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Reaction of the OH Radical with Tetrachloroethene and Trichloroacetaldehyde (Hydrate) in Oxygen-free Aqueous Solution
Mertens, Ralf,Sonntag, Clemens von
, p. 2181 - 2186 (2007/10/02)
Hydroxyl radicals are generated radiolytically in NO2-saturated aqueous solutions and are reacted with either tetrachloroethene or trichloroacetaldehyde hydrate.The primary products after irradiation are tetrachlorosuccinic acid (G = 2.3*10-7 mol J-1 and 2.7*10-7 mol J-1, respectively) and chloride ions (G = 11.6*10-7 mol J-1 and 7.3*10-7 mol J-1, respectively).The former decomposes into trichloroacrylic acid and carbon dioxide (Ea = 115 kJ mol-1).From tetrachloroethene some dichloroacetic acid is also formed (G = 0.2*10-7 mol J-1).Trichloroacetic acid is not among the products.Kinetic studies were carried out by pulse radiolysis.In the case of trichloroacetaldehyde hydrate there is a jump in conductivity increase of G(H+) ca. 2*G((OH(radical)) within not greater than 5 μs and no further increase in conductivity at later times.It is concluded that the OH-radicals abstract the carbon-bound H-atom and the resulting CCl3C(OH)2 radicals rapidly eliminate HCl forming CCl2CO2H radicals which dissociate and dimerize yielding tetrachlorosuccinic acid.In the case of the reaction of the OH-radical with tetrachloroethene the conductivity build-up is biphasic.In the first step one equivalent H+ is formed when the primary CCl2CCl2OH radical eliminates HCl yielding the CCl2C(O)Cl radical (k > 7*105 s-1).Upon dimerization (2 k = 6.9*108 dm3 mol-1 s-1) Cl(O)CCCl2CCl2C(O)Cl is formed which hydrolyses (k = 5 s-1).Only one conductivity equivalent is liberated, and it is concluded that the ensuing monoanion rapidly eliminates a chlorine ion (no conductivity change) yielding tetrachlorosuccinic anhydride.It subsequently hydrolyses at a later stage, not resolved by pulse radiolysis, to tetrachlorosuccinic acid.
