Evidence for Carbocation Intermediates in the TiO2-Catalyzed Photochemical Fluorination of Carboxylic Acids

Cuiwei Lai, Yeong Il Kim, Chong Mou Wang, Thomas E. Mallouk

研究成果: 雜誌貢獻文章

16 引文 斯高帕斯(Scopus)

摘要

Laser flash photolysis/transient absorbance spectroscopy was used to determine the mechanism of photo-Kolbe fluorination of carboxylic acids, RCOOH → RF, at colloidal TiO2 suspensions in acetonitrile. Transient absorption spectra of Ph3C+, Ph3C·, Ph2CH· and Ph2CH+ were observed from the photooxidation of Ph3CCOOH and Ph2CCOOH at TiO2 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 Ph3CCOOH to Ph3C· at TiO2 occurred via a single-photon process, while the formation of Ph3C+ 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 TiO2 surface in acetonitrile solution, the oxidizing power of photogenerated holes could be increased by coordinating HF to F, 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.

原文英語
頁(從 - 到)1393-1399
頁數7
期刊Journal of Organic Chemistry
58
發行號6
DOIs
出版狀態已發佈 - 1993 一月 1

ASJC Scopus subject areas

  • Organic Chemistry

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