DYNAMICS OF IONIZATION REACTIONS OF BETA-SUBSTITUTED RADICALS - SUBSTITUENT AND SOLVENT EFFECTS

The dynamics of reactions of carbon-centered 1-arylalkyl radicals with bromine or chlorine attached to the carbon adjacent (beta) to the rad ical center have been examined using nanosecond laser flash photolysis . The primary reaction of the radicals containing the electron-donatin g 4-methoxy group on the phenyl ring is highly dependent on the solven t composition. In weakly ionizing solvents such as acetonitrile, the r adicals decay in a second-order manner indicating that coupling of two radical centers is the primary mode of radical decay. However, when t he ionizing ability of the solvent is increased by addition of water, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), or 2,2,2-trifluoroethanol (T FE), heterolysis of the P-substituent becomes the dominant mode of dec ay. The occurrence of the heterolysis reaction is demonstrated unambig uously by direct observation of the radical cation produced as the pri mary, heterolysis product. The rate constants for and yield of the het erolysis reaction are found to be dependent on both the solvent ionizi ng ability and radical structure. In neat water or neat HFIP the react ions become extremely fast and occur with rate constants in the 10(7) s(-1) to greater than or equal to 10(8) s(-1) range. For the beta-brom ophenethyl and beta-bromo-4-methyphenethyl radicals, no heterolysis is observed even under strongly ionizing conditions, indicating that the rate constant for ionization is strongly influenced by the substituen t on the phenyl ring. For radicals with an additional beta-phenyl subs tituent, rapid heterolysis takes place leading to the formation of the stilbene radical cation. The formation of a radical/radical cation eq uilibrium was observed under the appropriate conditions only for the 4 -methoxyphenethyl radical derivatives.