Evidences in favour of a single electron transfer (SET) mechanism in the TiO2 sensitized photo-oxidation of α-hydroxy- and α,β-dihydroxybenzyl derivatives in water

• Published in: Physical chemistry chemical physics : PCCP Vol. 12; no. 2; pp. 5425 - 543
• Main Authors: Bettoni, Marta, Giacco, Tiziana Del, Elisei, Fausto, Rol, Cesare, Sebastiani, Giovanni V
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 01.01.2010
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Surface physical chemistry; Binary compound; Electron transfer; Photochemical reaction; Formaldehyde; Mechanism; Acetophenone; Chelation; Oxidation; Flash photolysis; Propanediol; Diol; Semiconductor materials; Benzaldehyde; Transition element compounds; Benzyl alcohol; Aldehyde; Ketone; Substrate; Adsorption; Benzenic compound; Kinetics; Aqueous solution; Titanium oxide; Laser photolysis; Electrons
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/b920785f

The TiO 2 photosensitized oxidation in water of a series of X-ring substituted benzyl alcohols gives the corresponding benzaldehyde. Kinetic evidence (from competitive experiments) suggests a single electron transfer (SET) mechanism with a changeover of the electron abstraction site from the aromatic moiety (X = 4-OCH 3 , 4-CH 3 , H and 3-Cl) to the hydroxylic group (X = 3-CF 3 and 4-CF 3 ), probably due to the preferential adsorption of the above OH group on the TiO 2 surface. The same photo-oxidation of a series of 1-(X-phenyl)-1,2-ethanediols and of 2-(X-phenyl)-1,2-propanediols gives the corresponding benzaldehyde and acetophenone, respectively, accompanied by formaldehyde, whereas a series of symmetrically X-ring-substituted 1,2-diphenyl-1,2-ethanediols yields the corresponding benzaldehyde (substrate/product molar ratio = 0.5). The relative rate values suggest a SET mechanism in all of the series, with electron abstraction from one of the two OH groups of all the considered diols, probably due to the much higher adsorption of the above groups (due to the chelation effect) on the semiconductor. Further confirmation of this mechanistic behaviour has been obtained from laser flash photolysis experiments. A SET mechanism from the aromatic ring or the OH group in the photo-oxidation of alcohols and 1,2-diols in water is suggested. The electron abstraction site is related to the extent of the OH adsorption.