Homolytic Bond Dissociation Energies of the Acidic C-H Bonds in α-Substituted and 10-Substituted 9-Methylanthracenes and Their Related Radical Anions

Article abstract of DOI:10.1021/jo00063a025

Equilibrium acidities (pK_HA), reduction potentials E_re(HA), oxidation potentials E_OX(HA), and oxidation potentials of the conjugate anions E_OX(A^-) have been measured in dimethyl sulfoxide (DMSO) solution for 13 α-substituted 9-methylanthracenes (9-GCH2An) and 11 10-substituted 9-methylanthracenes (10-G-9-MeAn).The equilibrium acidities have been shown to be linearly correlated with Hammett ?^- constants.The similar ρ^- values for the linear correlations of pK_HA vs ?^- for 9-GCH2An (ρ^- = 13.8) and 10-G-9-MeAn (ρ^- = 14.2) demonstrate that the like substituents in 9-GCH2An and 10-G-9-MeAn have similar large effects on the equilibrium acidities.Suitable combinations of these equilibrium acidities with the reduction and oxidation potentials led to estimates of (a) the homolytic bond dissociation energies (BDE_HA) of the acidic C-H bonds in 9-GCH2An and 10-G-9-MeAn, (b) the homolytic bond dissociation energies (BDE_HA*-%) of the acidic C-H bonds in 9-GCH2An^*- and 10-G-9-MeAn^*- radical anions, and (c) the radical cation acidities (pK_HA*+%) of the 9-GCH2An^*+ and 10-G-9-MeAn^*+ radical cations.Introduction of α-CN and α-NO2 groups had opposite effects on the strengths (BDE_HA) of the acidic C-H bonds in 9-GCH2An; the α-CN group decreased the BDE_HA value by 2.5 kcal/mol and the α-NO2 increased the BDE_HA value by 4.5 kcal/mol, but they both weakened the acidic C-H bonds in 9-GCH2An^*- radical anions (by 10.6 and 12.5 kcal/mol, respectively).The BDEs of the acidic C-H bonds in 10-G-9-MeAn were estimated to be 81.3 +/- 1.3 kcal/mol and those in the corresponding radical anions were estimated to be 58.5 +/- 2.5 kcal/mol.The 9-GCH2An^*+ and 10-G-9-MeAn^*+ radical cations are strongly acidic with the pK_HA*+% values estimated in the range of -5 to -11 pK_HA*+% units.Seven linear free energy relationships have been observed.