Levelling of substituent effects on acidities in the gas phase compared with those in solution

Abstract

Substituent effects on a series of ionization reactions of neutral and positively charged acids have been studied in different media (gas phase, water and dimethyl sulphoxide) through linear plots of relative free energies (δ_×ΔG) using the ionization of benzoic acids in the same medium as a reference. While the absolute magnitudes of δ_×ΔG values in solution are lower than in the gas phase (this effect being well known as ‘solvent attenuation’), we show that ρ values in the gas phase are levelled to values close to unity for all reactions considered, while in solution they are spread over a range of values. Therefore, if substituent effects in a given system are calculated by making reference to an equilibrium in the same system, structural effects on reactivities are actually better differentiated in solution than in the gas phase.