Triflic acid

Trifluoromethanesulfonic acid
IUPAC name Trifluoromethanesulfonic acid
Other names Triflic acid
Identifiers
CAS number	1493-13-6
PubChem	62406
ChemSpider	56192
ChEBI	CHEBI:48511
Jmol-3D images	Image 1
Properties
Molecular formula	CF3SO3H
Molar mass	150.08 g/mol
Appearance	Colorless liquid
Density	1.696 g/mL
Melting point	-40 °C
Boiling point	162 °C
Solubility in water	Miscible
Hazards
Main hazards	Corrosive, eye irritant
(what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.

Properties

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 an K_a = 8.0 ×10¹⁴ (pKa ~ −15) mol/kg, HOTf qualifies as a superacid. Triflic acid owes many of its useful properties to its great thermal and chemical stability. Both the acid and its conjugate base CF₃SO−
3, 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

Trifluouromethanesulfonic 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.

Syntheses

Trifluoromethanesulfonic acid was first synthesized in 1954 by Haszeldine and Kidd by the following reaction:^[4]

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:

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

Triflic acid is purified by distillation from triflic anhydride.

Uses

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

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

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

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.