Benzenesulfonic acid^[1]

Names
Preferred IUPAC name Benzenesulfonic acid[3]
Other names Benzene sulphonic acid; Benzenesulphonic acid; Phenylsulfonic acid; Phenylsulphonic acid; Besylic acid[2]
Identifiers
CAS Number	98-11-3 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:64455
ChEMBL	ChEMBL1422641
ChemSpider	7093 Y
ECHA InfoCard	100.002.399
EC Number	202-638-7
PubChem CID	7371
RTECS number	DB4200000
UNII	685928Z18A Y
UN number	2583, 2585, 1803
CompTox Dashboard (EPA)	DTXSID9024568
InChI InChI=1S/C6H6O3S/c7-10(8,9)6-4-2-1-3-5-6/h1-5H,(H,7,8,9) Y Key: SRSXLGNVWSONIS-UHFFFAOYSA-N Y InChI=1/C6H6O3S/c7-10(8,9)6-4-2-1-3-5-6/h1-5H,(H,7,8,9) Key: SRSXLGNVWSONIS-UHFFFAOYAJ
SMILES OS(=O)(=O)c1ccccc1
Properties
Chemical formula	C6H6O3S
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 water	Soluble
Solubility in other solvents	Soluble in alcohol, insoluble in non-polar solvents
Acidity (pKa)	−2.8[4]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Corrosive
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H290, H302, H314, H315, H319, H335
Precautionary statements	P234, P260, P261, P264, P270, P271, P280, P301+P312, P301+P330+P331, P302+P352, P303+P361+P353, P304+P340, P305+P351+P338, P310, P312, P321, P330, P332+P313, P337+P313, P362, P363, P390, P403+P233, P404, P405, P501
Flash point	> 113 °C
Safety data sheet (SDS)	External MSDS
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). Y verify (what is YN ?) Infobox references

Benzenesulfonic acid (conjugate base benzenesulfonate) is an organosulfur compound with the formula C₆H₆O₃S. 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.

Preparation

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

This conversion illustrates aromatic sulfonation, which has been called "one of the most important reactions in industrial organic chemistry".^[5]

Reactions

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:^[5]

C₆H₅SO₃H + H₂O → C₆H₅OH + H₂SO₄

For that reason, sulfonic acids are usually used as a protecting group, or as a meta director in electrophilic aromatic substitution.

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.

Uses

Salts of benzenesulfonic acid such as sodium benzenesulfonate (Ludigol) and monoethanolamine benzenesulfonate are used as surfactants in laundry detergent.^[6]

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