Trifluoromethanesulfonic acid

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Trifluoromethanesulfonic acid
CAS number 1493-13-6 YesY
PubChem 62406
ChemSpider 56192 YesY
ChEBI CHEBI:48511 YesY
Jmol-3D images Image 1
Molecular formula CF3SO3H
Molar mass 150.08 g/mol
Appearance Colorless liquid
Density 1.696 g/mL
Melting point −40 °C (−40 °F; 233 K)
Boiling point 162 °C (324 °F; 435 K)
Solubility in water Miscible
Main hazards Corrosive, eye irritant
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Trifluoromethanesulfonic acid, also known as triflic acid, TFMS, TFSA, HOTf or TfOH, is a sulfonic acid with the chemical formula CF3SO3H. It is one of the strongest acids. Triflic acid is mainly used in research as a catalyst for esterification.[1][2]


Triflic acid is a hygroscopic, colorless liquid at room temperature. It is soluble in polar solvents such as DMF, DMSO, acetonitrile, and dimethyl sulfone. Addition of triflic acid to polar solvents can however be dangerously exothermic.

With a Ka = 1 ×1012 (pKa ~ −12)[3] Triflic acid qualifies as a superacid. It owes many of its useful properties to its great thermal and chemical stability. Both the acid and its conjugate base CF3SO
, known as triflate, resist oxidation/reduction reactions, whereas many strong acids are oxidizing, e.g. HClO4 and HNO3. Further recommending its use, triflic acid does not sulfonate substrates, which can be a problem with sulfuric acid, fluorosulfuric acid, and chlorosulfonic acid. Below is a prototypical sulfonation, which HOTf does not undergo:

C6H6 + H2SO4 → C6H5(SO3H) + H2O

Triflic acid fumes in moist air and forms a stable solid monohydrate, CF3SO3H·H2O, melting point 34 °C.

Health Precautions[edit]

Trifluoromethanesulfonic acid is one of the strongest acids known. On eye contact it causes severe eye burns and may cause blindness. With contact with skin it causes severe burns with delayed tissue destruction. On inhalation it causes fatal spasms, inflammation and edema.[4]


Trifluoromethanesulfonic acid was first synthesized in 1954 by Haszeldine and Kidd by the following reaction:[5]

Synthesis Trifluoromethanesulfonic acid 1.svg

Trifluoromethanesulfonic acid is produced industrially by electrochemical fluorination (ECF) of methanesulfonic acid:

CH3SO3H + 4 HF → CF3SO2F + H2O + 1.5 H2

The resulting CF3SO2F is hydrolyzed, and the resulting triflate salt is preprotonated. Alternatively, trifluoromethanesulfonic acid arises by oxidation of trifluoromethylsulfenyl chloride:[6]

CF3SCl + 2 Cl2 + 2 H2O → CF3SO2OH + 4 HCl

Triflic acid is purified by distillation from triflic anhydride.[2]


In the laboratory, triflic acid is useful in protonations because the conjugate base of triflic acid is non-nucleophilic. It is also used as an acidic titrant in non-aqueous acid-base titration because it behaves as a strong acid in many solvents (acetonitrile, acetic acid, etc.) where common mineral acids (such as HCl or H2SO4) are only moderately strong.

Salt formation[edit]

Trifluoromethanesulfonic acid exothermically reacts with metal carbonates and hydroxides. Illustrative is the synthesis of Cu(OTf)2.[7]

CuCO3 + 2 CF3SO3H → Cu(O3SCF3)2 + H2O + CO2

Chloride ligands can be converted to the corresponding triflates:

3 CF3SO3H + [Co(NH3)5Cl]Cl2 → [Co(NH3)5O3SCF3](O3SCF3)2 + 3 HCl

This conversion is conducted in neat HOTf at 100 °C, followed by precipitation of the salt by the addition of ether.

Organic reactions[edit]

Triflic acid reacts with acyl halides to give mixed triflate anhydrides, which are strong acylating agents, e.g. in Friedel-Crafts reactions.

CH3C(O)Cl + CF3SO3H → CH3C(O)OSO2CF3 + HCl
CH3C(O)OSO2CF3 + C6H6 → CH3C(O)C6H5 + CF3SO3H

Triflic acid catalyzes the reaction of aromatic compounds with sulfonyl chlorides, probably also via the intermediacy of a mixed anhydride of the sulfonic acid.

Triflic acid promotes other Friedel-Crafts-like reactions including the cracking of alkanes and alkylation of alkenes, which are very important to the petroleum industry. These triflic acid derivative catalysts are very effective in isomerizing straight chain or slightly branched hydrocarbons that can increase the octane rating of a particular petroleum-based fuel.

Triflic acid reacts exothermically with alcohols to produce ethers and olefins.

triflic acid condensation reaction


  1. ^ Howells, R. D., McCown, J. D. (1977). "Trifluoromethanesulfonic Acid and Derivatives". Chemical Reviews 77 (1): 69–92. doi:10.1021/cr60305a005. 
  2. ^ a b Subramanian, L. R.; Martinez, A. G.; Hanack, M.; Prakash, G. K. S.; Hu, J. (2006). "Trifluoromethanesulfonic Acid". Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons. doi:10.1002/047084289X.rt246.pub2. ISBN 0-471-93623-5. 
  3. ^ Raamat, E.; Kaupmees, K.; Ovsjannikov, G.; Trummal, A.; Kütt, A.; Saame, J.; Koppel, I.; Kaljurand, I.; Lipping, L.; Rodima, T.; Pihl, V.; Koppel, I. A.; Leito, I. "Acidities of strong neutral Brønsted acids in different media." J. Phys. Org. Chem. 2013, 26, 162-170. doi:10.1002/poc.2946
  4. ^ "Trifluoromethanesulfonic acid MSDS". ChemCAS. 
  5. ^ Haszeldine, R. N.; Kidd, J. M. (1954). "Perfluoroalkyl derivatives of sulphur. Part I. Trifluoromethanesulphonic acid". Journal of the Chemical Society 1954: 4228–4232. doi:10.1039/JR9540004228. 
  6. ^ Siegemund, G.; Schwertfeger, W.; Feiring, A.; Smart, B.; Behr, F.; Vogel, H.; McKusick, B. (2000). "Fluorine Compounds, Organic". Ullmann's Encyclopedia of Industrial Chemistry. John Wiley & Sons. doi:10.1002/14356007.a11_349. 
  7. ^ Dixon, N. E.; Lawrance, G. A.; Lay, P. A.; Sargeson, A. M.; Taube, H. (1990). "Trifluoromethanesulfonates and trifluoromethanesulfonato-O complexes". Inorganic Syntheses 28: 70–76. doi:10.1002/9780470132593.ch16. ISBN 978-0-470-13259-3.