LEVELLING OF SUBSTITUENT EFFECTS ON ACIDITIES IN THE GAS PHASE COMPARED WITH THOSE IN SOLUTION

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 (deltaDeltaG) using the ionization of benzoic acids in the same medium as a reference. While the absolute magnitudes of deltaDeltaAG values in solution are lower than in the gas phase (this effect being well known as 'solvent attenuation'), we show that rho 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.