Carbon-Hydrogen Bond Dissociation Energies in Alkylbenzenes. Proton Affinities of the Radicals and the Absolute Proton Affinity Scale

Article abstract of DOI:10.1021/ja00365a002

Rate constants (k) were measured for proton-transfer reactions from alkylbenzene ions RH⁺ to a series of reference bases B, i.e., RH⁺ + B -> BH⁺ + R*.For exothermic reactions (ΔH -1.For example, the reaction C6H5CH3⁺ + B -> BH⁺ + C6H5CH2* is fast (reaction efficiency = k/k_col >/= 0.5) when B = MeO-t-Bu or stronger bases, but k/k_col is significantly smaller when B is n-Pr2O or weaker bases.From the falloff curve of reaction efficiency vs.PA(B), we find PA(n-Pr2O) = PA(C6H5CH2*) + 0.8 kcal mol^-1 = 200.0 kcal mol^-1.Since PA(C6H5CH2*) is obtained from known thermochemical data, this relation defines the absolute PA of n-Pr2O.Through a ladder of known PA, we then obtain PA(i-C4H8) = 186.8 kcal mol^-1; we also obtain the absolute PAs of other oxygen bases.Falloff curves of reaction efficiencies of 3-FC6H4CH3⁺, C6H5C2H5⁺, C6H5-n-C3H7⁺, and C6H5-i-C3H7⁺ with these reference bases give then the following PAs of R* and R-H bond dissociation energies (D^o) (all in kcal mol^-1) as R*, PA(R*), D^o(R-H): 3-FC6H4CH2*, 197.2, 89.4; , 197.9, 86.2; , 199.1, 86.1; , 199.6, 86.1.In similar manner, rate constants for H⁺ transfer from C6H5NH2⁺ to reference pyridines and amines yield PA(C6H5NH*) = 221.5 and D^o(C6H5NH-H) = 85.1 kcal mol^-1 (1 kcal mol^-1 = 4.18 kJ mol^-1).