Finding Fluorine: Photoproduct Formation during the Photolysis of Fluorinated Pesticides

Abstract

The photolysis of pesticides with different fluorine motifs was evaluated to quantify the formation of fluorinated products in buffered aqueous systems, advanced oxidation (AOP) and reduction processes (ARP), and river water. Simulated sunlight quantum yields at pH 7 were 0.0033, 0.0025, 0.0015, and 0.00012 for penoxsulam, florasulam, sulfoxaflor, and fluroxypyr, respectively. The bimolecular rate constants with hydroxyl radicals were 2 to 5.7 × 10 ¹⁰ M ^–1 s ^–1 and, with sulfate radicals, 1.6 to 2.6 × 10 ⁸ M ^–1 s ^–1 for penoxsulam, florasulam, and fluroxypyr, respectively. The rate constants of sulfoxaflor were 100-fold lower. Using quantitative ¹⁹F-NMR, complete fluorine mass balances were obtained. The maximum fluoride formation was 53.4 and 87.4% for penoxsulam and florasulam under ARP conditions, and 6.1 and 100% for sulfoxaflor and fluroxypyr under AOP conditions. Heteroaromatic CF ₃ and aliphatic CF ₂ groups were retained in multiple fluorinated photoproducts. Aryl F and heteroaromatic F groups were readily defluorinated to fluoride. CF ₃ and CF ₂ groups formed trifluoroacetate and difluoroacetate, and yields increased under oxidizing conditions. ¹⁹F-NMR chemical shifts and coupling analysis provided information on hydrogen loss on adjacent bonds or changes in chirality. Mass spectrometry results were consistent with the observed ¹⁹F-NMR products. These results will assist in selecting treatment processes for specific fluorine motifs and in the design of agrochemicals to reduce byproduct formation.

Abstract

Pesticides increasingly incorporate fluorine in their chemical structures. This study reports the photolysis kinetics of pesticides with different fluorinated functional groups and tracks fluorinated products using NMR and mass spectrometry.