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Photo‐oxidation of some benzylic alcohols sensitized by colloidal TiO 2 in CH 3 CN. A kinetic mechanistic study through quantum yield determinations
Author(s) -
Del Giacco Tiziana,
Ranchella Michele,
Rol Cesare,
Sebastiani Giovanni V.
Publication year - 2000
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/1099-1395(200011)13:11<745::aid-poc312>3.0.co;2-s
Subject(s) - chemistry , kinetic isotope effect , deprotonation , quantum yield , steric effects , intramolecular force , benzyl alcohol , electron transfer , photochemistry , yield (engineering) , reaction rate constant , deuterium , medicinal chemistry , adsorption , alcohol , stereochemistry , kinetics , catalysis , organic chemistry , thermodynamics , ion , physics , quantum mechanics , fluorescence
The quantum yields (ϕ obs ) relating to the colloidal TiO 2 ‐sensitized photo‐oxidation of ring methoxy‐substituted benzylic alcohols (D) were determined. The true quantum yields (ϕ 0 ) were obtained from a Langmuir–Hinshelwood isotherm treatment of ϕ obs at different [D]. In the light of the suggested photo‐oxidation mechanism, an equation was deduced where ϕ 0 is expressed as a function of the electron transfer ( k et ), back electron transfer ( k −et ) and benzylic deprotonation ( k p ) rate constants. In particular, the lower ϕ 0 value of 3‐CH 3 O‐ vs 4‐CH 3 O‐benzyl alcohol ( 1 , with lower E p ) should principally depend on the difference in k et , while the decrease in ϕ 0 on going from alcohol 1 to the α‐methyl derivative ( 4 ) should be due to the difference in k p (stereoelectronic effect). The adsorption equilibrium constants under irradiation ( K D ) were also obtained from the above equation and the values are similar for the considered substrates, except for 4 . In fact, this substrate shows a lower K D value, probably because of the steric hindrance of the methyl group to the OH adsorption (preferential site) on TiO 2 . Finally, both the inter‐ and intramolecular deuterium isotope effect ( k H / k D = 1.8 and 1.6, respectively) are consistent with a kinetically significant C α —H bond breaking following the electron‐transfer step. Copyright © 2000 John Wiley & Sons, Ltd.