Influence of TiO2 Surface on 1,2-Chlorine Shift in β-Chlorine Substituted Radicals As Studied by Radiation Chemistry and Photocatalysis

Article abstract of DOI:10.1021/j100200a058

The influence of surface-specific parameters on the photocatalytically induced oxidative degradation of halogenated hydrocarbons in aqueous TiO2 suspensions has been evaluated by comparing the results obtained in this heterogeneous system with those from γ-irradiated homogeneous aqueous solutions.A 1,2-chlorine shift known to occur in β-chlorinated alkyl radicals and the products obtained upon degradation of these radicals (particularly various chloroacetic acids) have been used as markers in these investigations.The results indicated that this chlorine shift, e.g., the rearrangement of CCl3-CH2(.) -> (.)CCl2-CH2Cl, occurs much slower (k in the order of 10⁶ s^-1) at the TiO2 surface than the homogeneous solution, where the present data confirm earlier rate constants of >/=10⁸ s^-1.This slowdown of the rearrangement process is attributed to steric hindrance in the surface-adsorbed state of the radicals.In the heterogeneous systems the rearrangement can, in fact, be interfered with by peroxidation of the unrearranged radical in the presence of molecular oxygen while such a competition cannot be achieved in the homogeneous solution even at high O2 concentrations.Experimentally, this has been demonstrated, for example, by the fate of the (.)CHCl-CCl3 radical generated upon oxidative C-H cleavage from 1,1,1,2-tetrachloroethane.Direct oxygen addition to this species yields the (.)OOCHCl-CCl3 peroxyl radical which eventually degrades into CCl3COOH.After rearrangement ((.)CHCl-CCl3 -> CHCl2-CCl2(.)) and subsequent peroxidation the then formed CHCl2-CCl2OO(.) peroxyl radical ends up in a completely different acid, namely, CHCl2COOH.It could further be deduced that the 1,2-chlorine shift occurs via a bridged mechanism without transient liberation of the chlorine atom, thereby rendering an alternatively possible chlorine elimination/readdition mechanism an unlikely event.Finally, a marked pH dependence of the product distribution in both the γ-radiolytic and photocatalytic systems is suggested to reflect acid/base catalyzed hydrolysis processes en route of the radical degradation to their final products.