Perfluoroalkyl carboxylic acids

Perfluoroalkyl carboxylic acids (PFCAs), or perfluorocarboxylic acids are compounds of the formula CnF(2n+1)CO2H that belong to the class of per- and polyfluoroalkyl substances. The simplest example is trifluoroacetic acid. These compounds are organofluorine analogues of ordinary carboxylic acids, but they are stronger by several pKa units and they exhibit great hydrophobic character. Perfluoroalkyl dicarboxylic acids (PFdiCAs) are also known, e.g. C2F4(CO2H)2.[1]

Perfluorononanoic acid, an example of a perfluoroalkyl carboxylic acid (PFCA)

Applications

Trifluoroacetic acid is a widely employed acid, used for example in the synthesis of peptides. Its esters are useful in analytical chemistry.

Longer-chain perfluoroalkyl carboxylic acids, e.g. with five to nine carbons, are useful fluorosurfactants and emulsifiers used in the production of polytetrafluoroethylene (Teflon) and related fluoropolymers.[1]

Production

These compounds are typically prepared by electrochemical fluorination of the carboxylic acid fluorides followed by hydrolysis:

CnH(2n+1)COF + (2n+1) HF → CnF(2n+1)COF + (2n+1) H2
CnF(2n+1)COF + H2O → CnF(2n+1)CO2H + HF

Environmental concerns

Long-chain PFCAs such as perfluorooctanoic acid (PFOA) are either banned or being under scrutiny because they are extremely persistent and bioaccumulative. Short-chain PFCAs (scPFCAs) are formed from atmospheric oxidation of fluorotelomer compounds and chlorofluorocarbon (CFC) replacements introduced as a result of the Montreal Protocol.[2][3]

Side-chain fluorinated polymers (SCFPs), in which fluorotelomers are attached to a polymer backbone, may release fluorotelomer alcohols through hydrolysis. The latter are then degraded to PFCAs.[4][5][6]

Simplified degradation pathway of urethane side-chain fluorinated polymers: hydrolysis yields fluorotelomer alcohols that are then degraded to PFCAs of different chain lengths (in the shown example PFHxA, PFHpA and PFBA).

Common examples
Name Abbreviation Molecular formula Molecular weight (g/mol) CAS No.
Trifluoroacetic acid TFA CF3COOH 114.02 76-05-1
Perfluoropropanoic acid PFPrA C2F5COOH 164.03 422-64-0
Perfluorobutanoic acid PFBA C3F7COOH 214.04 375-22-4
Perfluoropentanoic acid PFPeA C4F9COOH 264.05 2706-90-3
Perfluorohexanoic acid PFHxA C5F11COOH 314.05 307-24-4
Perfluoroheptanoic acid PFHpA C6F13COOH 364.06 375-85-9
Perfluorooctanoic acid PFOA C7F15COOH 414.07 335-67-1
Perfluorononanoic acid PFNA C8F17COOH 464.08 375-95-1
Perfluorodecanoic acid PFDA C9F19COOH 514.08 335-76-2
Perfluoroundecanoic acid PFUnDA C10F21COOH 564.09 2058-94-8
Perfluorododecanoic acid PFDoDA C11F23COOH 614.10 307-55-1
Perfluorotridecanoic acid PFTrDA C12F25COOH 664.10 72629-94-8
Perfluorotetradecanoic acid PFTeDA C13F27COOH 714.11 376-06-7

See also

References
  1. Siegemund G, Schwertfeger W, Feiring A, Smart B, Behr F, Vogel H, McKusick B (2002). "Fluorine Compounds, Organic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH: a11_349. doi:10.1002/14356007.a11_349. ISBN 978-3-527-30673-2.
  2. McGrath M (14 May 2020). "Ozone layer: Concern grows over threat from replacement chemicals". BBC News.
  3. Pickard HM, Criscitiello AS, Persaud D, Spencer C, Muir DC, Lehnherr I, et al. (28 May 2020). "Ice Core Record of Persistent Short‐Chain Fluorinated Alkyl Acids: Evidence of the Impact From Global Environmental Regulations". Geophysical Research Letters. 47 (10). doi:10.1029/2020GL087535.
  4. Synthesis Report on Understanding Side-Chain Fluorinated Polymers and Their Life Cycle (PDF). OECD Series on Risk Management. Vol. 73. Environment, Health and Safety, Environment Directorate, OECD. 2022.
  5. Letcher RJ, Chu S, Smyth SA (April 2020). "Side-chain fluorinated polymer surfactants in biosolids from wastewater treatment plants". Journal of Hazardous Materials. 388: 122044. doi:10.1016/j.jhazmat.2020.122044. PMID 31955025. S2CID 210830499.
  6. Brendel S, Fetter É, Staude C, Vierke L, Biegel-Engler A (2018-02-27). "Short-chain perfluoroalkyl acids: environmental concerns and a regulatory strategy under REACH". Environmental Sciences Europe. 30 (1): 9. doi:10.1186/s12302-018-0134-4. PMC 5834591. PMID 29527446.

Further reading
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