Reaction of Ozone with Ethene and Its Methyl- and Chlorine-Substituted Derivatives in Aqueous Solution

Byproduct formation in ozonation processes is of interest, especially in the area of drinking water, similar to the concerns regarding the health effects of disinfection byproducts generated by chlorine and other oxidants used for that purpose. Chlorinated olefins increasingly contaminate our drinking water resources, and the byproducts formed by ozone were determined here. For a better understanding of the underlying mechanisms, some other olefins were included in this study. The rate constants of the reaction of ozone with ethene and some of its methyl- and chlorine-substituted derivatives in aqueous solution have been measured by stopped-flow (rate constants in units of dm3 mol-1 s-1): tetramethylethene, >106; propene, 8 × 105; ethene, 1.8 × 105; buten-3-ol, 7.9 × 104; vinyl chloride, 1.4 × 104; trans-1,2-dichloroethene, 6.5 × 103; 1,1-dichloropropene, 2.6 × 103; cis-1,2-dichloroethene, 540; 1,1-dichloroethene, 110; trichloroethene, 14. From these data, it is concluded that the rate of reaction is mainly governed by the electron density of the CC double bond and that steric effects must be minor in comparison. In all cases, full material balances were obtained. They show that these olefins nearly exclusively follow the Criegee mechanism and cleave into a carbonyl compound and a hydroxyhydroperoxide, which in the case of a chlorine substituent in α-position rapidly loses HCl. The intermediate hydroperoxides were characterized by the kinetics of their reaction with molybdate-activated iodide ions. In unevenly substituted olefins, the substituent strongly decides on the pathway taken, e.g., propene yields formaldehyde and hydroxyethylhydroperoxide while vinyl chloride forms hydroxymethylhydroperoxide and formyl chloride (→ CO + HCl). Even with the highly substituted ethenes partial cleavage products (indicated by an asterisk) never exceed 10%. Intermediate ozonides must have a lifetime <2 ms as shown by stopped-flow conductometry. The products (yields per mole of ozone in parentheses) are the following. From ethene: formaldehyde (2.04) including hydroxymethylhydroperoxide (1.08) → formaldehyde + H2O2; propene: formaldehyde (1.03), hydroxyethylhydroperoxide (0.99) → acetaldehyde (0.97) + H2O2; vinyl chloride: HCl (1.05), hydroxymethylhydroperoxide (1.06) → formaldehyde (1.04) + H2O2, formic acid (0.06) precursors: formyl chloride (0.04) and formic peracid, 0.029, CO (1.01) precursor:formyl chloride (→ CO + HCl); cis- and trans-1,2dichloroethene (identical yields): HCl (2.02), formate (1.01) precursor: formic peracid (0.99); rate of hydrolysis 1.6 × 10-4 s-1, CO (1.08) precursor: formyl chloride, CO2 (0.02) precursor: formic peracid; 1,1-dichloroethene: HCl (1.95), formaldehyde (0.96, precursor: hydroxymethylhydroperoxide (0.96) → formaldehyde (0.96) + H2O2, CO2 (0.90) precursor: phosgene, chloroacetic acid* (0.08), glycolic acid* (0.07), formate (0.03), dichloroacetaldehyde* (≤0.01); 1,1-dichloropropene: HCl (2.05), CO2 (1.01) precursor: phosgene, hydroxyethylhydroperoxide (0.88) → acetaldehyde (1.03) + H2O2; trichloroethene: HCl (2.87), CO2 (0.95) precursor: phosgene, formate (0.82) precursor: formic peracid (0.88), CO (0.04) precursor: chloroformic peracid), dichloroacetic acid* (0.04). Some experiments were also done with tetramethylethene: acetone (1.74) [the precursor of half of the acetone, 2-hydroxypropyl-2-hydroperoxide reacts too slowly with activated iodide to be quantified, 2,3-dimethyl-2,3-dihydroxybutane* (ca. 0.1). These results show that full mineralization is not achieved.