Benign by Design: Which organofluorine structures are most degradable in the environment?

Driven to apply his undergraduate understanding of chemistry for the benefit of our environment, Thomas Mundhenke turned to Environmental Engineering for graduate school. Mundhenke joined the lab of William Arnold and stepped into research on the fate of fluorinated compounds. Rather than focus on poly- and perfluoroalkyl substances (PFAS) that have received so much attention, Mundhenke focused on other fluorinated chemicals – specifically the myriads of pesticides and pharmaceuticals that contain fluorine in their structures. 

PFAS have come to be known as “forever chemicals.” They are built around the carbon-fluorine bond, one of the strongest bonds in chemistry. The strong carbon-fluorine bond also helps chemists create many other useful products, specifically pharmaceuticals and pesticides. Because of the incorporation of fluorine in these products, Mundhenke sought to understand whether these chemicals break down to form other persistent fluorinated byproducts in the environment. 

Arnold and his research group focus on how light might affect pollutants in the environment. Mundhenke undertook to study the effects of photolysis on fluorinated organic molecules. His goals were to identify which chemical functional groups lead to fluoride, which is innocuous at low levels, and which groups led to persistent fluorinated byproducts that could be pollutants themselves. Understanding this environmental reactivity can also help synthetic chemists understand which fluorine molecules might be safer to use, and ultimately, to help guide them make choices that are safer for the environment. 

Mundhenke tested fluorinated compounds exposing them to a variety of photolysis conditions to determine what helps and what hinders the defluorination process. He measured reaction rates, quantum yields, and bimolecular rate constants to gain an understanding of the reactivity of these compounds. He used ¹⁹F nuclear magnetic resonance spectroscopy to complete fluorine mass balances, confirm organofluroine degradation products, and aid in predicting organofluorine degradation pathways. 

For most of the compounds and conditions, fluoride was a major degradation product. Fluoroacetic acids, fluorinated acetamides, and products retaining the parent compound fluorinated motifs were also observed, and all of these are potentially problematic from an environmental perspective.

The results of Mundhenke’s studies build our understanding of how organofluorine compounds will transform within the environment and give synthetic chemists an understanding of how adding various fluorinated functional groups in chemical compounds will impact the environment.

Thomas F. Mundhenke, Ph.D., was advised by William Arnold (Environmental Engineering) and William Pomerantz (Chemistry) at the University of Minnesota. Mundhenke's thesis is titled “Photolysis of Fluorinated Organic Molecules: Fluorine Mass Balances and the Roles of Nucleophiles, Rings, and Degree of Fluorination.”