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What is Benzenesulfonic Acid used for? Preparation, Properties, and uses

Benzenesulfonic acid Preparation

Benzenesulfonic acid is obtained from sulphonation of benzene. For this, benzene is heated with concentrated Sulfuric acid.

C6H6 + H2SO4(concentrated) C6H5SO3H + H2O

Laboratory Method

To make Benzenesulfonic acid in the laboratory, take a mixture of one part banzene and two parts concentrated sulfuric acid in a round bottom flask. Attaches a reflux condenser to the flask. Heat the flask at 80 -100°C for about two hours by placing it on a water heater.

After this, the mixture is cooled and put in water. It contains barium carbonate. Unused sulfuric acid precipitates as barium sulfate. Filter it and separate it. Berium salt of Benzenesulfonic acid is filtered due to dissolution in water.

On cooling after filtration, the cristal of Benzenesulfonic acid’s berium salt is obtained, separating these crystals and adding appropriate amounts of sulphuric acid. Berium sulfate precipitates and Benzenesulfonic acid is released.

The precipitates of berium sulfate are filtered and separated. Water containing crystals of Benzenesulfonic acid are obtained after cooling the filtrate. Anhydrous Benzenesulphonic(C6H5SO3H.3/2H2O) Acid is obtained after being isolated and heated.

Physical Properties

It is a colorless crystal solid. Its melting point is 65°C. Its water-containing crystals have a melting point of 47°C. It is more soluble in water.

Benzenesulphonic Acid

Chemical Properties

Its chemical reactions can be divided into three parts.

  • Reactions in which the -OH group of the -SO3H group participates.
  • Reactions in which the –SO3H group is displaced.
  • Replacement reactions of nuclei

Reactions in which the -OH group of the -SO3H group participates.

Acidic symptoms: It is an acid. Its aqueous solution is strongly acidic. The acid strength of its aqueous solution is as high as sulphuric acid. It reacts with bases to form salts.


C6H5SO3H + NaOH C6H5SO3Na + H2O

Reaction with Phosphorus pentachloride: In the reaction of Benzenesulfonic acid and phosphorus pentachloride, the -OH group of -SO3H group is displaced by the chlorine atom.

C6H5SO3H + PCl5 C6H5SO2Cl + POCl3 + HCl

Reaction with alcohols: Like other acids it reacts with alcohols to form ester.

C6H5SO3H + C2H5OH C6H5 – SO2 –O – C2H5 + H2O

Reactions in which the –SO3H group is displaced.

Displacement by hydrogen atom: On decomposition of benzene sulphonic acid with superheated steam or heating with dilute HCl or dilute H2SO4, its water decomposition occurs and benzene is obtained.

C6H5SO3H + H2O C6H6 + H2SO4

Displacement by Hydroxyl Group: Phenol is obtained by melting its sodium or potassium salts with solid NaOH or KOH.

C6H5SO3Na + NaOH C6H5OH + Na2SO3

Displacement by amines group: Aniline is obtained by melting its sodium or potassium salts with NaNH2 or KNH2.

C6H5SO3Na + NaNH2 C6H5NH2 + Na2SO3

Displacement by cyano group: Cyano benzene is obtained by melting its sodium or potassium salts with NaCN.

C6H5SO3Na + NaCN C6H5CN + Na2SO3

Displacement by carboxyle group: Its sodium or potassium salts are smelted with sodium phomate to obtain benzoic acid.


Displacement by -SN group: thaophenol is obtained by melting its sodium or potassium salts with KSH.


Replacement reactions of nuclei

The sulphonic acid group is a meta director group. Hence meta products are obtained from its halogenationnitration and sulphonation. Like all other meta director groups, it is also a deactivating group. This implies that its substitution reactions occur at a lower intensity than benzene. Following are its major nuclear substitution reactions.

Halogenation: When replicated with chlorine in the presence of halogen carrier, it forms meta chlorobenzene sulphonic acid.


Nitration: When heated with a mixture of concentrated NHO3 and concentrated H2SO4, it forms m nitrobenzene sulphonic acid.


sulphonation: when heated at 200°C with gentle sulphuric acid, it forms meta benzene di sulphonic acid.


Benzenesulfonic acid Uses

  • In the manufacture of dyes
  • Sulpha drugs are prominent in the manufacture of drugs.
  • In manufacture of saccharine
  • In the manufacture of detergents


Sanjay Bhandari

Hello Friends, My name is Sanjay Bhandari. I am a chemistry Teacher.

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