are relatively stable at 0-10 °C. Aryldiazonium salts are formed by diazotizing a primary aromatic amine. For example, p -methylaniline is diazotized to give p -methylbenzendiazonium chloride (Scheme 1). These reactions of diazonium salts are extremely useful in synthetic and material chemistry.

Scheme 1

When the acidic solution of this diazonium salt is warmed, the hydroxyl group of water replaces N₂ to give p -cresol (Scheme 2).

Scheme 2

Cuprous chloride reacts with diazonium salt to give p -chlorotoluene (Scheme 3). The use of cuprous salts to replace aryldiazonium group is called Sandmeyer reaction . Bromide and cyanide substituted aryl derivatives can be prepared by using cuprous bromide and cuprous cyanide, respectively.

Scheme 3

Aryl diazonium salt with fluoroboric acid (HBF₄) gives the diazonium fluoroborate, which could be converted to p -fluorotoluene under heating (Scheme 4).

Scheme 4

Iodobenzene derivatives can be prepared by treating the diazonium salts with potassium iodide (Scheme 5).