Illustrated Glossary of Organic Chemistry

Toluenesulfonic acid (p-toluenesulfonic acid; TsOH; p-TsOH): An aromatic sulfonic acid, often used as a strong acid catalyst. Most commonly refers to para-methylbenzenesulfonic acid monohydrate (CH₃C₆H₄SO₃H^.H₂O; pK_a -2), but the anhydrous form may also be used.

Molecular structure of toluenesulfonic acid.

cat. TsOH CH3OH

TsOH catalysis in the Fischer esterification reaction.