Preparation of phenol

In the early nineteenth century, phenol was selected from coal tar by destructive distillation. Now phenol is commercially produced synthetically. The laboratory methods of preparation of phenols are:

i) From aryl sulphonic acids

An aryl sulphonic acid yields the corresponding phenol on heating it with molten sodium hydroxide at 570 - 620 K. The sodium salt is obtained which is hydrolysed with acid to obtain free phenol.

ii) From haloarenes

Chlorobenzene (an haloarene) is hydrolysed by treating it with 10% NaOH at 623 K and 320 atmospheric pressure in presence of Cu catalyst. Phenol is obtained by acidification of sodium phenoxide.

iii) Hydrolysis of diazonium salts

A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (obtained from a mixture of NaNO₂ and HCl) at low temperature of 273 K to 278 K.

Diazonium salts are hydrolysed to phenols treating with dilute acids.