Perfluoroalkyl carboxylic acids

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

Perfluoroalkyl carboxylic acids (PFCAs), or perfluorocarboxylic acids are compounds of the formula C_nF_(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 pK_a units and they exhibit great hydrophobic character. Perfluoroalkyl dicarboxylic acids (PFdiCAs) are also known, e.g. C₂F₄(CO₂H)₂.[1]

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:

C_nH_(2n+1)COF + (2n+1) HF → C_nF_(2n+1)COF + (2n+1) H₂

C_nF_(2n+1)COF + H₂O → C_nF_(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 [de]	PFPeA	C4F9COOH	264.05	2706-90-3
Perfluorohexanoic acid	PFHxA	C5F11COOH	314.05	307-24-4
Perfluoroheptanoic acid [de]	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 [de]	PFUnDA	C10F21COOH	564.09	2058-94-8
Perfluorododecanoic acid [de]	PFDoDA	C11F23COOH	614.10	307-55-1
Perfluorotridecanoic acid [de]	PFTrDA	C12F25COOH	664.10	72629-94-8
Perfluorotetradecanoic acid [de]	PFTeDA	C13F27COOH	714.11	376-06-7

See also

• Per- and polyfluoroalkyl substances
• Surflon S-111

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. Retrieved 23 May 2024.
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

• "Chapter 2: Perfluoroalkyl carboxylic (a) and dicarboxylic (b) acids". Fact Cards of Major Groups of Per- and Polyfluoroalkyl Substances (PFASs). OECD Environment, Health and Safety Publications Series on Risk Management. Vol. 68. The Organisation for Economic Co-operation and Development (OECD). January 2022. pp. 16–32.
• Guo Z, Liu X, Krebs KA, Roache NF (March 2009). "Perfluorocarboxylic acid content in 116 articles of commerce". Research Triangle Park, NC: US Environmental Protection Agency.