Journal of Physical Chemistry p. 9526 - 9536 (1994)
Update date:2022-08-04
Topics: Solvents Substituents Excitation wavelength Photochemical Formation Electrophilic Reactivities Vinyl Cations Anionic Leaving Groups Photoheterolysis Photohomolysis
Verbeek, Jan-Maarten
Stapper, Marion
Krijnen, Erik S.
Loon, Jan-Dirk van
Lodder, Gerrit
Steenken, Steen
Using product analysis and time-resolved laser flash photolysis techniques, the photochemistry at λexc ca. 250 and 310 nm (exc = excitation) of 1-(p-R-phenyl)-2-(2,2'-biphenyldiyl)vinyl halides (R = H, Me, MeO) in methanol and acetonitrile at room temperature was studied.The title compounds undergo photoheterolysis and photohomolysis to give vinyl cations =C+- (carbenium ions) and vinyl radicals =C.-.The cation : radical ratio increases with electron-donating strength of the substituent R, H < Me < MeO, and with anionic leaving group power of the halide, F- < Cl- < Br- < I-, indicating that the cleavage of the C-X bond to yield cation and halide anion proceeds, in the rate-determining step, by heterolysis and not by homolysis followed by electron transfer in the radical pair.The cation : radical ratio is solvent dependent: e.g., for the vinyl bromide with R = MeO and with λexc = 308 nm, in CH2Cl2 as solvent, only radical is observed, in comparison with only cation in the much more polar solvent acetonitrile.In acetonitrile-methanol mixtures, the absolute yields of both cation and radical go through a maximum as the methanol content is increased, however, the catio : radical ratio decreases continuously with increasing
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