From Polyhalo-organic and Persistent Waste to Acetals: An Electrochemical Transfer Approach

Persistent legacy contaminants such as 1,2,3,4,5,6-hexachlorocyclohexane (HCH) and 1,2,5,6,9,10-hexabromocyclododecane (HBCD) remain a major environmental burden for the chemical industry. Their electrochemical valorization offers a promising strategy for remediation. In this work, the e-shuttle concept was successfully applied to the selective transformation of HCH and HBCD into acetals of chloroacetaldehyde and bromoacetaldehyde. Due to high selectivity and increasing industrial relevance, 2-chloromethyl-1,3-dioxepane was synthesized as a test substrate. Systematic optimization using fractional factorial designs and response surface methodology identified current density and the amount of applied charge as key influencing parameters. While ring sizes ≥7 afforded moderate to good yields, five-membered acetals were dominated by ring-opening reactions competing with the formation of esters. Complementary mechanism studies employing CV, IC, titration, and GC revealed that 50% of the released chloride is transferred to chloro acetals. Scaling up the reaction from a 0.218 g HCH to a 21.8 g HCH scale system with a stacked electrode arrangement increased the electrode surface area by a factor of 167, allowing a significant shortening of electrolysis time while maintaining product yields. For 2-chloromethyl-1,3-dioxepane, product accumulation was further investigated: repeated substrate addition enabled concentrations of up to 0.9 mol·L–1 within the electrolysis mixture. Direct electrolyte reusability was confirmed after more than 15 cycles. These results highlight the potential of electrosynthesis as a scalable and sustainable method for converting HCH to valuable platform chemicals. The integration of mechanistic insight, process optimization, and scale-up paves the way for future technical implementation.