How is Phenol Prepared from Benzene Sulphonic Acid?

Phenol can be prepared from benzene sulphonic acid by converting the sulphonic acid into its sodium salt, heating the sodium benzene sulphonate with molten sodium hydroxide, and then acidifying the sodium phenoxide formed in the reaction.

The complete conversion can be represented as:

Benzene sulphonic acid → Sodium benzene sulphonate → Sodium phenoxide → Phenol

The key reaction involves alkaline fusion with sodium hydroxide at about 573 K (300°C) followed by acidification. Educational chemistry references also describe the benzene sulphonic acid route using NaOH followed by dilute HCl under high-temperature conditions.

What Is Benzene Sulphonic Acid?

Benzene sulphonic acid is an aromatic sulphonic acid with the molecular formula:

C₆H₆O₃S

It consists of a benzene ring bearing a sulphonic acid group (–SO3H).

In the phenol-preparation reaction, the sulphonic acid is first converted to its sodium salt before alkaline fusion.

AAOL India works extensively with sulphonation chemistry and sulphonic-acid derivatives, making this reaction relevant to the broader chemistry of aromatic sulphonation.

For readers interested in the underlying chemistry, this section should internally link to AAOL India’s dedicated content on the sulphonation process and sulphonic acids.

Preparation of Phenol from Benzene Sulphonic Acid: Reaction

The preparation takes place in three logical stages.

Step 1: Formation of Sodium Benzene Sulphonate

Benzene sulphonic acid is first neutralized with sodium hydroxide to form sodium benzene sulphonate.

C₆H₅SO₃H + NaOH → C₆H₅SO₃Na + H₂O

Here:

• C6H5SO3H = benzene sulphonic acid
• C6H5SO3Na = sodium benzene sulphonate

This sodium salt is the intermediate used in the alkaline-fusion step.

Step 2: Alkaline Fusion with Sodium Hydroxide

Sodium benzene sulphonate is heated with molten sodium hydroxide at approximately 573 K or 300°C.

During this reaction, the sulphonate group is replaced and sodium phenoxide is formed.

C₆H₅SO₃Na + 2Na₂OH (573K)→ C₆H₅ONa + Na₂SO_{3 +} H₂O

This is the most important step in the conversion of benzene sulphonic acid to phenol.

Other chemistry resources similarly describe molten NaOH as the reagent required for the conversion and identify sodium phenoxide as the intermediate product.

Step 3: Acidification of Sodium Phenoxide

The sodium phenoxide formed in the previous step is then acidified, commonly using dilute hydrochloric acid.

C₆H₅ONa + HCl → C₆H₅OH + NaCl

The final product is phenol, also historically known as carbolic acid.

Therefore:

Phenol is prepared from benzene sulphonic acid by alkaline fusion of sodium benzene sulphonate with molten NaOH followed by acidification of sodium phenoxide.

Complete Conversion of Benzene Sulphonic Acid to Phenol

The full sequence can be written as:

C₆H₅SO₃H → C₆H₅SO₃Na → C₆H₅O₃Na → C₆H₅OH

Or in words:

Benzene sulphonic acid
↓ NaOH
Sodium benzene sulphonate
↓ molten NaOH, about 573 K
Sodium phenoxide
↓ dilute acid
Phenol

This is the clearest reaction pathway to remember when studying the preparation of phenol from benzene sulphonic acid.

Reaction Conditions at a Glance

The high-temperature alkaline-fusion step is essential to the conversion.

Why Does Benzene Sulphonic Acid Form Phenol with Molten NaOH?

The sulphonic acid group on the aromatic ring can ultimately be replaced by a hydroxyl group under strongly alkaline, high-temperature conditions.

When sodium benzene sulphonate is heated with molten sodium hydroxide, the reaction produces sodium phenoxide.

Sodium phenoxide is then protonated during acidification to form phenol.

The transformation is therefore a useful example of how one functional group attached to an aromatic ring can be converted into another through appropriate reaction conditions.

This is why the route is frequently taught in organic chemistry and in the Alcohols, Phenols and Ethers section of senior-secondary chemistry curricula.

Exam-Ready Answer: How Is Phenol Prepared from Benzene Sulphonic Acid?

For a short examination answer:

Benzene sulphonic acid is converted into sodium benzene sulphonate, which is fused with molten NaOH at about 573 K to form sodium phenoxide. On acidification with dilute HCl, sodium phenoxide gives phenol.

Reaction:

C₆H₅SO₃Na + 2NaOH (573 K)→ C₆H₅ONa + Na₂SO₃ + H₂O

followed by:

C₆H₅ONa + HCl → C₆H₅OH + NaCl

That is the answer I would place high on the page because the competing SERP is strongly exam-driven. Current ranking pages also emphasize the reagents, 573 K condition and acidification step.

How to Prepare Phenol from Benzene

Some users search more broadly for how to convert benzene into phenol.

If the starting material is benzene rather than benzene sulphonic acid, benzene must first be converted into benzene sulphonic acid through sulphonation.

The overall pathway becomes:

Benzene → Benzene sulphonic acid → Sodium benzene sulphonate → Sodium phenoxide → Phenol

This distinction is important because the standard benzene sulphonic acid route does not convert benzene directly into phenol in one step.

Is the Benzene Sulphonic Acid Route Used for Industrial Phenol Production?

The conversion of benzene sulphonic acid to phenol is an important historical and educational route in organic chemistry.

However, it should not be confused with the dominant modern large-scale method for producing phenol.

Modern industrial phenol production is commonly associated with the cumene process, where cumene is oxidized to cumene hydroperoxide and subsequently cleaved to produce phenol and acetone. Maharashtra Board chemistry material also identifies the cumene process as the commercial method of phenol preparation.

The benzene sulphonic acid route remains important because it demonstrates:

• • aromatic functional-group conversion
• • alkaline fusion chemistry
• • formation of sodium phenoxide
• • conversion of an aromatic sulphonate into phenol

This distinction should be made clearly so the article is chemically accurate rather than implying that this is the principal modern industrial process.

Benzene Sulphonic Acid Route vs Other Methods of Preparing Phenol

Phenol can be prepared through several different reactions.

The current article should not explain all of these in depth.

Their purpose here is only to show that the benzene sulphonic acid route is one of several established methods.

Conclusion

The preparation of phenol from benzene sulphonic acid involves converting benzene sulphonic acid to sodium benzene sulphonate, heating it with molten sodium hydroxide at approximately 573 K to form sodium phenoxide, and finally acidifying sodium phenoxide to obtain phenol.

The essential pathway is:

Benzene sulphonic acid → Sodium benzene sulphonate → Sodium phenoxide → Phenol

This reaction is an important example of aromatic functional-group transformation and remains widely taught in organic chemistry, even though modern bulk phenol production commonly uses other industrial processes.