Preparation of Phenol: Methods, Reactions and Mechanisms

Phenol, also known as hydroxybenzene, is an important aromatic organic compound with the molecular formula C₆H₅OH. It consists of a hydroxyl group (-OH) directly attached to a benzene ring. Phenol is widely used in the manufacture of resins, plastics, dyes, medicines and many other industrial chemicals.

There are several methods for the preparation of phenol. Some methods are particularly important for academic and competitive examinations, while the cumene process is an important industrial method.

1. Preparation of Phenol from Chlorobenzene: Dow Process

Phenol can be prepared from chlorobenzene by heating it with sodium hydroxide under high temperature and pressure. This method is known as the Dow process.

Step 1: Formation of Sodium Phenoxide

Chlorobenzene is treated with aqueous sodium hydroxide at about 623 K and high pressure. The reaction produces sodium phenoxide.

Reaction:

C₆H₅Cl + NaOH → C₆H₅ONa + HCl

Under industrial conditions, the reaction is commonly represented as:

C₆H₅Cl + NaOH → C₆H₅ONa + NaCl

Step 2: Acidification of Sodium Phenoxide

The sodium phenoxide formed in the first step is treated with an acid such as hydrochloric acid. Phenol is obtained as the final product.

C₆H₅ONa + HCl → C₆H₅OH + NaCl

Thus, chlorobenzene is converted into phenol through the intermediate sodium phenoxide.

2. Preparation of Phenol from Benzene Sulphonic Acid

Phenol can also be prepared from benzene sulphonic acid or its sodium salt. The process involves alkaline fusion followed by acidification.

Step 1: Formation of Sodium Phenoxide

Sodium benzene sulphonate is heated with molten sodium hydroxide. The sulphonate group is replaced by the hydroxyl group, producing sodium phenoxide.

C₆H₅SO₃Na + 2NaOH → C₆H₅ONa + Na₂SO₃ + H₂O

Step 2: Acidification

The sodium phenoxide is then acidified with hydrochloric acid to obtain phenol.

C₆H₅ONa + HCl → C₆H₅OH + NaCl

3. Preparation of Phenol from Benzene Diazonium Chloride

Benzene diazonium chloride is an important intermediate in aromatic chemistry. When it is heated with water, the diazonium group is replaced by a hydroxyl group and phenol is formed. Nitrogen gas is released during the reaction.

Reaction:

C₆H₅N₂⁺Cl^- + H₂O → C₆H₅OH + N₂ + HCl

The reaction is carried out by warming the aqueous solution of benzene diazonium chloride. Because nitrogen gas is evolved, the reaction proceeds readily.

Why is this method important?

This method demonstrates the replacement of the diazonium group by a hydroxyl group. It is an important example of the chemistry of diazonium salts.

4. Industrial Preparation of Phenol: Cumene Process

The cumene process is an important industrial method for manufacturing phenol. It is particularly significant because it produces phenol and acetone simultaneously.

Cumene is also known as isopropylbenzene.

Step 1: Formation of Cumene

Benzene reacts with propene in the presence of an acid catalyst to form cumene.

C₆H₆ + CH₃CH=CH₂ → C₆H₅CH(CH₃)₂

The product is called cumene.

Step 2: Oxidation of Cumene

Cumene is oxidized by air to form cumene hydroperoxide.

Cumene + O₂ → Cumene hydroperoxide

Step 3: Acidic Cleavage

Cumene hydroperoxide undergoes acid-catalysed cleavage to produce phenol and acetone.

Cumene hydroperoxide → Phenol + Acetone

In simplified form:

C₆H₅C(CH₃)₂OOH → C₆H₅OH + CH₃COCH₃

The formation of acetone as a valuable co-product makes the cumene process commercially important.

5. Comparison of Important Methods

Method	Starting Material	Important Intermediate	Final Product
Dow Process	Chlorobenzene	Sodium phenoxide	Phenol
Alkaline Fusion	Sodium benzene sulphonate	Sodium phenoxide	Phenol
Diazonium Salt Hydrolysis	Benzene diazonium chloride	Diazonium salt	Phenol
Cumene Process	Cumene	Cumene hydroperoxide	Phenol + Acetone

6. Important Reactions to Remember

From chlorobenzene:

C₆H₅Cl → C₆H₅ONa → C₆H₅OH

From benzene sulphonic acid:

C₆H₅SO₃Na → C₆H₅ONa → C₆H₅OH

From diazonium salt:

C₆H₅N₂⁺Cl^- → C₆H₅OH

By cumene process:

Cumene → Cumene hydroperoxide → Phenol + Acetone

7. Exam-Oriented Points

• The molecular formula of phenol is C₆H₅OH.
• The Dow process uses chlorobenzene and sodium hydroxide.
• Sodium phenoxide is an important intermediate in the Dow process.
• Acidification of sodium phenoxide gives phenol.
• Benzene diazonium chloride gives phenol on warming with water.
• The cumene process is an important industrial method for phenol preparation.
• The cumene process produces acetone as a valuable co-product.
• Cumene is also called isopropylbenzene.

Conclusion

Phenol can be prepared by several chemical routes. The preparation from chlorobenzene through the Dow process, alkaline fusion of benzene sulphonate, hydrolysis of benzene diazonium chloride, and the industrial cumene process are the most important methods to study. Among these, the cumene process is especially important industrially because it produces both phenol and acetone.

Understanding the starting material, reaction conditions, intermediate and final product makes these preparation methods much easier to remember and apply in organic chemistry problems.