Quick Search
p-Toluenesulfonic acid (commonly abbreviated PTSA, p-TsOH or TsOH) is a widely used, strong organic sulfonic acid.
In practice the commercially important form is the monohydrate (p-TsOH·H₂O).
The monohydrate combines the practical advantages of a solid, highly acidic catalyst with convenient handling compared to liquid mineral acids.
CAS numbers: 104-15-4 (parent/anhydrous p-toluenesulfonic acid), 6192-52-5 (monohydrate as a substance).
Molecular formula (monohydrate): C₇H₈O₃S · H₂O; molar mass ≈ 190.22 g·mol⁻¹ (monohydrate).
Nomenclature, synonyms and registry numbers
IUPAC / systematic name
4-Methylbenzene-1-sulfonic acid monohydrate (for the hydrated solid).
SYNONYMS
p-Toluenesulfonic acid monohydrate; para-toluenesulfonic acid monohydrate; p-TsOH·H₂O; pTsOH monohydrate; PTSA monohydrate; Tosic acid (common name, less used); TsOH monohydrate; 4-methylbenzenesulfonic acid hydrate.
This article compiles nomenclature and identifiers, physical and chemical properties, preparation routes, analytical and spectroscopic characteristics, synthetic uses (as acid catalyst, dehydrating agent, tosylation chemistry precursor), industrial applications, handling and safety, environmental fate, and representative literature and supplier data.
Key quantitative data — CAS numbers, molecular weights, melting point, solubility and acid strength — are provided with references.
Key identifiers (load-bearing):
Preferred name: p-Toluenesulfonic acid monohydrate (p-TsOH·H₂O).
Registry numbers
CAS (anhydrous): 104-15-4.
CAS (monohydrate): 6192-52-5.
Molecular and structural data
Formulae & weights
Anhydrous p-TsOH: C₇H₈O₃S (M ≈ 172.20 g·mol⁻¹).
Monohydrate: C₇H₈O₃S·H₂O (M ≈ 190.22 g·mol⁻¹).
Aromatic benzene ring with a para-methyl substituent and a sulfonic acid substituent (-SO₃H) at the 1-position; sulfonic group is S(VI) tetrahedral in geometry with three O and one C attachments (one of the O atoms bears the acidic hydrogen in the –SO₃H unit).
The water in the monohydrate is hydrogen-bonded in the crystal lattice and affects melting behavior and hygroscopicity.
Crystallography
Crystalline solid as monohydrate under typical ambient storage; crystals are hygroscopic.
X-ray analyses indicate strong hydrogen bonding networks between sulfonic oxygens and the crystallization water; multiple polymorphs/solvates have been reported in literature for sulfonic acids in general (see crystallographic literature for precise unit cell parameters). (Representative supplier datasheets provide practical crystallographic descriptors.)
Physical properties (practical, measured)
Values below are typical ranges reported by suppliers and databases; small differences arise with purity and hydration state.
Appearance: white to off-white crystalline solid (monohydrate).
Melting point: monohydrate typically ~96–105 °C (literature/supplier values ~96–105 °C); anhydrous form melts much lower (~38 °C for the anhydrous material which sublimes/decomposes).
Density: ≈1.24 g·cm⁻³ (solid).
Solubility: highly soluble in water and polar organic solvents (e.g., alcohols, acetone, acetonitrile); reported water solubility large — tens of g per 100 mL (e.g., ~67 g/100 mL reported in some references for the anhydrous form in water at ambient temperature).
pKₐ / acidity: a strong organic acid — in water the pKₐ is very low (reported values for benzenesulfonic acids are typically ≤ −2 to −3 in aqueous media; pTSA is effectively a strong acid comparable to mineral acids in many organic reaction media). In non-aqueous solvents acidities differ (e.g., pK_a in acetonitrile ≈ 8.5 on the acetonitrile scale using specific conventions).
Stability: stable solid under normal conditions; hygroscopic and deliquescent under humid conditions if impure; decomposes at elevated temperatures leaving sulfonated residues; avoid strong oxidants under heating.
Synthetic routes and production
Industrial manufacture (overview)
Industrial manufacture of p-toluenesulfonic acid derives from electrophilic sulfonation of toluene using sulfur trioxide (SO₃) or concentrated sulfuric acid/SO₃–fuming sulfuric acid systems to introduce the sulfonic acid group at the para position predominantly.
Reaction control (temperature, stoichiometry, catalyst/promoter, absorption of heat) and subsequent work-up (neutralization, concentration, crystallization) deliver the sulfonic acid which is then crystallized as the monohydrate for handling and commercial distribution.
Process variants exist to favor para isomer and to minimize polysulfonation and oxidation.
After sulfonation, purification steps include aqueous work-up, removal of residual SO₂/SO₃, neutralization of free mineral acid, concentration, and crystallization.
Laboratory preparations
Direct sulfonation of toluene: classical route using fuming sulfuric acid or SO₃ gas to produce p- and o-toluenesulfonic acids; para-isomer predominates at controlled temperatures; subsequent separation/purification by recrystallization from solvent yields monohydrate.
Alternative synthetic methods: sulfonation of activated aromatic substrates under milder conditions using chlorosulfonic acid followed by hydrolysis; these are less common for large-scale manufacture due to handling and byproduct profiles.
Purification & conversion to monohydrate
After reaction and separation of inorganic acids, p-TsOH is typically isolated by concentration and crystallization (water presence leads to monohydrate formation).
Suppliers subsequently dry to specified residual moisture content to supply either the monohydrate or anhydrous grades depending on required application.
Analytical methods and characterization
Chromatography & purity assays
HPLC / ion chromatography: quantitative assays for p-TsOH in mixtures; UV detection at aromatic absorbance wavelengths, or conductivity detection in ion chromatography, are commonly used for assay and impurity profiling.
GC: not typically used for the intact sulfonic acid (non-volatile) but used for toluene or volatile impurities.
Titration: acid-base titration (aqueous) for total acidity; coulometric titration for high accuracy.
Spectroscopy & structural characterization
¹H NMR (DMSO-d₆ / D₂O): aromatic signals consistent with para-substituted benzene (two sets of aromatic protons), methyl singlet around δ ~2.3 ppm; the acidic proton often broad and exchangeable in protic solvents.
¹³C NMR: resonances for aromatic carbons, methyl carbon ~δ 21 ppm; characteristic sulfonated aromatic carbon shifts.
IR (FTIR): strong S=O stretching bands (asymmetric and symmetric sulfonyl stretches) typically near 1150–1350 cm⁻¹, strong O–H (sulfonic acid and water) stretching broad band in 3200–3600 cm⁻¹ region, aromatic C–H stretches, and methyl bending modes.
Mass spectrometry (ESI/HRMS): molecular ion of the deprotonated sulfonate anion often observed in negative mode; fragmentation patterns reflect loss of SO₃/H₂O under strong ionization.
Elemental analysis: supports composition and hydration; accurate weighing and moisture analysis (Karl Fischer) used to determine water content (monohydrate stoichiometry).
Melting point / DSC: DSC often shows endotherm corresponding to melting/hydrate transitions (monohydrate behavior).
X-ray crystallography: used to confirm lattice water and hydrogen bonding; available in literature for solid forms.
SAFETY INFORMATION ABOUT P-TOULENE SULFONIC ACID MONOHYDRATE
First aid measures:
Description of first aid measures:
General advice:
Consult a physician.
Show this safety data sheet to the doctor in attendance.
Move out of dangerous area:
If inhaled:
If breathed in, move person into fresh air.
If not breathing, give artificial respiration.
Consult a physician.
In case of skin contact:
Take off contaminated clothing and shoes immediately.
Wash off with soap and plenty of water.
Consult a physician.
In case of eye contact:
Rinse thoroughly with plenty of water for at least 15 minutes and consult a physician.
Continue rinsing eyes during transport to hospital.
If swallowed:
Do NOT induce vomiting.
Never give anything by mouth to an unconscious person.
Rinse mouth with water.
Consult a physician.
Firefighting measures:
Extinguishing media:
Suitable extinguishing media:
Use water spray, alcohol-resistant foam, dry chemical or carbon dioxide.
Special hazards arising from the substance or mixture
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas
Advice for firefighters:
Wear self-contained breathing apparatus for firefighting if necessary.
Accidental release measures:
Personal precautions, protective equipment and emergency procedures
Use personal protective equipment.
Avoid breathing vapours, mist or gas.
Evacuate personnel to safe areas.
Environmental precautions:
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Methods and materials for containment and cleaning up:
Soak up with inert absorbent material and dispose of as hazardous waste.
Keep in suitable, closed containers for disposal.
Handling and storage:
Precautions for safe handling:
Avoid inhalation of vapour or mist.
Conditions for safe storage, including any incompatibilities:
Keep container tightly closed in a dry and well-ventilated place.
Containers which are opened must be carefully resealed and kept upright to prevent leakage.
Storage class (TRGS 510): 8A: Combustible, corrosive hazardous materials
Exposure controls/personal protection:
Control parameters:
Components with workplace control parameters
Contains no substances with occupational exposure limit values.
Exposure controls:
Appropriate engineering controls:
Handle in accordance with good industrial hygiene and safety practice.
Wash hands before breaks and at the end of workday.
Personal protective equipment:
Eye/face protection:
Tightly fitting safety goggles.
Faceshield (8-inch minimum).
Use equipment for eye protection tested and approved under appropriate government standards such as NIOSH (US) or EN 166(EU).
Skin protection:
Handle with gloves.
Gloves must be inspected prior to use.
Use proper glove
removal technique (without touching glove's outer surface) to avoid skin contact with this product.
Dispose of contaminated gloves after use in accordance with applicable laws and good laboratory practices.
Wash and dry hands.
Full contact:
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
Splash contact
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
It should not be construed as offering an approval for any specific use scenario.
Body Protection:
Complete suit protecting against chemicals, The type of protective equipment must be selected according to the concentration and amount of the dangerous substance at the specific workplace.
Respiratory protection:
Where risk assessment shows air-purifying respirators are appropriate use a fullface respirator with multi-purpose combination (US) or type ABEK (EN 14387) respirator cartridges as a backup to engineering controls.
If the respirator is the sole means of protection, use a full-face supplied air respirator.
Use respirators and components tested and approved under appropriate government standards such as NIOSH (US) or CEN (EU).
Control of environmental exposure
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Stability and reactivity:
Chemical stability:
Stable under recommended storage conditions.
Incompatible materials:
Strong oxidizing agents:
Hazardous decomposition products:
Hazardous decomposition products formed under fire conditions.
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas.
Disposal considerations:
Waste treatment methods:
Product:
Offer surplus and non-recyclable solutions to a licensed disposal company.
Contact a licensed professional waste disposal service to dispose of this material.
Contaminated packaging:
Dispose of as unused product
