Benzenesulfonic acid

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Benzenesulfonic acid[1]
Skeletal formula of benzenesulfonic acid
Ball-and-stick model of the benzenesulfonic acid molecule
Preferred IUPAC name
Benzenesulfonic acid
Other names
Benzene sulphonic acid; Benzenesulphonic acid; Phenylsulfonic acid; Phenylsulphonic acid; Besylic acid[2]
3D model (JSmol)
ECHA InfoCard 100.002.399
EC Number 202-638-7
RTECS number DB4200000
UN number 2583, 2585, 1803
Molar mass 158.17 g·mol−1
Appearance Colorless crystalline solid
Density 1.32 g/cm3 (47 °C)
Melting point * 44 °C (hydrate)
  • 51 °C (anhydrous)
Boiling point 190 °C (374 °F; 463 K)
Solubility in other solvents Soluble in alcohol, insoluble in non-polar solvents
Acidity (pKa) −2.8[3]
Main hazards Corrosive
Safety data sheet External MSDS
GHS pictograms The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The exclamation-mark pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word Danger
H290, H302, H314, H315, H319, H335
P234, P260, P261, P264, P270, P271, P280, P301+312, P301+330+331, P302+352, P303+361+353, P304+340, P305+351+338, P310
Flash point > 113 °C
Related compounds
Related sulfonic acids
Sulfanilic acid
p-Toluenesulfonic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Benzenesulfonic acid (conjugate base benzenesulfonate) is an organosulfur compound with the formula C6H5SO3H. It is the simplest aromatic sulfonic acid. It forms white deliquescent sheet crystals or a white waxy solid that is soluble in water and ethanol, slightly soluble in benzene and insoluble in nonpolar solvents like diethyl ether. It is often stored in the form of alkali metal salts. Its aqueous solution is strongly acidic.


Benzenesulfonic acid is prepared from the sulfonation of benzene using concentrated sulfuric acid:

Aromatic sulfonation of benzene

This conversion illustrates aromatic sulfonation, which has been called "one of the most important reactions in industrial organic chemistry."[4]


Benzenesulfonic acid exhibits the reactions typical of other aromatic sulfonic acids, forming sulfonamides, sulfonyl chloride, and esters. The sulfonation is reversed above 220 °C. Dehydration with phosphorus pentoxide gives benzenesulfonic acid anhydride ((C6H5SO2)2O). Conversion to the corresponding benzenesulfonyl chloride (C6H5SO2Cl) is effected with phosphorus pentachloride.

It is a strong acid, being almost fully dissociated in water.

Benzenesulfonic acid and related compounds undergo desulfonation when heated in water near 200 °C. The temperature of desulfonation correlates with the ease of the sulfonation:[4]

C6H5SO3H + H2O → C6H6 + H2SO4H

The alkali metal salt of benzenesulfonic acid was once used in the industrial production of phenol. The process, sometimes called alkaline fusion, initially affords the phenoxide salt:

C6H5SO3Na + 2 NaOH → C6H5ONa + Na2SO3
C6H5ONa + HCl → C6H5OH + NaCl

The process has been largely displaced by the Hock process, which generates less waste.


Benzenesulfonic acid is mainly consumed by conversion to other specialty chemicals.

A variety of pharmaceutical drugs are prepared as benzenesulfonate salts and are known as besilates (INN) or besylates (USAN).

In a diluted form, it is also used as a polymer remover stripping agent.


  1. ^ Benzenesulfonic acid[dead link], Sigma-Aldrich
  2. ^ Besylic acid, ChemIndustry
  3. ^ Guthrie, J. P. Hydrolysis of esters of oxy acids: pKa values for strong acids Can. J. Chem. 1978, 56, 2342-2354. doi:10.1139/v78-385
  4. ^ a b Otto Lindner, Lars Rodefeld "Benzenesulfonic Acids and Their Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a03_507