Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.4 Mixed solvent influence on the chemical equilibrium 531
former highly differ one from another, so ε is one of the main factors which affects equilibrium,
such as in equations [9.60, 9.61].
If equations of processes are identical to that of scheme [9.45] in the form and maintenance,
it is possible to apply the equations from Section 9.4.1 to calculate the equilibrium
constants of the investigated process.
Abrahem 20 worked out details of the solvent effect theory on changing equilibrium.
This theory also accounts for the quadruple interactions with media dipoles. It demonstrates
that the accuracy of this theory equations is not better than that obtained from an ordinary
electrostatic model.
Because of low energy of [9.60, 9.61] processes, it is not possible to assign any solvents
strictly to a universal or specific group. That is why only a limited number of binary
solvents can be used for the analysis of universal media influence on conformer equilibrium
constants. It is interesting to note that over the years, the real benzene basicity in liquid
phase has not been sufficiently investigated. There were available conformer equilibrium
constants in benzene, toluene, etc., highly differing from those in other low-polar media.
Such phenomenon is proposed to be called the “benzene effect”. 21
Sometimes rather than equilibrium constants, the differences in rotamer energies - for
example, gosh- and trans-isomers - were calculated from the experiment. It is evident that
these values are linearly proportional to the equilibrium constant logarithm.
Different conformers or different intermediate states are characterized by highly distinguished
values of dipole moments. 22 Indeed, the media permittivity, ε, change highly influences
the energy of dipole-dipole interaction. Therefore, according to [9.53a], it is
expected that conformer transformation energies and energies of intermediate processes in
universal solvents are inversely proportional to permittivity. But equilibrium constants of
reactive processes are exponent dependent on1/ε value, i.e., there is a linear correlation between
lnK conf and reciprocal permittivity:
E = A+ B ε [9.62]
conf ( turning)
/
ln K = a + b / ε [9.63]
conf ( turning )
The analysis of experimental data on conformer and intermediate equilibrium in universal
media demonstrates that they can be described by these equations with sufficient accuracy
not worse than the accuracy of experiment.
The differences in rotamer energies of 1-fluoro-2-chloroetane in mixed solvents such
as alkane-chloroalkane can be described by equation: 20
ΔE =E gosh -E trans = -2.86 + 8.69/ε, kJ/mol
According to equation [9.51], the permittivity increase leads to decreasing absolute
value of electrostatic components of conformer transformations free energies in universal
solvents. For instance, conformer transformation free energy of α-bromocyclohexanone in
cyclohexane (ε=2) is 5.2 kJ/mol, but in acetonitrile (ε=36) it is -0.3 kJ/mol.
Specific solvation is the effective factor which controls conformer stability. The correlation
between equilibrium constants of the investigated processes [9.60] and [9.61] and polarity
of solvation-inert solvents is very indefinite. For instance, diaxial conformer of
4-methoxycyclohexanone in acetone (as solvent) (ε=20.7) is sufficiently more stable than in