ASSESSMENT OF THE IMPORTANCE OF CHANGES IN GROUND-STATE ENERGIES ON THE BOND-DISSOCIATION ENTHALPIES OF THE O-H BONDS IN PHENOLS AND THE S-H BONDS IN THIOPHENOLS

The homolytic bond dissociation enthalpies (BDEs) for the S-H bonds in 13 thiophenols, 2-naphthiol, and phenylmethanethiol have been estimated by combination of the values of their equilibrium acidities (pK(HA)s in DMSO) with the oxidation potentials of their conjugate anions, E(ox)(A(-)). An estimate of the average BDE for the S-H bond in seven thiophenols bearing meta- or para-electron acceptor substituents gave 79.8 +/- 1.6 kcal/mol; p-MeO and p-NH2 points deviate from this average by 2.9 and 10 kcal/mol, respectively. A Hammett plot of the BDE values for the S-H bonds in m- and p-GC(6)H(4)SH thiophenols (excluding the p-NH2 point) was linear with a slope of rho = 3.2 (r = 0.95). Similar plots for GC(6)H(4)NH(2) anilines and GC(6)H(4)OH phenols gave rho = 5.5 (r = 0.92) and rho = 7.3 (r = 0.95), respectively. From an analysis of these and related Hammett plots, and a comparison with similar plots for the GC(6)H(4)CH(2)CN family, the following conclusions were drawn: (a) meta- and para-electron-withdrawing substituents interacting with the S-H, N-H, and O-H dipoles cause lowering of ground-state energies leading to increases in BDEs in the order S-H < N-H < O-H, and (b) para-donor substituents exert effects to decrease BDEs of the acidic H-A bonds primarily by stabilizing the corresponding radicals but also to some extent by raising the ground-state energies in the ArOH, ArSH, and ArNH2 families. A plot of rho(ArX-Y) values for Hammett BDE correlations versus the differences in the electronegativities of the atoms forming the bonds being cleaved (Delta nu) is surprisingly linear for ArO-H, ArNH-H, ArS-H, ArO-CH3, ArCH(CN)-H, ArCH2-H, ArCH2-Br, and Ar-Cl bonds. Not unexpectedly, a plot of the oxidation potentials, E(ox)(HA), for 12 ortho-, meta-, or para-substituted thiophenols versus their pK(HA.)+ values is linear.