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Preparation of Phenol

Synthesis from Haloarenes, Sulphonic Acids, Diazonium Salts, and Cumene.

By chemca Team • Updated Jan 2026

Phenol (Carbolic Acid) is an aromatic alcohol where the -OH group is directly attached to the benzene ring. It is prepared industrially and in the laboratory using several key methods.

1. From Haloarenes (Dow's Process)

Industrial Method

Chlorobenzene is fused with NaOH at very high temperature (623 K) and pressure (300 atm) to form Sodium Phenoxide, which on acidification yields Phenol.

$$ \underset{\text{Chlorobenzene}}{C_6H_5Cl} + 2NaOH \xrightarrow{623K, 300 atm} C_6H_5O^-Na^+ + NaCl + H_2O $$ $$ C_6H_5O^-Na^+ \xrightarrow{HCl} \underset{\text{Phenol}}{C_6H_5OH} + NaCl $$

Note: Nucleophilic substitution on the benzene ring is difficult and requires drastic conditions unless Electron Withdrawing Groups are present.

2. From Benzene Sulphonic Acid

Alkali Fusion

Benzene is sulphonated with Oleum to form Benzene Sulphonic Acid. This is neutralized with NaOH and then fused with solid NaOH at 573 K to form Sodium Phenoxide.

$$ C_6H_6 + H_2S_2O_7 \text{ (Oleum)} \rightarrow C_6H_5SO_3H $$ $$ C_6H_5SO_3H + NaOH \xrightarrow{\Delta} C_6H_5SO_3Na $$ $$ C_6H_5SO_3Na + 2NaOH \xrightarrow{\text{Fuse, } 573K} C_6H_5ONa + Na_2SO_3 + H_2O $$ $$ C_6H_5ONa \xrightarrow{H^+} C_6H_5OH $$

3. From Diazonium Salts

Laboratory Method

Aniline is diazotized with $NaNO_2/HCl$ at 0-5°C. The resulting diazonium salt is hydrolyzed by warming with water or dilute acids to give Phenol.

$$ \underset{\text{Aniline}}{C_6H_5NH_2} \xrightarrow{NaNO_2/HCl, 273K} C_6H_5N_2^+Cl^- $$ $$ C_6H_5N_2^+Cl^- + H_2O \xrightarrow{\text{Warm}} C_6H_5OH + N_2 \uparrow + HCl $$

4. From Cumene (Isopropyl Benzene)

Most Important Industrial Method

Cumene is oxidized by air to Cumene Hydroperoxide. Treatment with dilute acid converts it into Phenol and Acetone (a valuable byproduct).

Step 1: Oxidation

$$ \underset{\text{Cumene}}{C_6H_5-CH(CH_3)_2} + O_2 \xrightarrow{\text{Air}} \underset{\text{Cumene Hydroperoxide}}{C_6H_5-C(CH_3)_2-O-OH} $$

Step 2: Hydrolysis

$$ C_6H_5-C(CH_3)_2-O-OH \xrightarrow{H^+/H_2O} \underset{\text{Phenol}}{C_6H_5OH} + \underset{\text{Acetone}}{CH_3COCH_3} $$

This method is widely used because both products (Phenol and Acetone) are commercially valuable.

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