A study of the OH-initiated oxidation of chlorinated ethenes in the gas phase

Abstract

Rate constants for the reaction of OH radicals with trans-1,2-dichloroethene and 1,1-dichloroethene have been determined at a pressure of 1.6 Torr and at T = 298 ± 2 K using the discharge-flow technique. The same method was used to study for the first time the kinetics of the reactions of atomic Cl with trans-1,2-dichloroethene and with 1,1-dichloroethene at low pressure. The values obtained for the rate constants for the Cl-atom reactions are significantly lower than those published previously, which refer to atmospheric pressure. The rate coefficients are therefore likely to be strongly pressure dependent over the range 1.6–760 Torr. The value obtained at low pressure may be considered as the upper limit of the rate constant for the abstraction channel. The kinetic data obtained are summarised below.

The release of atomic chlorine following the reaction of OH radicals with trans-1,2-dichloroethene, 1,1-dichloroethene and vinyl chloride has been observed by a direct method for the first time. The reaction of trans-1,2-dichloroethene with OH radicals gives a yield of Cl atoms that is essentially unity. The yield is independent of the presence of molecular oxygen, suggesting that Cl is released promptly after the addition of OH. Some release of Cl following the reaction of OH with 1,1-dichloroethene occurs in the absence of oxygen, although the release is enhanced by the presence of NO₂ in the system. The dominant channel leading to the formation of Cl appears not to be the initial reaction with OH, but rather the decomposition of an alkoxy radical that is formed in a secondary process. The presence of molecular oxygen increases the yield of Cl atoms, probably as a result of increased formation of the alkoxy radical. The lower limit of the yield in the absence of O₂ and NO₂ is estimated to be 0.03; the corresponding limit when O₂ is present is 0.05. Roughly 0.09 Cl atoms are released from vinyl chloride for each OH radical consumed. The release is due to the initial reaction of OH radicals, and the yield appears to correspond to the branching ratio for addition of OH to the more substituted end of the carbon–carbon double bond. FTIR spectroscopic studies were made of the products of reaction of Cl with 2-chloroethanol in the presence of O₂. This reaction provides a mimic of the reaction of OH with vinyl chloride in that the same peroxy radical species, CH₂(OH)CHClO₂, is formed. The only organic products observed are formyl chloride and chloroacetaldehyde. The formyl chloride results from the abstraction of an H atom from the carbon atom bearing the Cl substituent. Formaldehyde formed in the system is photolysed to produce CO, an observed product. Chloroacetaldehyde is formed following the abstraction of a hydrogen from the carbon atom bearing the OH group. The oxidation of vinyl chloride by OH in the atmosphere proceeds ia the formation of CH₂(OH)CH₂ClO₂. In the absence of NO_x, the peroxy radical reacts with RO₂ to yield the corresponding alkoxy radical. This alkoxy radical appears to decompose in exactly the same way as it does when it has been formed by the reaction of CH₂(OH)CH₂ClO₂ with NO.