Trifluoromethanesulfonic acid
[CAS# 1493-13-6]

Identification

• Classification: Chemical reagent >> Organic reagent >> Fatty acid
• Name: Trifluoromethanesulfonic acid
• Synonyms: Triflic acid
• Molecular Formula: CHF₃O₃S
• Molecular Weight: 150.07
• CAS Registry Number: 1493-13-6
• EC Number: 216-087-5
• SMILES: C(F)(F)(F)S(=O)(=O)O

Properties

• Density: 1.9±0.1 g/cm³ Calc.^*, 1.706 g/mL (Expl.)
• Melting point: -40 °C (Expl.)
• Boiling point: 162.0 °C 760 mmHg (Calc.)^*, 162 °C (Expl.)
• Solubility: water: soluble (Expl.)
• Index of refraction: 1.354 (Calc.)^*, 1.327 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS05;GHS07 Danger
• Risk Statements: H290-H302-H314-H335
• Safety Statements: P234-P260-P261-P264-P270-P271-P280-P301+P317-P301+P330+P331-P302+P361+P354-P304+P340-P305+P354+P338-P316-P319-P321-P330-P363-P390-P403+P233-P405-P406-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Acute toxicity	Acute Tox.	4	H302
Specific target organ toxicity - single exposure	STOT SE	3	H335
Skin corrosion	Skin Corr.	1B	H314
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Skin corrosion	Skin Corr.	1C	H314
Serious eye damage	Eye Dam.	1	H318
Skin corrosion	Skin Corr.	1A	H314
Acute toxicity	Acute Tox.	4	H312
Specific target organ toxicity - single exposure	STOT SE	3	H371
Acute toxicity	Acute Tox.	4	H332
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413

• Transport Information: UN 3265

Discovery and Applications

Trifluoromethanesulfonic acid (CF₃SO₃H), commonly known as triflic acid, is one of the strongest known simple acids and a key reagent in modern synthetic and industrial chemistry. It was first described in the late 1950s by Robert Haszeldine and co-workers during investigations into fluorinated sulfonic compounds derived from the reaction of fluorocarbons with sulfur trioxide. Since its introduction, triflic acid has become a cornerstone reagent in both laboratory and large-scale chemical transformations due to its exceptional acidity, stability, and versatility.

The molecular structure of trifluoromethanesulfonic acid features a strongly electron-withdrawing trifluoromethyl group bonded to a sulfonic acid moiety. The high electronegativity of the fluorine atoms delocalizes charge through the sulfur–oxygen bonds, greatly stabilizing the conjugate base, the triflate anion (CF₃SO₃⁻). This stabilization is the key to its superacidic strength, which surpasses that of concentrated sulfuric acid and rivals other superacids such as fluorosulfonic acid and antimony pentafluoride mixtures. In pure form, triflic acid is a colorless, hygroscopic liquid that fumes in moist air and dissolves readily in polar solvents, including water, acetonitrile, and sulfur dioxide.

The discovery of triflic acid marked a significant advance in the field of acid chemistry, leading to the development of a wide range of triflate-based reagents and catalysts. Its strong acidity and non-oxidizing character make it ideal for catalyzing electrophilic substitution reactions, including alkylation, acylation, and rearrangement reactions in organic synthesis. In contrast to other strong mineral acids, triflic acid is remarkably stable and resistant to oxidation and reduction, which allows it to be used under conditions where other acids would decompose or interfere with sensitive substrates. It also forms stable salts and esters, which are widely used as leaving groups in nucleophilic substitution reactions and as components of ionic liquids.

In industrial and laboratory settings, triflic acid serves as a catalyst in processes such as the isomerization and alkylation of hydrocarbons, esterification, polymerization, and the synthesis of fine chemicals and pharmaceuticals. It is particularly valuable in generating carbocations under mild conditions, enabling transformations that would otherwise require harsher reagents. The conjugate base, triflate, has become a widely used counterion in organometallic and coordination chemistry, owing to its weakly coordinating nature and chemical inertness. Metal triflates such as scandium triflate and ytterbium triflate, derived from triflic acid, are important Lewis acid catalysts in a variety of stereoselective and environmentally friendly reactions.

Triflic acid has also found extensive application in electrochemistry, particularly in the preparation of conductive materials and as an additive in high-performance batteries and capacitors. Its ability to protonate aromatic compounds and stabilize carbocation intermediates under anhydrous conditions has made it an indispensable reagent in advanced synthetic methodologies. Moreover, due to its chemical robustness, triflic acid is recyclable and can be recovered for repeated use in catalytic systems.

Despite its usefulness, handling triflic acid requires careful precautions because of its extreme corrosiveness and reactivity with water. Contact with organic matter or moisture can produce heat and toxic fumes. Proper protective equipment and dry, inert conditions are essential for laboratory operations involving this compound. Its reactivity and strong proton-donating ability continue to make it an active area of study in acid catalysis and materials chemistry.

References

Haszeldine RN & Kidd J M (1954) Perfluoroalkyl derivatives of sulphur. Part I. Trifluoromethanesulphonic acid. Journal of the Chemical Society 1954 4228–4231. DOI: 10.1039/JR9540004228

Olah GA, Prakash GK & Sommer J (1979) Superacids. Science 206(4414) 13–20. DOI: 10.1126/science.206.4414.13

Tachrim ZP, Wang L, Murai Y, Yoshida T, Kurokawa N, Ohashi F & Hashimoto M (2017) Trifluoromethanesulfonic acid as acylation catalyst: special feature for C- and/or O-acylation reactions. Catalysts 7(2) 40. DOI: 10.3390/catal7020040