Stable isotope labelling to elucidate ring cleavage mechanisms of disinfection by-product formation during chlorination of phenols

Despite decades of research on the formation of toxic disinfection by-products (DBPs) during water disinfection with chlorine, considerable uncertainties remain regarding the formation mechanism of toxic DBPs from phenolic precursors. Here we report the use of a series of synthesized ethylparabens containing stable isotope (13C) labels at different positions of the molecule to ascertain DBP formation mechanisms from phenols, including those of regulated chloroacetic acids and recently identified α,β-unsaturated dialdehydes and dicarboxylic acids. Our results highlight the involvement of four general ring cleavage pathways. Three of the DBP formation pathways involve carbons originating from the aromatic ring, while the fourth pathway involves the substituent carboxylic ester carbon in the formation of dichloroacetic acid and C4-dicarboxylic acids. Quantitative comparison of the 13C-labelled DBPs enabled further assessment of the contribution from each of these distinct pathways, providing novel insights into ring cleavage reaction mechanisms that have eluded previous DBP investigations. The pathways by which aromatic compounds transform into acyclic by-products in chlorinated waters have important implications for water treatment and public health. Ethyl parabens with stable isotope labels at different carbon positions yield insights into how aromatic compounds can transform into lower-molecular-weight disinfection by-products.