Chlorine Atom Initiated Oxidation of Chlorinated Ethenes:  Results for 1,1-Dichloroethene (H₂CCCl₂), 1,2-Dichloroethene (HClCCClH), Trichloroethene (HClCCCl₂), and Tetrachloroethene (Cl₂CCCl₂)

Abstract

Experiments have been carried out on the oxidation of the four chlorinated ethenes H2CCCl2, HClCCClH, HClCCCl2, and Cl2CCCl2. Reaction was initiated by continuous photolysis of Cl2, and product yields were measured using Fourier transform infrared (FTIR) spectroscopy. In all cases, experiments were performed at different concentrations of the chlorinated ethenes (down to 3 × 1013 molecule cm-3), Cl2 and O2. In the cases of H2CCCl2 and HClCCCl2, the product yields were invariant with these changes, consistent with them being determined by competitive unimolecular dissociations of the chlorinated alkoxy radical; that is, H2ClCCCl2O (HCl2CCCl2O) (+M) → H2ClCCOCl (HCl2CCOCl) + Cl (+M) and H2ClCCCl2O (HCl2CCCl2O) (+M) → COCl2 + CH2Cl (CHCl2) (+M), and the subsequent formation of HCOCl and/or COCl2 from CH2Cl and CHCl2. In the case of HCl2CCHClO formed from HClCCClH, H atom abstraction by O2 is competitive with CCl and CC bond cleavage, so the yields depended on the concentration of O2. With C2Cl4, the only products observed were CCl3COCl and COCl2, consistent with competitive dissociations of perchloroethyl radicals: CCl3CCl2O (+M) → CCl3COCl + Cl (+M); CCl3CCl2O (+M) → COCl2 + CCl3 (+M). However, the relative yields of CCl3COCl and COCl2 were found to depend on the initial concentration of C2Cl4 which is incompatible with the simple, and generally accepted, mechanism. To investigate this unexpected result further, experiments were performed on pentachloroethane (C2Cl5H). Again, the product yields depended on the initial concentration of the chlorinated compound. In addition, product yields from C2Cl4 were measured using two different rates of photolysis of Cl2, at three temperatures, 298, 353, and 393 K, and in the presence of added NO and added HCl. Although the experimental results could be modeled when bimolecular reactions of perchlorinated methoxy and ethoxy radicals with C2Cl4 and HCl were included in the reaction mechanisms, the values of the rate constants which were needed seem unrealistically large if the currently accepted rate for the unimolecular decomposition of CCl3CCl2O is correct. This observation on C2Cl4 is important since it casts doubt on the current view of the oxidative chemistry of this compound, which is released in significant amounts into the atmosphere.

Kinetics of the Self-Reactions of Peroxy Radicals Arising from Chlorine-Initiated Oxidation of Chloroethenes

“…2 Absorption traces exhibited a decay at all wavelengths, from 230 to 280 nm, followed by an apparent residual absorption, as shown in Figure 3. This absorption was accounted for by the regeneration of RO 2 due to the chain reaction initiated by the decomposition of the CHCl 2 O radical (reaction 10) into Cl atoms and CHClO.…”

“…The only end-product study available for this reaction was performed by Hasson and Smith. 2 As no evidence for the molecular channel 1b has been clearly characterized, we have considered that reaction 1a is the dominant channel in reaction 1 and thus, that no CHCl 2 C(O)Cl was produced from channel 1b. This assumption seems reasonable as similar branching ratios k 1a /k 1 ≈ 1 have been found in the case of most halogenated alkylperoxy radical self-reactions.…”

“…Nevertheless, experimental traces could never be satisfactorily simulated using the branching ratios proposed by Hasson and Smith. 2 In particular, the branching ratio k 7a /k 7 had to be set at a smaller value, with an upper limit of 0.65. Taking into account the complete mechanism, detailed in Table 3, the following rate constant was derived from seven measurements of k 1 , all listed in Table 4, along with the experimental conditions k 1 ) (3.6 ( 0.9) × 10 -12 cm 3 molecule -1 s -1 at 298 K, where the quoted uncertainties represent 95% confidence limits.…”

“…The endproducts of reaction 3 have been studied by several groups. 1,2,17,18 The alkoxy radical CCl 3 CCl 2 O presents two different decomposition pathways, channel 16a leading to the formation of CCl 2 O and channel 16b resulting in the formation of CCl 3 C(O)Cl and Cl atoms, leading to CCl 3 CCl 2 O 2 regeneration.…”

Matrix isolation and theoretical study on the photolysis of trichloroacetyl chloride

“…Since chlorinated acetyl chlorides play important roles in the photochemical reactions of chlorinated ethenes [1][2][3][4][5], their photolysis reactions have been studied actively, especially in environmental concern. In the photochemical reaction of acetyl chloride, CH 3 COCl, in matrices [6,7], acetyl chloride was photoexcited by irradiation of 266 nm laser to the S 1 state, which suffered internal conversion and decomposed to the HCl·ketene complex in the S 0 state [7]; four-center elimination mechanism was proposed for this process.…”