Figure 4 shows a representation of the solvent effect. The dielectric constant of the solvent has a direct influence on the reaction spontaneity, with water having a higher effect, followed by tetrahydrofuran and then toluene. Although this would be expected, considering the values of the dielectric constant for these solvents, it is interesting to observe that although the tetrahydrofuran dielectric constant is closer to that of toluene than to the dielectric constant of water, the effect of tetrahydrofuran on the reaction Gibbs free energy is closer to that of water than to that of toluene. Additionally, the effect is higher for the smaller anions, which are capable of polarizing the solvent more strongly than the bigger ones.
Figure 4. Solvent effect on the Gibbs free energy for the A- + CO2 reactions. The solvent effect is the difference between the Gibbs free energy in the solvent and the corresponding one in the gas-phase.
The interaction with the solvent decreases the basicity and, consequently, the reactivity of the anions. The free anions are therefore more strongly stabilized by the solvent than the adducts, resulting in a less spontaneous reaction. In turn, the solvent effect is higher for smaller anions such as F-, H- and OH- because they have a more concentrated charge density, increasing polarization of the solvent. However, the reactions are still exergonic in all solvents, except for C2H3S- and SH- which are the weakest bases.
The Gibbs free energy changes for the SiH3-, C2H3NH-, C2H3PH- and CH3PH- anions are the less strongly influenced by the solvent. For C2H3NH-, C2H3PH- and CH3PH-, as the charge is already dispersed in the free anion, both the anion and the adduct are equally stabilized by the solvent. For SiH3-in toluene the free anion and the adduct are equally stabilized while in water the adduct is slightly more stabilized than the anion.