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CAS number:
6192-52-5 (monohydrate)



p-Toluenesulfonic acid (PTSA) 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. 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 are solid and, hence, conveniently weighed. Also, unlike some of the 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 concentrated hydrochloric acid followed by azeotropic drying.
Toluenesulfonic acid finds use in organic synthesis as an "organic-soluble" acid catalyst. Examples of uses:

Acetalization of an aldehyde.
Esterification of carboxylic acids.
Transesterification of an ester.

Tosylate esters

Ball-and-stick model of the tosylate anion

A tosylate ester has a limited shelf life in air due to its ready hydrolysis in the presence of light. The tosyl group is electron-withdrawing which makes tosylates excellent leaving groups. 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. Toluenesulfonate esters undergo nucleophilic attack or elimination.


p-Toluenesulfonic acid may be converted to p-toluenesulfonic anhydride by heating with phosphorus pentoxide.

When toluene sulphonic acid (o-, m-, p-,) is boiled with HCl or treated with super-heated steam, toluene is formed along with H2SO4

CH3C6H4SO3H (Toluene Sulphonic acid) + HOH (steam) → C6H5CH3 (Toluene) + H2SO4 (Sulfuric acid)


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