Overlooked Chlorination of Aqueous Alcohols: Aldehyde Formation and Artifactual Chlorine Consumption

In environmental chemistry research, stock solutions of organic compounds are commonly prepared in alcohols (e.g., methanol), including during experiments to quantify oxidation kinetics and disinfection byproduct formation. Lacking an obviously oxidizable functional group, alcohols are tacitly assumed to be inert with respect to common water treatment oxidants and favored for their low cost, low toxicity, and miscibility with water. While attempting to duplicate a previous result reporting formaldehyde as a product of dimethylamine chlorination, we found that a substantial amount of formaldehyde was generated when the dimethylamine dosing solution was prepared in methanol but not when it was prepared in water. We further found that under conditions typical of chlorination kinetics experiments, aqueous methanol concentrations as low as 0.4% (v/v) could significantly deplete chlorine compared to methanol-free controls. In the presence of increasing methanol concentrations, chlorine depletion half-lives decreased to ∼4 h at 2% methanol. Finally, we examined the chlorination of five primary alcohols and one secondary alcohol, which all formed the corresponding carbonyls at comparable rates. These findings raise doubts about the routine use of alcohols as carrier solvents for the preparation of stock solutions in research on aqueous chlorination reactions and highlight alcohols as a potential source of aldehyde formation during chlorination.