Trifluoromethanesulfonic acid

Structural formula
General
Surname	Trifluoromethanesulfonic acid
other names	TfOH TFMSS TFMSA TFSA
Molecular formula	CF 3 SO 3 H
Brief description	colorless, pungent smelling, hygroscopic liquid
External identifiers / databases
CAS number 1493-13-6 EC number 216-087-5 ECHA InfoCard 100.014.625 PubChem 62406 Wikidata Q304850
properties
Molar mass	150.08 g mol −1
Physical state	liquid
density	1.71 g cm −3 (20 ° C)
Melting point	−40 ° C
boiling point	162 ° C
Vapor pressure	10 h Pa (55 ° C)
pK s value	−20
solubility	soluble in water and polar organic solvents such as DMF, DMSO, sulfolane, acetonitrile
safety instructions
GHS labeling of hazardous substances danger H and P phrases H: 290-302-314-335 P: 280-301 + 330 + 331-303 + 361 + 353-304 + 340-305 + 351 + 338-310
Toxicological data	1605 mg kg −1 ( LD 50 ,  rat ,  oral )
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

The trifluoromethanesulfonic acid CF ₃ SO ₃ H is a so-called super acid and plays a role in organic chemistry . Their anion, the trifluoromethanesulfonate CF ₃ SO ₃ ^- , usually called triflate , is also important. As the conjugate base of a super acid, it is a very stable anion. Trifluoromethylsulfonic acid is used in many different ways in organic chemistry, the triflate group is often used as a leaving group and its salts are used in catalysts . The abbreviation “OTf” is mostly used for the triflate group in specialist publications; the abbreviation “Tf” stands for the trifluoromethanesulfonyl group –SO ₂ CF ₃ , also known as the triflyl group.

properties

Physical Properties

Trifluoromethanesulfonic acid is a clear, colorless liquid under standard conditions with a boiling point of 162 ° C. The compound smokes in the air, forming the stable monohydrate (actually: Oxonium trifluoromethylsulfonate), which has a melting point of 34 ° C and is highly hygroscopic .

Chemical properties

Trifluoromethanesulfonic acid is soluble in all polar solvents. However, in esters , ethers , alcohols or ketones, it cannot always be assumed that trifluoromethanesulfonic acid is inert towards the solvent. Both the acid and the conjugate base, the triflate anion, are resistant to most oxidizing and reducing agents , while many other strong acids have an oxidizing effect, e.g. B. perchloric acid or nitric acid . The triflate anion resists even strong nucleophiles . In addition, trifluoromethanesulfonic acid does not sulfonate substrates, which can certainly happen with sulfuric acid .

Extraction and presentation

Trifluoromethanesulfonic acid was first presented in 1954 by Haszeldine and Kidd through the following reaction:

Further synthesis options are electrochemical fluorination (ECF):

or the oxidation of methyl trifluoromethyl sulfide :

Industrial production takes place via the electrochemical fluorination of methanesulfonyl chloride . This synthesis is expensive, so alternatives are sought. For example, it has been possible to produce trifluoromethanesulfonic acid from trifluoromethane in a strongly acidic or strongly basic solution. This is a radical reaction, for example with SO ₃ or SO ₂ Cl ₂ . However, the yields are low. More favorable syntheses are still being sought.

use

Trifluoromethanesulfonic acid and triflates (see below) are widely used in organic chemistry, the acid itself mainly as a catalyst in the polymer, fuel, pharmaceutical and sugar industries. The acid is particularly suitable for protonation, since the conjugate base, the triflate, does not react further.

Salt formation

Trifluoromethanesulfonic acid reacts, among other things, with metal carbonates and hydroxides in aqueous solution exothermically to form the triflate salts . An example is the synthesis of copper (II) triflate from copper (II) carbonate :

${\displaystyle \mathrm {CuCO_{3}+2\ CF_{3}SO_{3}H\rightarrow \ Cu(O_{3}SCF_{3})_{2}+\ H_{2}O+\ CO_{2}} }$

Reactions in Organic Chemistry

The reactions of this connection are manifold and can only be shown here to a small extent. For further information, please refer to the literature.

Mixed anhydrides can be obtained from trifluoromethanesulfonic acid by adding acid anhydrides or chlorides. These are strong acylating reagents and can be used, for example, in Friedel-Crafts acylations :

${\displaystyle \mathrm {1)\ CH_{3}COCl+\ CF_{3}SO_{3}H\rightarrow \ CH_{3}COOSO_{2}CF_{3}+\ HCl} }$

${\displaystyle \mathrm {2)\ CH_{3}COOSO_{2}CF_{3}+\ C_{6}H_{6}\rightarrow \ CH_{3}COC_{6}H_{5}+\ CF_{3}SO_{3}H} }$

Trifluoromethanesulfonic acid catalyzes the reaction of aromatics with sulfonyl chlorides, presumably also via intermediate anhydride formation.

In the case of acid-catalyzed dehydrations, the addition of trifluoromethanesulfonic acid causes an immediate isomerization of the alkenes formed in favor of the thermodynamically more stable alkene.

Triflates

The triflate group is widely used as a leaving group in organic chemistry. The reason for this is the extreme stability of the triflate anion, on the one hand due to the mesomeric distribution of the negative charge over the three oxygen atoms and the sulfur:

In addition, this charge is stabilized by the powerful electron-withdrawing effect (high electronegativity of the three fluorine atoms) of the trifluoromethyl group . Triflate is therefore a better leaving group than, for example, the related nucleofuge tosylate and mesylate .

Triflate salts are often very thermally stable; the melting points of the anhydrous salts are sometimes above 350 ° C ( Na , Ba , Ag salts). As already mentioned above, they can be obtained directly from the reaction of the acid with metal hydroxides or metal carbonates in aqueous solution. Triflate salts have been used increasingly and successfully as Lewis acids in many reactions in recent years . The advantage of these salts is their stability in water, which is common for many classic Lewis acids such as. B. AlCl _{3 is} not given. Lanthanide salts of the Ln (OTf) _{3 type} , in particular the scandium salt Sc (OTf) _3, are particularly suitable . This compound can be used for a wide range of reactions, such as aldol reactions and Diels-Alder reactions (for further reactions see name reaction or Lewis acid).

safety instructions

Trifluoromethanesulfonic acid smokes in the air, is corrosive and must therefore be handled with care. Work under the fume cupboard is essential.

Individual evidence

1. ↑ ^{a b c d e} Entry on trifluoromethanesulfonic acid. In: Römpp Online . Georg Thieme Verlag, accessed on July 25, 2016.
2. ↑ ^{a b c d e} data sheet trifluoromethanesulfonic acid (PDF) from Merck , accessed on April 24, 2011.
3. ↑ pKs overview in patent EP0955562
4. ↑ ^{a b} e-EROS Encyclopedia of Reagents for Organic Synthesis , 1999-2013, John Wiley and Sons, Inc., entry for Trifluoromethanesulfonic Acid, accessed January 6, 2015 .
5. ↑ ^{a b} Entry on trifluoromethanesulphonic acid in the GESTIS substance database of the IFA , accessed on January 8, 2020(JavaScript required) .
6. ↑ ^{a b c} R.D. Howells, JD Mc Cowna: Trifluoromethanesulfonic Acid and Derivatives , Chemical Reviews 77 (1977) 69-92. doi: 10.1021 / cr60305a005 .
7. ^ A. Streitwieser Jr., CL Wilkins, E. Kiehlmann: Kinetics and Isotope Effects in Solvolyses of Ethyl Trifluoromethanesulfonate , Journal of the American Chemical Society 90 (1968) 1598-1601. doi: 10.1021 / ja01008a601 .
8. ^ S. Mukhopadhyay, AT Bell, RV Srinivas, GS Smith: Synthesis of Trifluoromethanesulfonic Acid from CHF ₃ . In: Organic Process Research & Development 4/8/2004. Pp. 660-662, doi: 10.1021 / op040007r .
9. ↑ E. Vedejs, DA Engler, MJ Mullins: Reactive triflates Alkylating Agents . In: The Journal of Organic Chemistry 19/42/1977. Pp. 3109-3113 doi: 10.1021 / jo00439a001 .