Stabilizing and destabilizing effects of structural changes on radical stabilities

Abstract

A simple method of estimating homolytic bond dissociation energies (BDEs) of the acidic H-A bonds in weak acids has been developed by combining their pK HA values in DMSO with the oxidation potentials of their conjugate bases, A - . Differences in BDEs between H-A 1 and H-A 2 bonds have been found to give good measures of the relative radical stabilization energies (RSEs) of the corresponding radicals, A 1 * and A 2 *. Application of the method to weak acids with remote or proximate substituents has revealed that substituents play a dual role in affecting radical stabilities - stabilizing to the extent that they delocalize an odd electron, and destabilizing to the extent that they exert an electron-withdrawing effect. Estimates of BDEs (or RSEs) have provided new insights into the stabilizing effects in radicals of the type A-C*-D and A-N*-D, where A is an acceptor group, such as PhCO, and D is a donor group, such as Me 2 N. The stabilities of heteroallylic radicals of the type 0 = C-0*, 0 = C-C*, S = C-N* and O-C-N* have been found to vary with the electronegativities of the atoms at the termini in support of a theoretical prediction.