Handbook of Solvents - George Wypych - ChemTech - Ventech!

546 Y. Y. Fialkov, V. L. Chumak
-δΔG cov
Solvent ε
el
−δΔG ε=1
σΔG el in process
[9.102] [9.104]
HCl HOSO3CH3 CF3COOH 8.2 17.7 -38.1
128 66.0 -100.6
In contrast to the processes considered earlier, the vacuum electrostatic component of
resolvation process has high value whereas −δΔG el values are comparable with the covalent
component, δΔG cov .
δΔG values according to [9.51] are equal to:
( + + )
δΔG = ΔG + ΔG − ΔG + ΔG
HB A HA B sol
For small additions of B (to component A), (ΔG A) sol=0 and (ΔG HA) sol=0 , then
δΔG =(ΔG HB+ + ΔG B) sol
Solvation energy of complex HB + by solvent A is small because coordination vacancies
of the proton are saturated to a considerable extent. Therefore the interaction energy between
A and B influences significantly the value of σΔG. That is why, the mixed solvents
(alcohol-water and alcohol-pyridine, for instance) are different because of the proton
resolvation process. This can be explained in terms of higher energy of heteromolecular association
for the alcohol-water in comparison with alcohol pyridine.
The concept of solvent effect on the proton resolvation process was confirmed by
quantum chemical calculations. 51 Above phenomena determine the dependence of
resolvation constant on physical and chemical properties.
Let the resolvation process proceeds at substantial abundance of the component A in
mixed solvent and initial concentrations HA (HA + ) and B to be equal. The output of the process
can be calculated from the equation similar to equation [9.66]. The large value of Kus in
all considered processes of proton resolvation indicates the effect of permittivity change on
the yield of complex HB + formation. The output of resolvated proton in process [9.104] proceeding
in methanol equals 100%, whereas in the same process in low polarity solvent (e.g.,
methanol-hexane), with abundance of the second component, the equilibrium is shifted to
the left, resulting in solvate output of less than 0.1%. Kus values in single alcohol solvents
are large, thus the output of reaction does not depend on solvent exchange.
The process H•DMSO + +B↔HB + + DMSO may be considered as an example of the
effect of chemical properties of B on the output of the reaction [9.99]. The output of HB +
equals to 98%, if pyridine is included in the process at initial concentration of 0.1M. Use of
diphenylamine, having lower donor properties, decreases the output to 60%. The output differs
even more at smaller concentrations of component, such as 10 -3 M, which gives yields
of 83 and 33%, respectively.
9.4.5 MIXED SOLVENT EFFECT ON THE ION ASSOCIATION PROCESS
The ion association process (or opposite to it - ion dissociation process) has been studied in
detail in comparison with other types of chemical equilibrium in solutions. The modern