Perfluoroalkyl carboxylic acids explained

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]}

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
	PFPeA	C4F9COOH	264.05	2706-90-3
Perfluorohexanoic acid	PFHxA	C5F11COOH	314.05	307-24-4
	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
	PFUnDA	C10F21COOH	564.09	2058-94-8
	PFDoDA	C11F23COOH	614.10	307-55-1
	PFTrDA	C12F25COOH	664.10	72629-94-8
	PFTeDA	C13F27COOH	714.11	376-06-7

See also

• Per- and polyfluoroalkyl substances
• Surflon S-111

Further reading

• Book: Fact Cards of Major Groups of Per- and Polyfluoroalkyl Substances (PFASs). OECD Environment, Health and Safety Publications Series on Risk Management . 68 . ENV/CBC/MONO(2022)1 . 16–32 . Chapter 2: Perfluoroalkyl carboxylic (a) and dicarboxylic (b) acids . January 2022 . English . The Organisation for Economic Co-operation and Development (OECD) .
• Web site: Guo Z, Liu X, Krebs KA, Roache NF . Perfluorocarboxylic acid content in 116 articles of commerce. . Research Triangle Park, NC . US Environmental Protection Agency . March 2009 .

Notes and References

1. Siegemund G, Schwertfeger W, Feiring A, Smart B, Behr F, Vogel H, McKusick B . Fluorine Compounds, Organic . Ullmann's Encyclopedia of Industrial Chemistry . 2002 . a11_349 . 10.1002/14356007.a11_349 . Wiley-VCH . Weinheim . 978-3-527-30673-2 .
2. Web site: McGrath M . Ozone layer: Concern grows over threat from replacement chemicals . BBC News . 14 May 2020. 23 May 2024.
3. Pickard HM, Criscitiello AS, Persaud D, Spencer C, Muir DC, Lehnherr I, Sharp MJ, De Silva AO, Young CJ . 6 . 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. 10.1029/2020GL087535. free .
4. Book: Synthesis Report on Understanding Side-Chain Fluorinated Polymers and Their Life Cycle . OECD Series on Risk Management . 73 . Environment, Health and Safety, Environment Directorate, OECD . 2022.
5. Letcher RJ, Chu S, Smyth SA . Side-chain fluorinated polymer surfactants in biosolids from wastewater treatment plants . Journal of Hazardous Materials . 388 . 122044 . April 2020 . 31955025 . 10.1016/j.jhazmat.2020.122044 . 210830499 .
6. Brendel S, Fetter É, Staude C, Vierke L, Biegel-Engler A . Short-chain perfluoroalkyl acids: environmental concerns and a regulatory strategy under REACH . Environmental Sciences Europe . 30 . 1 . 9 . 2018-02-27 . 29527446 . 5834591 . 10.1186/s12302-018-0134-4 . free .