An approach for assessing the relative importance of field and σ inductive contributions to polar substituent effects based on the non-proportionality of field and σ inductive substituent constants

Abstract

Evidence is presented showing that field effects and σ inductive effects are not proportional to one another and that field effects depend on the substituent group dipole moment while σ inductive effects depend upon group electronegativity. Correlations with σ_I and ι indicate that polar effects in a variety of systems are basically field effects. Problems associated with a priori calculations of field and σ inductive effects are discussed. In particular, it is suggested that the effective dielectric constant, D_E, should be evaluated empirically from gas-phase and solution acidities.