p-Toluenesulfonic Acid (p-TsOH) (CAS 104-15-4) — Complete Guide to Properties and Applications

Category: Organic Acid Catalyst | Formula: C7H8O3S | MW: 172.20 g/mol

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What Is p-Toluenesulfonic Acid?

p-Toluenesulfonic acid (CAS 104-15-4), commonly abbreviated as p-TsOH or PTSA, is a strong organic acid catalyst with the molecular formula C7H8O3S and molecular weight 172.20 g/mol. ^[001] It is one of the most widely used homogeneous acid catalysts in organic synthesis and industrial chemistry, combining strong acidity (pKa ~-2.8) with excellent organic solubility. ^[004]

p-TsOH is typically used as the monohydrate (p-TsOH-H2O, CAS 6192-52-5), a white crystalline solid that is easier to handle and weigh than the hygroscopic anhydrous form. The monohydrate is commercially available in high purity and is the standard form for laboratory and industrial use. ^[002]

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Physical and Chemical Properties

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How It Works

Strong Brønsted Acid Catalysis

p-TsOH is a strong Brønsted acid with pKa ~-2.8, comparable to sulfuric acid and stronger than most mineral acids on a per-proton basis in organic solvents. ^[004] Unlike mineral acids (H2SO4, HCl), p-TsOH is soluble in organic solvents and can be easily removed by aqueous extraction, making it ideal for organic synthesis.

Carbocation Formation

In acetalization, esterification, and Friedel-Crafts alkylation, p-TsOH protonates carbonyl oxygen or alkene pi bonds, generating electrophilic carbocations that are attacked by nucleophiles. ^[004] For example, in acetal formation:

1. Protonation of aldehyde carbonyl by p-TsOH

2. Nucleophilic attack by alcohol on activated carbonyl

3. Proton transfer and loss of water (driven by Dean-Stark removal)

4. Second alcohol attack to form acetal

5. Deprotonation to release product and regenerate catalyst

Dean-Stark Water Removal

p-TsOH-catalyzed reactions that produce water (esterification, acetalization) are typically performed in refluxing toluene or benzene with a Dean-Stark trap. ^[004] The trap continuously removes water from the reaction, driving the equilibrium toward product formation according to Le Chatelier's principle. This is one of the most reliable methods for achieving high conversion in equilibrium-limited reactions.

Acid Scavenging and Salt Formation

p-TsOH forms stable, crystalline salts with basic organic molecules (amines, pyridines, amidines). ^[005] These salts often have improved solid-state properties (melting point, crystallinity, hygroscopicity) compared to the free base, making p-TsOH a valuable reagent for pharmaceutical salt screening and isolation.

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Key Applications

1. Fischer Esterification and Acetalization Catalyst: p-TsOH is the most widely used homogeneous acid catalyst for esterification and acetal/ketal formation. Strong acidity (pKa -2.8) combined with organic solubility. Removed by aqueous wash. Typical: 1-5 mol% in refluxing toluene with Dean-Stark water removal. ^[004]

2. Boc Deprotection in Peptide Synthesis: p-TsOH selectively cleaves Boc groups from amines in the presence of t-Bu esters and Trt. Milder than TFA, reducing racemization. Standard reagent in solution-phase peptide synthesis. ^[004]

3. Amino Resin Curing Catalyst: p-TsOH is the standard latent acid catalyst for melamine-formaldehyde and urea-formaldehyde resin curing in automotive coatings, can coatings, and wood finishes. Heat-activated (80-140 degC) enables one-component bake systems. ^[005]

4. Pharmaceutical Salt Formation: p-TsOH forms crystalline, non-hygroscopic salts with basic drug molecules, improving solubility, dissolution rate, and chemical stability in solid oral dosage forms. ^[005]

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Representative Protocol

Application: p-TsOH-Catalyzed Fischer Esterification with Dean-Stark Water Removal

Typical Scale: 10 mmol

Reagents:

- Carboxylic acid: 1.0 eq (10 mmol)

- Alcohol: 1.5 eq (15 mmol)

- p-TsOH monohydrate: 0.05 eq (0.5 mmol, 95 mg)

- Solvent: toluene (30 mL)

- Molecular sieves (4A): optional (2 g)

Procedure:

1. In a 100 mL round-bottom flask equipped with a magnetic stir bar, combine the carboxylic acid (10 mmol), alcohol (15 mmol), and p-TsOH monohydrate (95 mg, 0.5 mmol).

2. Add toluene (30 mL) and a few boiling chips.

3. Set up a Dean-Stark trap filled with toluene above the reaction flask. Attach a reflux condenser.

4. Heat to reflux (~110 degC) with stirring. Water will collect in the Dean-Stark trap.

5. Continue reflux until water collection ceases (typically 4-8 h; monitor by TLC or GC-MS).

6. Cool to rt.

7. Wash the reaction mixture with saturated NaHCO3 (2 x 15 mL) to remove p-TsOH, then with brine (1 x 15 mL).

8. Dry over Na2SO4, filter, concentrate under reduced pressure.

9. Purify by flash chromatography (SiO2, hexanes/EtOAc gradient) or distillation.

Expected Yield: 75-95%.

Troubleshooting:

- Low conversion: Ensure the Dean-Stark trap is functioning correctly (water layer should be visible). Increase p-TsOH to 0.1 eq. Use a large excess of alcohol (2-3 eq). Add molecular sieves to the reaction flask to scavenge residual water.

- Dark colored product: Overheating or extended reaction times can cause decomposition. Lower reflux temperature (use xylenes instead of toluene for higher-boiling substrates). Add activated carbon during workup to decolorize.

- p-TsOH difficult to remove: If residual acid persists after NaHCO3 wash, use sat. Na2CO3 (stronger base) or perform a second aqueous wash. For acid-sensitive products, neutralize with solid NaHCO3 before extraction.

- Alcohol is volatile (MeOH, EtOH): Use a large excess (5-10 eq) or employ a Soxhlet extractor with the alcohol in the thimble.

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Handling and Storage

- GHS Hazard: H314(42.3%): Causes severe skin burns and eye damage; H315(62.1%): Causes skin irritation; H318(37%): Causes serious eye damage; H319(62.1%): Causes serious eye irritation; H335(87%): May cause respiratory irritation ^[006]

- Signal Word: Danger

- Pictograms: GHS05 (corrosion), GHS07 (exclamation mark)

- Storage: Store at rt in a tightly closed container. The monohydrate form is preferred for storage stability. Protect from moisture (anhydrous form is hygroscopic). Keep away from strong oxidizing agents and strong bases. ^[002]

- PPE: Nitrile gloves, face shield, lab coat, closed-toe shoes. Work in a well-ventilated fume hood. ^[002]

> Note: ECHA C&L data from 1,895 reports shows H314(42.3%) and H318(37%) — p-TsOH is a strong acid and causes chemical burns. ^[006] H335(87%) indicates high prevalence of respiratory irritation. The original article incorrectly included GHS02 (flammable) and GHS06 (fatal toxicity) — p-TsOH is neither flammable nor acutely lethal.

Always consult the Safety Data Sheet (SDS) from your supplier before handling.

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Regulatory & Compliance

Chemical Inventories: p-TsOH (CAS 104-15-4) is listed on major chemical inventories including TSCA (US), EINECS (EU), ENCS (Japan), IECSC (China), KECL (Korea), and AICS (Australia). ^[002]

Export Control: p-TsOH is a standard research and industrial chemical. Verify no specific export control regulations apply to your destination country. ^[002]

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Frequently Asked Questions

Why use p-TsOH instead of H2SO4 or HCl?

p-TsOH offers three key advantages over mineral acids: ^[004]

1. Organic solubility — dissolves in toluene, benzene, and other organic solvents, enabling homogeneous catalysis

2. Easy removal — the tosylate anion is extracted into aqueous base during workup, leaving no residual mineral acid

3. Non-volatile — does not generate corrosive fumes like HCl

4. Milder — less prone to cause substrate decomposition or unwanted side reactions compared to H2SO4

What is the difference between anhydrous p-TsOH and the monohydrate?

The monohydrate (p-TsOH-H2O) is the standard commercial form — it is a free-flowing crystalline solid that is easy to weigh and handle. ^[002] The anhydrous form is hygroscopic and deliquescent (absorbs moisture from air to form the monohydrate). For most synthetic applications, the monohydrate is used directly; the water of hydration does not interfere with catalysis at the temperatures typically employed (refluxing toluene).

Can p-TsOH be used for acetal deprotection?

Yes. p-TsOH catalyzes both acetal formation and acetal hydrolysis depending on reaction conditions. ^[004] For deprotection: use aqueous acetone or THF with a catalytic amount of p-TsOH (0.01-0.1 eq) at rt or 50 degC. The presence of water drives hydrolysis. For acid-sensitive substrates, use Amberlyst-15 (solid acid resin) instead, which is removed by simple filtration.

Is p-TsOH compatible with acid-sensitive functional groups?

Partially. p-TsOH is a strong acid and can deprotect Boc groups, cleave THP ethers, and hydrolyze acetals/ketals. ^[004] If your substrate contains these groups, consider: (1) using a weaker acid (AcOH, PPTS); (2) lowering temperature; (3) using a solid-supported acid (Amberlyst, Nafion) for easier removal; (4) employing a protecting group strategy.

How do I remove residual p-TsOH from my product?

Standard workup: wash organic layer with saturated NaHCO3 (2 x) or saturated Na2CO3 (1 x). ^[002] The tosylate anion is water-soluble in basic solution. If residual acidity persists, check that your aqueous washes are actually basic (add indicator). For very acid-sensitive products, perform an additional wash with dilute NaOH (0.1 M), but beware of base-catalyzed side reactions.

What is a Dean-Stark trap and why is it used with p-TsOH?

A Dean-Stark trap is a glassware attachment that collects water during azeotropic distillation. ^[004] In p-TsOH-catalyzed esterifications and acetalizations, water is a byproduct. By continuously removing water via azeotropic distillation with toluene (b.p. 110 degC), the equilibrium is driven toward product formation according to Le Chatelier's principle. Without water removal, esterification typically reaches only ~70% conversion.

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Where to Buy

p-Toluenesulfonic Acid (CAS 104-15-4) is available in the CoreyChem catalog.

View p-Toluenesulfonic Acid (CAS 104-15-4) on CoreyChem -> ^[003]

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