New evidence for through-space transmission of substituent effects in benzene derivatives

Abstract

Electrostatic interaction energies between dipolar substituents and dipolar or charged reaction sites are re-examined. At short interaction distances and given orientations, the point-dipole approximation is shown to introduce important errors. Exact expressions are derived for correcting current equations for both interaction types, including the Kirkwood–Westheimer equation. The parameter λ for describing para–meta ratios of substituent inductive or Electra effects in benzene derivatives is modelled in terms of electrostatic interaction energy. Using the new equations it is shown that parameter λ can take values smaller or greater than unity, approaching a value of 2 at very large distances. Experimental λ values are calculated using reliable literature data for the ionisation equilibria of substituted phenylethanoic and 3-phenylpropanoic acids in ethanol–water mixtures. Theoretical λ values for these reactions are obtained taking into account rotational and conformational changes in the corresponding carboxylate anions. From the good agreement between experimental and theoretical λ values, it is concluded that there is through-space transmission of substituent effects in benzene derivatives.