p-Toluenesulfonic acid

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p-Toluenesulfonic acid

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Template:Longitem	C7H8O3S
Molar mass	172.20 g/mol (anhydrous) 190.22 g/mol (monohydrate)
Appearance	colorless (white) solid
Density	1.24 g/cm3
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Acidity (pKa)	−2.8 (water) reference for benzenesulfonic acid, 8.5 (acetonitrile)
Template:Longitem	tetrahedral at S
Template:Longitem	Benzenesulfonic acid Sulfanilic acid

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

As with other aryl sulfonic acids, TsOH is a strong organic acid. It is about one million times stronger than benzoic acid. It is one of the few strong acids that is solid and therefore is conveniently weighed and stored.

Preparation and uses

TsOH is prepared on an industrial scale by the sulfonation of toluene. Common impurities include benzenesulfonic acid and sulfuric acid. TsOH is most often supplied as the monohydrate, and it may be necessary to remove the complexed water before use. Impurities can be removed by recrystallization from its concentrated aqueous solution followed by azeotropic drying with toluene.

TsOH finds use in organic synthesis as an "organic-soluble" strong acid. Examples of uses include:

• Acetalization of an aldehyde.
• Fischer–Speier esterification
• Transesterification reactions

Tosylates

Alkyl tosylates are alkylating agents because tosylate is electron-withdrawing as well as a good leaving group. Tosylate is a pseudohalide. 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.

File:Non-classical 7-norbornenyl Structure.png

Structures of the 7-norbornenyl cation with p-orbital stabilization.

In a famous and illustrative use of tosylate as a leaving group, the 2-norbornyl cation was formed by an elimination reaction of 7-norbornenyl tosylate. The elimination occurs 10¹¹ times faster than the solvolysis of anti-7-norbornyl Para-toluenesulfonate.

Tosylates are also protecting groups for alcohols. They are prepared by combining the alcohol with 4-toluenesulfonyl chloride in the presence of a base. These reactions are usually performed in an aprotic solvent, often pyridine, which additionally acts as a base.

Reactions

• TsOH may be converted to Para-toluenesulfonic anhydride by heating with phosphorus pentoxide.
• When heated with acid and water, TsOH undergoes hydrolysis to toluene:

CH₃C₆H₄SO₃H + H₂O → C₆H₅CH₃ + H₂SO₄

This reaction is general for aryl sulfonic acids.

See also

• Tosyl
• Collidinium p-toluenesulfonate