Effects of adjacent acceptors and donors on the stabilities of carbon-centered radicals

The radical stabilization energies (RSEs) for 21 radicals, GCH{sub 2}{sup {sm_bullet}}, where G is an electron donor or acceptor group, were estimated from differences in homolytic C-H bond dissociation energies ({Delta}BDEs relative to that of methane). These RSEs were found to agree reasonably well in order with theoretically calculated RSEs that have been reported and with those obtained by averaging RSEs from nine different methods (a literature RRS{sub x} scale). But the RSEs estimated from {Delta}BDEs were almost without exception larger because the theoretical calculations and RRS{sub x} method greatly underestimate the size of most RSEs. The RSEs for GCH{sub 2}P{sup {sm_bullet}} radicals were found to be enhanced to the extent that G is able to delocalize an odd electron on an adjacent carbon atom and were diminished by the presence of electron-withdrawing properties in G. Most groups stabilize the radical, but when the electron-withdrawing effect of the group is large, as in F{sub 3}C{sup {sm_bullet}} and Me{sub 3}N{sup +}CH{sub 2}{sup {sm_bullet}} radicals, the net effect is to destabilize the radical relative to the methyl radical. The RSEs of 12 radicals of the type RSC{sup {sm_bullet}}HG or PhSC{sup {sm_bullet}}HG, relative to that of the GCH{sub 2}{sup {sm_bullet}} radical, were foundmore » to increase progressively as the acceptor group G was changed along the series Ph, fluorenyl (Fl), CO{sub 2}Et, CN, COMe, and COPh. The RSEs of these donor-acceptor radicals were all smaller than the sum of the RSEs of the singly-substituted radicals, GCH{sub 2}{sup {sm_bullet}} and PhSCH{sub 2}{sup {sm_bullet}}. The effects of a second PhS or like donor and of a second acceptor on the RSEs were also determined. 39 refs., 6 tabs.« less