P-Toluenesulfonic acid

p-Toluenesulfonic acid[1]
IUPAC name 4-methylbenzenesulfonic acid
Other names Tosylic acid tosic acid PTSA
Identifiers
CAS number	104-15-4 Y,  6192-52-5 (monohydrate)
PubChem	6101
ChemSpider	5876 Y
DrugBank	DB03120
KEGG	C06677 Y
ChEBI	CHEBI:27849 N
ChEMBL	CHEMBL541253 Y
Jmol-3D images	Image 1
SMILES Cc1ccc(cc1)S(=O)(=O)O
InChI InChI=1S/C7H8O3S/c1-6-2-4-7(5-3-6)11(8,9)10/h2-5H,1H3,(H,8,9,10) Y Key: JOXIMZWYDAKGHI-UHFFFAOYSA-N Y InChI=1/C7H8O3S/c1-6-2-4-7(5-3-6)11(8,9)10/h2-5H,1H3,(H,8,9,10) Key: JOXIMZWYDAKGHI-UHFFFAOYAG
Properties
Molecular formula	CH3C6H4SO3H
Molar mass	172.20 g/mol (anhydrous) 190.22 g/mol (monohydrate)
Appearance	colorless (white) solid
Density	1.24 g/cm3
Melting point	38 °C, 311 K (anhydrous) [2] 103-106 °C, 376-379 K (monohydrate)
Boiling point	140 °C; 284 °F; 413 K (at 20 mmHg)
Solubility in water	67 g/100 mL
Acidity (pKa)	-2.8 (water),[3] 8.5 (acetonitrile)[4]
Structure
Molecular shape	tetrahedral at S
Hazards
MSDS	External MSDS
R-phrases	R36/37/38
S-phrases	S26
Main hazards	skin irritant
Related compounds
Related sulfonic acids	Benzenesulfonic acid Sulfanilic acid
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

p-Toluenesulfonic acid (PTSA or pTsOH) or tosylic acid (TsOH) is an organic compound with the formula CH₃C₆H₄SO₃H. It is a white solid that is soluble in water, alcohols, and other polar organic solvents. The 4-CH₃C₆H₄SO₂- group is known as tosyl group and is often abbreviated as Ts or Tos. Most often, TsOH refers to the monohydrate, TsOH^.H₂O.

TsOH is a strong organic acid, about a million times stronger than benzoic acid. It is one of the few strong acids that is solid and, hence, conveniently weighed. Also, unlike some strong mineral acids (especially nitric acid, sulfuric acid, and perchloric acid), TsOH is non-oxidizing.

Preparation and handling

TsOH is prepared on an industrial scale by the sulfonation of toluene. It hydrates readily. Common impurities include benzenesulfonic acid and sulfuric acid. Impurities can be removed by recrystallization from its concentrated aqueous solution followed by azeotropic drying with toluene.^[5]

Toluenesulfonic acid finds use in organic synthesis as an "organic-soluble" acid catalyst. Examples of uses:

• Acetalization of an aldehyde.^[6]
• Esterification of carboxylic acids.^[7]
• Transesterification of an ester.^[8]

Tosylate esters

Ball-and-stick model of the tosylate anion

Tosylate esters are used as alkylating agents because the tosyl group is electron-withdrawing, which makes the tosylate anion a good leaving group. The tosyl group is also a protecting group for alcohols and amines, prepared by combining the alcohol with 4-toluenesulfonyl chloride, usually in an aprotic solvent, often pyridine, the basicity of which activates the reaction.^[9] Toluenesulfonate esters undergo nucleophilic attack or elimination. Reduction of tosylate esters gives the hydrocarbon. Thus, tosylation followed by reduction allows for the deoxygenation of alcohols.

Reactions

• p-Toluenesulfonic acid may be converted to p-toluenesulfonic anhydride by heating with phosphorus pentoxide.^[10]

• When TsOH is heated with acid and water, a hydrolysis reaction takes place and toluene is formed:

CH₃C₆H₄SO₃H + H₂O → C₆H₅CH₃ + H₂SO₄

This reaction is general for aryl sulfonic acids, but the rate at which it occurs depends upon the structure of the acid, the temperature and the nature of the catalyzing acid. For example p-TsOH is unaffected by cold concentrated hydrochloric acid, but hydrolyzes when heated to 186°C in concentrated phosphoric acid.^[11][12]

See also

• Tosyl
• Collidine p-toluenesulfonate

References