HYDRIDE AFFINITIES OF ORGANIC RADICALS IN SOLUTION - A COMPARISON OF FREE-RADICALS AND CARBENIUM IONS AS HYDRIDE ION ACCEPTORS

Hydride affinities of free radicals (R.) in solution can be obtained from a thermochemical cycle which gives rise to (i) (-DELTAG(hydride)(R.)s = DELTAG(hom)(R-H)S - FDELTAE-degrees(NHE)[(H>/H-) - (R-H/R-H.-)]S), in which -DELTAG(hydride(R.)S is equated to the difference between the free energy change of homolytic bond dissociation in solution (DELTAG(hom)(R-H)S) and a term involving the standard potentials of the (H./H-) and (R-H/R-H.-) couples, FDELTAE-degrees(NHE)[(H./H-) + (R-H/R-H.-)]S. Substitution in (i) for DELTAG(hom)(R-H)S results in (ii) (-DELTAG(hydride(R.)S = 2.303RT(pK(a))(R-H)S + FE-degrees(NHE)[(R./R-) + (R-H/R-H.-)]S + C), which equates DELTAG(hydride)(R.)s to experimentally accessible quantities plus a constant C. For a given solvent S, C is equal to -FE-degrees(NHE)[(H+/H.) + (H./H-)]s, which has previously been evaluated for water, acetonitrile, and dimethyl sulfoxide. The difference in the free energy changes for heterolytic dissociation of the R-H bond in a neutral compound and the corresponding anion radical is given by (iii) (DELTADELTAG(hydride)(R. - R+)S = -FDELTAE-degrees(NHE)[(R-H/R-H.-)-(R+/R.)]S). Since the differences in standard potentials for reduction of R+ and R-H are large, usually greater than 2 V, DELTADELTAG(hydride)(R.-R+)S will generally equal greater-than-or-equal-to 50 kcal/mol. Equations ii and iii are applied to the reactions of 10-substituted 9-methylanthracene and substituted toluene radical anions in dimethyl sulfoxikde and acetonitrile. A general relationship between DELTAG(hydride(R. or R+)S and the electron affinity (of R-H or R+) in solution, which arises from the near cancellation of the other terms in (ii), is discussed. Absolute values for the hydride affinities of several NAD+ models in acetonitrile are estimated.