LEVELING 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 elative free energies (delta(x)DELTA-G) using the ionization of benzoic acids in the same medium as a reference. While the absolute magnitudes of delta(x)DELTA-G 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.