Inductive effects on proton affinity of benzene derivatives: analysis using fictitious hydrogen atoms

Pure inductive effects on the gas-phase basicity of seven benzene derivatives (3- and 4-substitution) are monitored in a continuous way using fictitious hydrogen atoms bearing an adjustable nuclear charge Z*. This approach (H* method) affords three main advantages over existing treatments: such entities are by definition purely inductive (without any underlying assumptions), use of empirical parameters is circumvented, and yet the method has been designed to remain particularly easy to use. We directly establish the linear dependence of proton affinities on inductive effects and, more quantitatively, measure accurate sensitivities rho(I)* analogous to Taft's coefficients. Functional centers exhibit contrasted values, up to a factor of 3, which finds an interpretation within the framework of the HSAB theory. The sensitivities rho(I)* for 3- and 4-substitution are quantified. The associated para/meta rho(I)* ratio ranges from 1.02 to 1.16 according to the functional center. These values, always slightly superior to unity, denote a contribution of pi electrons in the transmission of the inductive effect. This effect, first identified by Exner, is shown to account for ca. 30% of the basicity of benzoic acid, which is taken as an example.