Handbook of Solvents - George Wypych - ChemTech - Ventech!

28 Estanislao Silla, Arturo Arnau and Iñaki Tuñón
ing superficially the influence that the solvent has on one of the equilibria of greatest relevance,
the acid-base equilibrium.
The solvent can alter an acid-base equilibrium not only through the acid or basic character
of the solvent itself, but also by its dielectric effect and its capacity to solvate the different
species which participate in the process. Whilst the acid or basic force of a substance
in the gas phase is an intrinsic characteristic of the substance, in solution this force is also a
reflection of the acid or basic character of the solvent and of the actual process of solvatation.
For this reason the scales of acidity or basicity in solution are clearly different from
those corresponding to the gas phase. Thus, toluene is more acid than water in the gas phase
but less acid in solution. These differences between the scales of intrinsic acidity-basicity
and in solution have an evident repercussion on the order of acidity of some series of chemical
substances. Thus, the order of acidity of the aliphatic alcohols becomes inverted on passing
from the gas phase to solution:
R'
R'
in gas phase: R CH2 OH < R CH OH < R C OH
R'
R'
in solution: R CH2 OH > R CH OH >
R C OH
The protonation free energies of MeOH to t-ButOH have been calculated in gas phase
and with a continuum model of the solvent. 61 It has been shown that in this case continuum
models gives solvation energies which are good enough to correctly predict the acidity ordering
of alcohols in solution. Simple electrostatic arguments based on the charge
delocalization concept, were used to rationalize the progressive acidity of the alcohols when
hydrogen atoms are substituted by methyl groups in the gas phase, with the effect on the solution
energies being just the opposite. Thus, both the methyl stabilizing effect and the electrostatic
interaction with the solvent can explain the acid scale in solution. As both terms are
related to the molecular size, this explanation could be generalized for acid and base equilibria
of homologous series of organic compounds:
AH ⇔ A - +H +
B+H + ⇔BH +
In vacuo, as the size becomes greater by adding methyl groups, displacement of the
equilibria takes place toward the charged species. In solution, the electrostatic stabilization
is lower when the size increases, favoring the displacement of the equilibria toward the neutral
species. The balance between these two tendencies gives the final acidity or basicity ordering
in solution. Irregular ordering in homologous series are thus not unexpected taking
into account the delicate balance between these factors in some cases. 62
2.1.4.2 Solvent effects on the rate of chemical reactions
When a chemical reaction takes place in the midst of a solution this is because, prior to this,
the molecules of the reactants have diffused throughout the medium until they have met.
This prior step of the diffusion of the reactants can reach the point of conditioning the performance
of the reaction, especially in particularly dense and/or viscous surroundings. This
is the consequence of the liquid phase having a certain microscopic order which, although
R''
R''