Mechanistic studies and applications of photochemical fluorination of organic compounds using TiO[sub 2]

Thesis/Dissertation · Fri Jan 01 04:00:00 EST 1993

OSTI ID:7242849

Lai, C

The mechanism of the photo-Kolbe fluorination of carboxylic acids, RCOOH [yields] RF, at colloidal TiO[sub 2] suspensions in acetonitrile was studied by laser flash photolysis/transient absorbance spectroscopy. Transient absorption spectra of Ph[sub 3]C[sup +], Ph[sub 3]C, Ph[sub 2]CH and Ph[sub 2]CH[sup +] were observed from photo-oxidation of Ph[sub 3]CCOOH and Ph[sub 2]CCOOH at TiO[sub 2] using 355 nm excitation. The carbocations reacted rapidly with fluoride, but the neutral radicals did not. The photo-oxidation of Ph[sub 3]CCOOH to Ph[sub 3]C at TiO[sub 2] occurred via a single-photon process, while the formation of PH[sub 3]C[sup +] required two photons. Fluoride is strongly adsorbed at the TiO[sub 2] surface in both aqueous and acetonitrile solutions, causing a negative shift in the flat-band potential of TiO[sub 2] relative to that found with non-adsorbing anions. In the photochemical fluorination reaction, the oxidizing power of photogenerated holes at the semiconductor surface 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. The adsorption of fluoride ion at the surface of TiO[sub 2] in aqueous solutions was studied by electrochemical methods. The method of photo-Kolbe decarboxylation of carboxylic acids, RCOOH, at illuminated TiO[sub 2] to produce an intermediate R radical was applied to the trifluoromethylation reaction. Silver trifluoroacetate was used as a source of CF[sub 3] and powdered TiO[sub 2] as photocatalyst. Electrophilic substitution of the trifluoromethyl group for hydrogen occurred, preferentially at electron-rich positions on arene rings.

• Research Organization: Texas Univ., Austin, TX (United States)
• OSTI ID: 7242849
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400500* -- Photochemistry
ABSORPTION SPECTRA
CARBOXYLIC ACIDS
CHALCOGENIDES
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHEMISORPTION
FLUORINATION
HALOGENATION
KINETICS
ORGANIC ACIDS
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
REACTION KINETICS
SEPARATION PROCESSES
SORPTION
SPECTRA
TITANIUM COMPOUNDS
TITANIUM OXIDES
TRANSITION ELEMENT COMPOUNDS