Methods of Preparation of Phenols

Phenol was historically extracted from coal tar, but essentially all of today's industrial supply

comes from one dominant synthetic route, with two further routes remaining useful for smaller-scale

or laboratory preparation.

The cumene process (industrial route). Benzene is first alkylated with propene under

Friedel-Crafts conditions (an acid catalyst such as phosphoric acid or $\text{AlCl}_3$) to give

cumene (isopropylbenzene):

Cumene is then oxidised by atmospheric oxygen (air is simply bubbled through it) to cumene

hydroperoxide, $\text{C}_6\text{H}_5\text{C(CH}_3)_2\text{OOH}$, and this hydroperoxide is finally

treated with dilute acid, which triggers a rearrangement that cleaves it to give phenol

together with acetone as a valuable co-product:

This route dominates world phenol production precisely because it delivers two commercially

valuable products, phenol and acetone, from cheap starting materials (benzene and propene) in one

integrated process.

From diazonium salts. Aniline (aminobenzene) is first converted to benzenediazonium chloride

by treatment with sodium nitrite and dilute hydrochloric acid at a low temperature ($0$–$5\,° \text{C}$, since the diazonium salt is unstable above this range):

Warming (boiling) this diazonium salt with water then hydrolyses it, releasing nitrogen gas and

substituting the diazonium group with $-\text{OH}$:

From haloarenes (the Dow process). Chlorobenzene, which does not undergo ordinary nucleophilic

substitution under mild conditions (the $\text{C--Cl}$ bond is strengthened by partial double-bond

character from resonance with the ring, and a simple $S_N1$/$S_N2$ pathway is not available at an

$sp^2$ carbon), can nonetheless be forced to react with sodium hydroxide under much harsher forcing

conditions -- fusion (heating together) at about $623\ \text{K}$ and $300$ atmospheres pressure --

to give sodium phenoxide, which is then acidified to liberate phenol: …