Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.4 Mixed solvent influence on the chemical equilibrium 545
+ +
ROH + Py ↔ HPy + ROH
[9.104]
2
The solutions of trifluoroacetic acid in ethanol and methanol in the temperature range
273.15-323.15K were investigated. The dependence of K us on permittivity and temperature
is described by the equation:
ln K us = 9.44 + 1768/T + 77.8/ε - 11076/εT
Unlike the processes considered above, in the case of process [9.104] permittivity decrease
leads to decreasing K us. The explanation of the results based on covalent and electrostatic
components of resolvation process enthalpy is given. 50
The reaction [9.104] also has been studied for isodielectric mixtures of alcoholchlorobenzene
with ε=20,2 (permittivity of pure n-propanol) and ε=17.1 (permittivity of
pure n-butanol) to investigate the relative effect of universal and specific solvation on the
resolvation process. The mixtures were prepared by adding chlorobenzene to methanol, ethanol,
and C 1-C 3 alcohol. Alcohol is a solvate-active component in these isodielectric solvents.
K us data are given in Table 9.6.
Table 9.6. Equilibrium constants of the process [9.104] in isodielectric solvents
Solvents with ε=20.2 Kus×10 -5
Solvents with ε=17.1 Kus×10 -5
Methanol +23.5% chlorobenzene 3.9 Methanol + 32.8% chlorobenzene 1.4
Ethanol + 13% chlorobenzene 4.9 Ethanol + 24% chlorobenzene 2.0
n-Propanol 7.3 n-Butanol 4.9
Insignificant increase of K us in n-butanol (or n-propanol) solution in comparison to
methanol is due to relaxation of the proton-alcohol bond, when the distance of ion-dipole interaction
increases.
The change of donor property of the solvate-active component is not significant. The
equations relating K us to ε permit to divide free energy of resolvation process into the components.
Corresponding data are presented in Table 9.7.
Table 9.7. The components of free energy (kJ mol -1 ) of proton resolvation process at
298.15K
Solvent ε
σΔG el in process
[9.102] [9.104]
HCl HOSO3CH3 CF3COOH Methanol 32.6 3.9 2.0 -3.1
Ethanol 24.3 5.3 2.7 -4.1
n-Propanol 20.1 6.4 3.3 -5.0
n-Butanol 17.1 7.5 3.9 -5.9
n-Pentanol 14.4 8.9 4.6 -