Evidence for carbocation intermediates in the TiO[sub 2]-catalyzed photochemical fluorination of carboxylic acids
- Univ. of Texas, Austin (United States)
In the past few years, laser flash photolysis studies of colloidal TiO[sub 2] suspensions have been used to determine the mechanism of photochemical reactions relevant both to solar energy conversion and to environmental remediation. Laser flash photolysis/transient absorbance spectroscopy was used to determine the mechanism of photo-Kolbe fluorination of carboxylic acids, RCOOH [yields] RF, at colloidal TiO[sub 2] suspensions in acetonitrile. Transient absorption spectra of Ph[sub 3]C[sup +], Ph[sub 3]C[sup [sm bullet]], Ph[sub 2]CH[sup [sm bullet]] and Ph[sub 2]CH[sup +] were observed from the photooxidation of Ph[sub 3]CCOOH and Ph[sub 2]CCOOH at TiO[sub 2] using 355-nm excitation. Transient decays, monitored in the presence and absence of fluoride ions, showed that the carbocations reacted rapidly with fluoride, but the neutral radicals did not. By varying the laser intensity, it was found that the photooxidation of Ph[sub 3]CCOOH to Ph[sub 3]C[sup [sm bullet]] at TiO[sub 2] occurred via a single-photon process, while the formation of Ph[sub 3]C[sup +] required two photons. This finding is in agreement with the parabolic light intensity dependence of initial reaction rates in bulk photolysis experiments. Although fluoride is strongly adsorbed on the TiO[sub 2] surface in acetonitrile solution, the oxidizing power of photogenerated holes could be increased by coordinating HF to F[sup [minus]], and therefore the threshold for oxidative photochemical fluorination was extended to more positive potentials. In this way less easily oxidized carboxylic acids RCOOH could be converted to RF. 20 refs., 8 figs., 2 tabs.
- OSTI ID:
- 6398404
- Journal Information:
- Journal of Organic Chemistry; (United States), Journal Name: Journal of Organic Chemistry; (United States) Vol. 58:6; ISSN 0022-3263; ISSN JOCEAH
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400201* -- Chemical & Physicochemical Properties
ABSORPTION SPECTROSCOPY
ACETONITRILE
BOSONS
CARBOXYLIC ACIDS
CATALYSIS
CATALYTIC EFFECTS
CHALCOGENIDES
CHARGED PARTICLES
CHEMICAL REACTION YIELD
CHEMICAL REACTIONS
CHEMISTRY
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
ENERGY-LEVEL TRANSITIONS
EXCITATION
FLUORINATION
HALOGENATION
HOLES
IONS
LASER RADIATION
MASSLESS PARTICLES
NITRILES
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC FLUORINE COMPOUNDS
ORGANIC HALOGEN COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PHOTOCHEMISTRY
PHOTONS
POTENTIALS
RADIATIONS
RADICALS
SPECTROSCOPY
TITANIUM COMPOUNDS
TITANIUM OXIDES
TRANSITION ELEMENT COMPOUNDS
YIELDS