Homolytic Cleavage Energies of R−H Bonds Centered on Carbon Atoms of High Electronegativity:  First General Observations of <i>O</i>-type Variation on C−H BDEs and the Implication for the Governing Factors Leading to the Distinct <i>O/S</i> Patterns of Radical Substituent Effects

Abstract

A systematic investigation of the methylene C−H bond dissociation energies (BDEs) of the onium-substituted toluene series (1−6) and of the adjacent electron-pulling group-substituted acetophenone and fluorene series (7 and 8) (Chart 1) was carried out using a modified eletrochemical method (eq 1) incorporating the pKa's measured in DMSO solution. This provided the first opportunity to examine the stabilization effect of substituents on carbon radicals (or C−H bonds) of varying degrees of electron deficiency. The relative BDE (ΔBDE) values estimated for these substrates within a family showed that for most systems, the adjacent or remote electron-withdrawing groups (EWGs) were found to strengthen the benzylic (or methylene) C−H bonds (i.e., the O-type), which is in sharp contrast to the universally observed C−H bond-weakening effect of EWGs (i.e., the S-type) in the literature. This general phenomenon reveals that it is the apparent electronegativity of the methylene carbon, rather than the nonbonded electron pair as suggested in the literature, that governs the direction of radical substituent effects.