p-Toluenesulfonic acid monohydrate
[CAS 6192-52-5]

Identification

• Classification: Chemical reagent >> Organic reagent >> Sulfonate / sulfinate
• Name: p-Toluenesulfonic acid monohydrate
• Synonyms: Toluene-4-sulfonic acid monohydrate; 4-Methylbenzenesulfonic acid monohydrate; PTSA
• Molecular Formula: C₇H₈O₃S.H₂O
• Molecular Weight: 190.21
• CAS Registry Number: 6192-52-5
• EC Number: 612-715-0
• SMILES: CC1=CC=C(C=C1)S(=O)(=O)O.O

Properties

• Density: 1.24 g/mL (Expl.)
• Melting point: 103 - 106 °C (Expl.)
• Boiling point: 280.2 °C (Expl.)
• Flash point: 184 °C (Expl.)
• Solubility: water: soluble (Expl.)

Safety Data

• Hazard Symbols: GHS05;GHS07 Danger
• Risk Statements: H314-H315-H318-H319-H335
• Safety Statements: P260-P261-P264-P264+P265-P271-P280-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P351+P338-P305+P354+P338-P316-P317-P319-P321-P332+P317-P337+P317-P362+P364-P363-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Specific target organ toxicity - single exposure	STOT SE	3	H335
Skin irritation	Skin Irrit.	2	H315
Serious eye damage	Eye Dam.	1	H318
Eye irritation	Eye Irrit.	2	H319
Skin corrosion	Skin Corr.	1B	H314
Skin corrosion	Skin Corr.	1C	H314
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Acute toxicity	Acute Tox.	4	H302
Skin corrosion	Skin Corr.	1	H314
Chronic hazardous to the aquatic environment	Aquatic Chronic	3	H412
Specific target organ toxicity - single exposure	STOT SE	3	H336
Eye irritation	Eye Irrit.	2A	H319
Acute toxicity	Acute Tox.	4	H312
Specific target organ toxicity - repeated exposure	STOT RE	2	H373
Specific target organ toxicity - single exposure	STOT SE	2	H371

• Transport Information: UN 1759; UN 2585

chemBlink Chemical Story

p-Toluenesulfonic acid monohydrate, CAS 6192-52-5, commonly abbreviated p-TsOH.H2O or PTSA monohydrate, is the crystalline monohydrate of p-toluenesulfonic acid. PubChem records the composition C7H8O3S.H2O and molecular weight 190.22. It combines the strong acidity of an arylsulfonic acid with the practical convenience of a crystalline organic reagent.

p-Toluenesulfonic acid is widely used as a strong acid catalyst in organic synthesis. Unlike mineral acids such as sulfuric acid, its aromatic sulfonic-acid structure makes it soluble in many organic reaction media and easier to weigh and handle as a solid. The monohydrate is especially common commercially because it forms stable crystals containing one equivalent of water.

Acid-catalyzed esterification, acetal formation and hydrolysis are among the familiar transformations in which p-TsOH is used. It can protonate carbonyl or oxygen-containing functional groups, increasing electrophilicity or improving leaving-group ability without introducing a strongly nucleophilic counterion. It is also used in polymer and resin chemistry where acid catalysis is needed.

The water of hydration deserves attention. CAS 6192-52-5 is specifically the monohydrate, not anhydrous p-toluenesulfonic acid. In reactions that are sensitive to water, this distinction can matter to stoichiometry or equilibrium. Conversely, for many routine acid-catalyzed processes, the stable crystalline monohydrate is preferred precisely because it is convenient to store and dispense.

The compound illustrates how reagent form can be almost as important as reagent identity. The sulfonic-acid group supplies strong Brønsted acidity, the tolyl group gives an organic-compatible framework, and one defined molecule of water produces a readily handled crystalline form. Those features made p-TsOH monohydrate one of the laboratory's standard organic acid catalysts.

References:
1. PubChem, p-Toluenesulfonic acid monohydrate, CID 521998, CAS 6192-52-5.
2. Smith MB, March J. March's Advanced Organic Chemistry. 7th ed. Wiley, 2013.
3. TCI America, p-Toluenesulfonic Acid Monohydrate, CAS 6192-52-5.

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