Trifluoromethanesulfonic acid

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ImageFile = Triflic-acid.png ImageFile1 = TfOH-3D-ball-&-stick.png ImageFile2 = TfOH-3D-vdW.png ImageSize =
IUPACName = Trifluoromethanesulfonic acid
OtherNames = Triflic acid
Section1 = Chembox Identifiers
CASNo = 1493-13-6
PubChem =
SMILES =
Section2 = Chembox Properties
Formula = CF₃SO₃H
MolarMass = 150.08 g/mol
Appearance = Colorless liquid
Density = 1.696 g/mL
MeltingPt = -40 °C
BoilingPt = 162 °C
Solubility = Miscible
Section3 = Chembox Hazards
MainHazards = Corrosive, eye irritant
FlashPt =
Autoignition =

Trifluoromethanesulfonic acid, also known as triflic acid or TfOH, is a sulfonic acid with the chemical formula CF₃SO₃H . It is often regarded as one of the strongest acids, and is one of a number of so-called "superacids". It is about 1000 times stronger than sulfuric acid. Triflic acid is widely used especially as a catalyst and a precursor in organic chemistry. [cite journal | author = Howells, R. D., McCown, J. D. | title = Trifluoromethanesulfonic Acid and Derivatives | journal = Chemical Reviews | year = 1977 | volume = 77 | pages = 69–92 | doi = 10.1021/cr60305a005] cite encyclopedia | year = 2006 | title = Trifluoromethanesulfonic Acid | encyclopedia = Encyclopedia of Reagents for Organic Synthesis | publisher = John Wiley & Sons, Ltd | doi = 10.1002/047084289X.rt246.pub2]

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 be dangerously exothermic.

With an K_a = 8.0 *10¹⁴ (~pKa=-15) mole kg^-1, 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 CF₃SO₃^-, known as triflate, resist oxidation/reduction reactions, whereas many strong acids are oxidizing, e.g. HClO₄ and HNO₃. The triflate anion is immune to attack by even strong nucleophiles. Because of it resistance to oxidation and reduction, triflic acid is a very useful and versatile reagent. 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.

History and syntheses

Trifluoromethanesulfonic acid was first synthesized in 1954 by Haszeldine and Kidd by the following reaction:cite journal | author = R. N. Haszeldine and J. M. Kidd | title = Perfluoroalkyl derivatives of sulphur. Part I. Trifluoromethanesulphonic acid
journal = J. Chem. Soc. | year = 1954 | pages = 4228–4232 | doi = 10.1039/JR9540004228]

Other ways to synthesize trifluoromethanesulfonic acid including electrochemical fluorination (ECF).Fact|date=June 2007

The industrial synthesis involves hydrolysis of CF₃SO₂F, followed by acidification. Triflic acid is purified by distillation from triflic anhydride.

Uses

Triflic acid is useful in protonations because the conjugate base of triflic acid will not react with other reagents.

alt formation

Trifluoromethanesulfonic acid exothermically reacts with metal carbonates and hydroxides. Illustrative is the synthesis of Cu(OTf)₂.Fact|date=June 2007::CuCO₃ + 2 CF₃SO₃H → Cu(O₃SCF₃)₂ + H₂O + CO₂Far more interesting to the synthetic chemist is the conversion of chloro complexes to the corresponding triflates. Illustrative is the synthesis of [Co(NH₃)₅OTf] ²⁺:::3 CF₃SO₃H + [Co(NH₃)₅Cl] Cl₂ → [Co(NH₃)₅O₃SCF₃] (O₃SCF₃)₂ + 3 HClThis 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 anhydrides, which are strong acylating agents, e.g. in Friedel-Crafts reactions.Fact|date=June 2007::CH₃C(O)Cl + CF₃SO₃H → CH₃C(O)OSO₂CF₃ + HCl::CH₃C(O)OSO₂CF₃ + C₆H₆ → CH₃C(O)C₆H₅ + CF₃SO₃HTriflic acid catalyzes the reaction of aromatic compounds with sulfonyl chlorides, probably also via the intermediacy of a mixed anhydride.Fact|date=June 2007

This is a very similar reaction to what would be done if one wanted to create polymers using triflic acid in the synthesis. Other Friedel-Crafts type reactions using triflic acid include 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.Fact|date=June 2007 It can be used as a catalyst for the condensation of alcohols and carboxylic acids.

References

Further reading

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