Reaction of the OH radical with tetrachloroethene and trichloroacetaldehyde (hydrate) in oxygen-free aqueous solution
Hydroxyl radicals are generated radiolytically in N2O-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+)≈ 2 ×G(˙OH) within ⩽ 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 chloride ion (no conductivity change) yielding tetrachlorosuccinic anhydride. It subsequently hydrolyses at a later stage, not resolved by pulse radiolysis, to tetrachlorosuccinic acid.