Benzenesulfonic Acid: Production, Reactions and Uses

Benzenesulfonic acid structure

Benzenesulfonic acid is a chemical compound with the formula C6H5SO3H and a molecular weight of 158.17 g/mol. It was first synthesized in 1834 by E. Mitscherlich through the reaction of benzene and concentrated sulfuric acid, yielding benzenesulfonic acid and diphenyl sulfone.

A notable industrial application of benzenesulfonic acid is the production phenol by alkali fusion, a reaction pioneered by A. Wurtz and A. Kekulé in 1867 and later refined by P. O. Degener in 1878.

Historically, benzenesulfonic acid had an important role in phenol production. However, in recent times, alternative phenol synthesis methods have gained favor in industrial processes.

Table of Contents

1. Physical Properties of Benzenesulfonic Acid

Crystallization of benzenesulfonic acid from an aqueous solution forms a hydrate containing 1.5 water molecules, resulting in deliquescent needles with a melting point of 43-44 °C. The monohydrate has a melting point of 45-46 °C.

The anhydrous acid is distillable without decomposition at 171-172 °C (0.13 mbar). It demonstrates high solubility in water and ethanol, moderate solubility in benzene, and insolubility in diethyl ether and carbon disulfide.

Sodium benzenesulfonate decomposes at approximately 450 degrees Celsius. It is soluble in 1.75 parts of water at 30 °C and 0.8 parts of boiling water. Crystallization from aqueous solutions yields the salt with one mole of water of crystallization. The calcium and barium salts are also water-soluble.

Physical Properties of Benzenesulfonic Acid
Property Value
Appearance White deliquescent crystals or waxy solid
Molecular weight 158.17 g/mol
Density 1.48 g/cm³
Melting point (hydrate) 43-44 °C
Melting point (monohydrate) 45-46 °C
Melting point (anhydrous) 65-66 °C
Boiling point 274 °C (decomposes)
Distillation point (anhydrous) 171-172 °C (0.13 mbar)
Flash point 113 °C
Solubility in water High
Solubility in alcohol High
Solubility in benzene Moderate
Solubility in diethyl ether Insoluble
Solubility in carbon disulfide Insoluble
Dielectric constant (aqueous solution) 0.2
pKa −2.8
Odor Pungent
Decomposition temperature (sodium benzenesulfonate) 450 °C
Solubility of sodium benzenesulfonate in water 1.75 parts at 30 °C
0.8 parts at boiling point
Solubility of calcium and barium salts in water Water-soluble
Hazards Corrosive to eyes, skin, and respiratory tract

2. Chemical Reactions of Benzenesulfonic Acid

Benzenesulfonic acid exhibits typical reactions of a strong aromatic sulfonic acid. Acid hydrolysis at 175 °C yields benzene and sulfuric acid.

hydrolysis of Benzenesulfonic acid

Further sulfonation with fuming sulfuric acid produces 1,3-benzenedisulfonic acid, which can be converted to 1,3,5-benzenetrisulfonic acid and diphenyl sulfone disulfonic acid.

sulfonation of Benzenesulfonic acid

Benzenesulfonic acid undergoes a Friedel-Crafts-type reaction with benzene to form diphenyl sulfone.

Friedel-Crafts reaction of Benzenesulfonic acid with benzene

Benzenesulfonic acid reacts with alkali sodium hydroxide at 320-350 °C to produce sodium phenolate according to the following equation:

C6H5SO3Na + 2 NaOH → C6H5ONa + Na2SO3 + H2O

This reaction formed the basis of the first industrial production of phenol.

Alkali fusion of Benzenesulfonic acid to produce phenol

Treating benzenesulfonic acid with phosphorus halogenides (PCl5, PBr5), chlorosulfuric acid, thionyl chloride, or phosgene yields sulfonyl halides:

C6H5SO2OH + PCl5 → C6H5SO2Cl + POCl3 + HCl

When an excess of PCl5 is used, chlorobenzene is formed according to:

C6H5SO2Cl + PCl5 → C6H5Cl + SOCl2 + POCl3

reaction of benzenesulfonic acid with phosphorus halogenides
Benzenesulfonic anhydride is synthesized by reacting benzenesulfonic acid with P2O5, SOCl2, or SO3. It can also be formed as side product during the preparation of benzenesulfonic acid or benzenesulfonyl chloride. However, in these cases, they are often not observed due to their rapid hydrolysis in aqueous acid.
production of Benzenesulfonic anhydride

Benzenesulfonic esters are produced by the reaction of sulfonic acid or sulfonyl chlorides with alcohols or ethers.

3. Production of Benzenesulfonic Acid

Benzenesulfonic acid is produced by an exothermic reaction between benzene and sulfuric acid, as described by the equation:

C6H6 + H2SO4 → C6H5SO3H + H2O

Production of Benzenesulfonic Acid

CROOKS and WHITE investigated this reaction, examining the impact of temperature and sulfonation mixture composition on reaction rate. It is noted that the reaction ceases at a specific sulfuric acid concentration, typically between 74 and 78%, depending on temperature and water generation.

In industrial settings, a portion of sulfuric acid (usually 45% in the classical process) remains unconverted and serves as a solvent and diluent. This limits byproduct formation, particularly diphenyl sulfone, to a low yield. Excess acid can be removed industrially by adding calcium carbonate.

Various methods have been employed to reduce excess sulfuric acid and enhance sulfone formation:

  1. Employing oleum or sulfur trioxide instead of sulfuric acid.
  2. Extracting benzenesulfonic acid from the reaction mixture with benzene.
  3. Removing reaction water azeotropically with benzene.

a. The classical process, involved the sulfonation of benzene to produce benzenesulfonic acid which was used in the synthesis of phenol then the byproduct sulfite formed is used to neutralize benzenesulfonic acid. This process is only of historical interest.

b. A more modern approach is Continuous Sulfonation with Oleum, also known as the Monsanto process. This involves pumping benzene and oleum into a cascade of six sulfonation vessels. Connected by overflow pipes, these vessels allow the reaction mixture to flow from one to the next.

The first two vessels are cooled, while the others are heated. Neutralizing the reaction mixture with sodium sulfite or sodium hydroxide solution removes excess sulfuric acid. Sodium sulfate precipitates and can be separated in centrifuges. The resulting sodium benzenesulfonate solution can be concentrated and converted directly to phenol or dried.

This process uses oleum containing 35.6% sulfur trioxide and maintains specific temperature conditions. Only about 1% of the benzene is converted to diphenyl sulfone in this process.

c. Another method is the Continuous Extraction Process, which involves introducing excess benzene and sulfur trioxide into a vessel under vigorous stirring. A benzene layer saturated with benzenesulfonic acid forms in the vessel’s upper part and is continuously washed with water or sodium hydroxide solution in a second agitator vessel.

The separated benzene can be returned to the reaction vessel. In this process, sulfuric acid consumption is 1260 kg for every 1000 kg of converted benzene, and diphenyl sulfone formation is below 2%.

d. The Azeotropic Removal of Reaction Water method involves heating sulfuric acid with a concentration of, for example, 79% to 170 °C in a sulfonation vessel and introducing finely divided benzene vapor.

This process sulfonates a portion of the benzene vapor, while the unconverted portion continuously removes water from the reaction mixture. The condensed mixture of benzene and water vapor can be separated, and the benzene is returned to the evaporator.

Continuous operation of this process yields a final product containing 80.2% benzenesulfonic acid and 14.3% sulfuric acid, while batch operation results in a final product containing 93.1% of sulfonic acid and 4.8% sulfuric acid. Adding sodium benzenesulfonate to the sulfonation mixture can keep sulfone formation below 2%.

4. Uses of Benzenesulfonic Acid

Benzenesulfonic acid is used for diverse applications in various industries as follow:

  1. Phenol production: Benzenesulfonic acid is used to produce phenol by fusion with sodium hydroxide or hydrolysis of its salts, typically the sodium salt.

  2. Surfactant synthesis: Benzenesulfonic acid is a key component in the creation of surfactants, particularly when combined with metal or amine salts. Salts of benzenesulfonic acid, such as sodium benzenesulfonate (Ludigol) and monoethanolamine benzenesulfonate, are employed as surfactants in laundry detergent formulations.

  3. Pharmaceutical drug synthesis: Benzenesulfonic acid is involved in pharmaceutical drug synthesis, where these drugs are produced as benzenesulfonate salts. These salts are recognized by the International Nonproprietary Name as besilates or the United States Adopted Name as besylates.

  4. Acid catalyst: The acidic nature of benzenesulfonic acid makes it valuable as an acid catalyst in various chemical reactions.

  5. Dye standardization: The sodium salt of benzenesulfonic acid plays a role in standardizing dyes, ensuring the accuracy of coloration processes.

  6. Surfactant-enhanced oil recovery (SEOR): Benzenesulfonic acid is essential in SEOR, a technique that involves the use of surfactants to facilitate the extraction of oil from reservoirs, also known as surfactant flushing.

5. Toxicology of Benzenesulfonic Acid

Lethal Dose 50 (LD50) in Rats (Oral): 1170 milligrams per kilogram

Short-term Exposure Effects:

Benzenesulfonic acid is a corrosive substance that can cause severe damage to the eyes, skin, and respiratory tract. It is also corrosive if swallowed.

Symptoms of Exposure:

  • Inhalation: Cough, sore throat, shortness of breath, labored breathing, burning sensation, headache, nausea
  • Skin: Redness, burning sensation, pain, skin burns
  • Eyes: Redness, pain, severe deep burns
  • Ingestion: Sore throat, burning sensation in the throat and chest, abdominal pain, shock, or collapse


  1. Benzenesulfonic Acids and Their Derivatives; Ullmann’s Encyclopedia of Industrial Chemistry. –
  5. Benzenesulfonic Acid (Surfactant)