Chlorine atom initiated oxidation of chlorinated ethenes:: Results for 1,1-dichloroethene (H2C=CCl2), 1,2-dichloroethene (HClC=CClH), trichloroethene (HClC=CCl2), and tetrachloroethene (Cl2C=CCl2)

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 Cl-2, 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 x 10(13) molecule cm(-3)), Cl-2 and O-2. 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 O-2 is competitive with CCl and CC bond cleavage, so the yields depended on the concentration of O-2. With C2Cl4, the only products observed were CC3COCl 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 Cl-2, 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.