HOMOLYTIC BOND-DISSOCIATION ENERGIES OF ACIDIC C-H BONDS ACTIVATED BY ONE OR 2 ELECTRON-ACCEPTORS

Estimates of homolytic bond dissociation energies (BDEs) have been made for the acidic H-C bonds in CH3SO2Ph, CH3SO2CF3, CH3CO2Et, CH3COCH3, CH3COPh, and CH3NO2 by combining their pK(HA) values with the E(ox)(A-) values of their conjugate bases, both measured in DMSO. Similar estimates were made for the BDEs of the acidic H-C bond in 17 compounds of the type GCH2G and GCH2G' where G and G' are both one of these functions or SO2CH3 or CN. Most of the BDEs for these methylene compounds bearing two electron acceptors fall in the range of 90-103 kcal/mol. The BDEs are generally within 3 kcal/mol of half the sum of the GCH2-H and G'CH2-H BDEs. The small effects caused by introduction of a second electron acceptor are rationalized in terms of the leveling effect of the first substituent and the dual nature of the effects most acceptors have on the stabilities of adjacent radicals, i.e., stabilizing by virtue of their delocalizing ability and destabilizing by virtue of their field/inductive electron-withdrawing ability. The E(ox)(A-) values of PhCH2SO2G compounds, where G = Me, t-Bu, N(Me)Ph, CH2Ph, Ph, OPh, and CF3, plot linearly with Pk(HA) and all have BDEs of 90 +/- 1 kcal/mol.