Mechanistic insights into (toxic) carbonyl compound formation during ozonation of substituted phenols

To avoid unintended toxic byproduct formation when ozone is applied in water treatment, it is critical to understand reaction mechanisms between ozone and relevant organic precursors. Among these byproducts, (di)carbonyl compounds are particularly concerning due to their toxicity. However, knowledge about their formation mechanisms remains limited. As phenolic compounds are ubiquitous in water systems and show high ozone reactivity, we investigated their fate during ozonation to explore how substituent type and position impact yields of dicarbonyl compounds glyoxal, 2-butene-1,4-dial (BDA), and their substituted analogues. Experiments were performed in the presence and absence of the hydroxyl radical (^•OH) scavenger tert‑butanol (t-BuOH) to assess the formation of dicarbonyl compounds via direct ozonation and ^•OH reaction, respectively. Glyoxal was a common product for all investigated phenols, with the highest yield for o-cresol (26.5%). Formation of BDA and its substituted analogues was observed for all tested phenols except 4-nitrophenol and 4-hydroxybenzonitrile, with phenol exhibiting the highest yield (4.8%). Overall, total dicarbonyl product yields were enhanced in the presence of t-BuOH regardless of substituent effects, highlighting the importance of direct ozonation for their formation. Furthermore, quantum chemical calculations aided in identifying potential dicarbonyl precursors and supported their ring-cleavage formation pathways.