Competitive kinetic experiments in the TiO2-sensitized photooxidation of some series of differently ring-substituted benzylic derivatives (ArCHROR) in aerated and/or deaerated CH3CN and in the presence of Ag2SO4 were carried out. From logkrel vs Ep plots it was hypothesized that a changeover of the electron abstraction site occurs on going from electron-donating to electron-withdrawing groups on the ring, from the aromatic moiety to the OCH3 group of benzyl methyl ethers, probably owing to the preferential adsorption of this group on TiO2 with respect to the aromatic ring. This phenomenon is not observed with benzyltrimethylsilanes because the orbital resulting from the overlap between the C— Si bond and the aromatic system is the only adsorption site. The steric hindrance of an - methyl group (in benzylic alcohols) or a tert-butyl group (in benzyl tert-butyl ethers) reduces the degree of adsorption but does not influence the shape of the plot because OR should still be the preferential adsorption group. This also provides useful information about the surface structure of TiO2 suspended in CH3CN.
Involvement of adsorption effects on the TiO2 sensitized photooxidation rate of benzylic derivatives in CH3CN
DEL GIACCO, Tiziana;ROL, Cesare;SEBASTIANI, Giovanni Vittorio
2003
Abstract
Competitive kinetic experiments in the TiO2-sensitized photooxidation of some series of differently ring-substituted benzylic derivatives (ArCHROR) in aerated and/or deaerated CH3CN and in the presence of Ag2SO4 were carried out. From logkrel vs Ep plots it was hypothesized that a changeover of the electron abstraction site occurs on going from electron-donating to electron-withdrawing groups on the ring, from the aromatic moiety to the OCH3 group of benzyl methyl ethers, probably owing to the preferential adsorption of this group on TiO2 with respect to the aromatic ring. This phenomenon is not observed with benzyltrimethylsilanes because the orbital resulting from the overlap between the C— Si bond and the aromatic system is the only adsorption site. The steric hindrance of an - methyl group (in benzylic alcohols) or a tert-butyl group (in benzyl tert-butyl ethers) reduces the degree of adsorption but does not influence the shape of the plot because OR should still be the preferential adsorption group. This also provides useful information about the surface structure of TiO2 suspended in CH3CN.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
