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.