Dynamics of Ionization Reactions of β-Substituted Radicals. Substituent and Solvent Effects
journal contributionposted on 05.11.1997, 00:00 by Frances L. Cozens, Melanie O'Neill, Roumiana Bogdanova, Norman Schepp
The dynamics of reactions of carbon-centered 1-arylalkyl radicals with bromine or chlorine attached to the carbon adjacent (β) to the radical center have been examined using nanosecond laser flash photolysis. The primary reaction of the radicals containing the electron-donating 4-methoxy group on the phenyl ring is highly dependent on the solvent composition. In weakly ionizing solvents such as acetonitrile, the radicals decay in a second-order manner indicating that coupling of two radical centers is the primary mode of radical decay. However, when the 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 (TFE), heterolysis of the β-substituent becomes the dominant mode of decay. The occurrence of the heterolysis reaction is demonstrated unambiguously by direct observation of the radical cation produced as the primary, heterolysis product. The rate constants for and yield of the heterolysis reaction are found to be dependent on both the solvent ionizing ability and radical structure. In neat water or neat HFIP the reactions become extremely fast and occur with rate constants in the 107 s-1 to ≥ 108 s-1 range. For the β-bromophenethyl and β-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 substituent on the phenyl ring. For radicals with an additional β-phenyl substituent, rapid heterolysis takes place leading to the formation of the stilbene radical cation. The formation of a radical/radical cation equilibrium was observed under the appropriate conditions only for the 4-methoxyphenethyl radical derivatives.