Defluorination of Omega-Hydroperfluorocarboxylates (ω-HPFCAs): Distinct Reactivities from Perfluoro and Fluorotelomeric Carboxylates

Omega-hydroperfluorocarboxylates (omega-HPFCAs, HCF2-(CF2)(n-1)-COO-) are commercially available in bulk quantities and have been applied in agrochemicals, fluoropolymer production, and semiconductor coating. In this study, we used kinetic measurements, theoretical calculations, model compound experiments, and transformation product analyses to reveal novel mechanistic insights into the reductive and oxidative transformation of omega-HPFCAs. Like perfluorocarboxylates (PFCAs, CF3-(CF2)(n-1)-COO-), the direct linkage between HCnF2n- and -COO- enables facile degradation under UV/sulfite treatment. To our surprise, the presence of the H atom on the remote carbon makes omega-HPFCAs more susceptible than PFCAs to decarboxylation (i.e., yielding shorter-chain omega-HPFCAs) and less susceptible to hydrodefluorination (i.e., H/F exchange). Like fluorotelomer carboxylates (FTCAs, CnF2n+1-CH2CH2-COO-), the C-H bond in HCF2-(CF2)(n-1)-COO- allows hydroxyl radical oxidation and limited defluorination. While FTCAs yielded PFCAs in all chain lengths, omega-HPFCAs only yielded -OOC-(CF2)(n-1)-COO- (major) and -OOC-(CF2)(n-2)-COO- (minor) due to the unfavorable beta-fragmentation pathway that shortens the fluoroalkyl chain. We also compared two treatment sequences.UV/sulfite followed by heat/persulfate and the reverse.toward complete defluorination of.-HPFCAs. The findings will benefit the treatment and monitoring of H-containing per- and polyfluoroalkyl substance (PFAS) pollutants as well as the design of future fluorochemicals.

Automated summary

The direct linkage in Ω-HPFCAs enables easier degradation under UV/sulfite treatment compared to PFCAs, while the presence of the H atom makes Ω-HPFCAs more susceptible to decarboxylation but less susceptible to hydrodefluorination than PFCAs. Experimental results showed that Ω-HPFCAs only yield shorter-chain products during transformation, rather than longer-chain PFCAs.