Mechanistic studies on photohydration reactions

The detailed mechanism of aromatic alkene photohydration was studied through investigation of the photochemical and photophysical processes of cyclic and acyclic styryl compounds. Cis-trans isomerization was determined to be a significant energy-wasting process for styrene in aqueous solution. Regeneration of reactant through deprotonation of the protonated cationic intermediate was shown not to be a significant pathway for the styryl compounds. Cyclic styrenes were shown to undergo photohydration reactions producing anti-Markovnikov addition products, as well as Markovnikov addition products which were seen in the reaction of acyclic styrenes. A mechanism is presented which accounts for the formation of these reaction products. An analysis of the remaining deactivational pathways of the excited alkenes was presented. From all these results, a complete energy reaction coordinate diagram with all energy gaps and possible pathways was constructed. Rates are given for all photochemical and photophysical pathways.;9-Alkyl and 9-arylxanthenols were shown to photodehydroxylate adiabatically in neutral aqueous solution to generate a cationic intermediate. From this result, it was generalized that cation formation from aromatic alkene photoprotonation may also occur adiabatically.;The photochemistry of thermally generated xanthyl cations was also studied. The cations were found to undergo excited-state quenching by water. Quenching rate constants were determined. For the 9-alkyl substituted xanthyl cations, the magnitude of the quenching rate constant corresponded to the degree of steric hindrance at the 9-position. An electronic effect was found for the 9-aryl substituted xanthyl cations. The quenching rates were found to be competitive with fluorescence rate constants. Quenching by water was invoked to explain the very weak fluorescence observed by the adiabatically formed cation.