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Through Acid, Catalysis and Hygroscopy
Synthesis
Trifluoromethanesulfonic acid is produced industrially by electrochemical fluorination (ECF) of methanesulfonic acid to triflyl fluoride:
- CH3SO3H + 4 HF → CF3SO2F + H2O + 3 H2
The acyl fluoride is hydrolyzed, and the resulting triflate salt is heated in sulfuric acid, resulting in the distillation of HOTf. Alternatively, trifluoromethanesulfonic acid arises by oxidation of trifluoromethylsulfenyl chloride:
- CF3SCl + 2 Cl2 + 3 H2O → CF3SO3H + 5 HCl
Triflic acid is purified by distillation from triflic anhydride.
Historical
Trifluoromethanesulfonic acid was first synthesized in 1954 by Haszeldine and Kidd by the following reaction:
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Reactions
As an acid
In the laboratory, triflic acid is useful in protonations because the conjugate base of triflic acid is nonnucleophilic. It is also used as an acidic titrant in nonaqueous 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.
With a Ka = 5×1014, pKa = −14.7±2.0, 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−
3, known as triflate, resist oxidation/reduction reactions, whereas many strong acids are oxidizing, such as perchloric or nitric acid. 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 triflic acid does not undergo:
- C6H6 + H2SO4 → C6H5(SO3H) + H2O in SO3
Triflic acid fumes in moist air and forms a stable solid monohydrate, CF3SO3H·H2O, melting point 34 °C. Both the hydrate and the acid can be distilled with phosphorus pentoxide, yielding triflic anhydride.
Salt and complex formation
In its coordination complexes, the triflate ligand is labile, reflecting its low basicity. Trifluoromethanesulfonic acid reacts exothermically with metal carbonates, hydroxides, and oxides. Illustrative is the synthesis of Cu(OTf)2.
- Cu2CO3(OH)2 + 4 CF3SO3H → 2 Cu(O3SCF3)2 + 3 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 upon the addition of ether.
Organic chemistry
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 through 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.
Dehydration gives the acid anhydride, trifluoromethanesulfonic anhydride, (CF3SO2)2O.
Safety
Triflic acid is one of the strongest acids. Contact with skin causes severe burns with delayed tissue destruction. On inhalation it causes fatal spasms, inflammation and edema.
Like sulfuric acid, triflic acid must be slowly added to polar solvents to prevent thermal runaway.