Middle oil of coal-tar distillation comprise naphthalene and phenolic elements. Phenolic compounds are isolated in subsequent steps.

Step a: Middle oil is washed along with H₂SO₄. It dissolves basic impurities such as pyridine (base).

Step b: Extreme cooling separates naphthalene (a low melting solid)

Step c: Filtrate of step b is treated with aqueous NaOH when phenols dissolve as phenoxides. Carbon dioxide is then given by the solution to liberate phenols.

Step d: Crude phenol (of step c) is subjected to fractional distillation.