Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 613
involves interactions not only between the solute and solvent but also among different molecules
present in the mixture; the latter type of contribution also plays a central role in the
solvation process. Among others, it results in significant deviations of the vapor pressure of
a mixture with respect to the ideal behavior established by Raoult’s law.
Solvation studies of solutes in mixed solvents have led to the conclusion that the
above-mentioned divergences may arise from the fact that the proportion of solvent components
may be significantly different around the solute and in the bulk solution. This would
be the case if the solute were preferentially surrounded by one of the mixture components,
which would lead to a more negative Gibbs energy of solvation. 1,96 Consequently, the solvent
shell around the solute would have a composition other than the macroscopic ratio.
This phenomenon is known as “preferential solvation”, a term that indicates that the solute
induces a change with respect to the bulk solvent in its environment; however, such a
change takes place via either non-specific solute-solvent interactions called “dielectric enrichment”
or specific solute-solvent association (e.g. hydrogen bonding).
Preferential solvation has been studied in the light of various methods, most of which
are based on conductance and transference measurements 96 , NMR measurements of the
chemical shift of a nucleus in the solute 97 or measurements of the solvatochromism of a solute
in the IR 98 or UV-Vis spectral region. 99 Plots of the data obtained from such measurements
against the composition of the bulk solvent (usually as a mole fraction) depart clearly
from the ideal behavior and the deviation is ascribed to the presence of preferential solvation.
Several reported methods aim to quantify preferential solvation; 96-100 none, however,
provides an acceptable characterization facilitating a clear understanding of the phenomenon.
If preferential solvation is so strongly dictated by the polar or ionic character of the
solute, then characterizing a mixture of solvents by using a molecular probe will be utterly
impossible since any conclusions reached could only be extrapolated to solutes of identical
nature as regards not only polarity and charge, but also molecular size and shape.
However, the experimental evidence presented below allows one to conclude that this
is not the case and that solvent mixtures can in fact be characterized in as simple and precise
terms as can a pure a solvent.
In the light of the previous reasoning, describing the solvolysis of tert-butyl chloride
or the decarboxylation kinetics of 3-carboxybenzisoxazole in mixed solvents in terms of
SPP, SB and SA for the mixtures appeared to be rather difficult owing to the differences between
the processes concerned and the solvatochromism upon which the scales were constructed.
However, the results are categorical as judged by the following facts:
(a) The solvolysis rate of tert-butyl chloride in 27 pure solvents and 120 binary mixtures
of water with methanol (31 mixtures), ethanol (31), isopropyl alcohol (1),
trifluoroethanol (8), dioxane (13), acetone (27) and acetic acid (9), in addition to the datum
for the gas phase, all conform to the following equation: 92
log k = 10.62(±0.44)SPP + 1.71(±0.22)SB + 7.89(±0.17)SA - 20.07(±0.34) [10.3.32]
with n = 148, r = 0.99 and sd = 0.40.
(b) The decarboxylation rate of 3-carboxybenzisoxazole in 24 pure solvents and 36
mixtures of DMSO with diglyme (4 mixtures), acetonitrile (4), benzene (7), dichloro-