Handbook of Solvents - George Wypych - ChemTech - Ventech!

532 Y. Y. Fialkov, V. L. Chumak
methanol (ε=32.6). But the maximum stability of that isomer is reached in low-polar
trifluoroacetic acid as solvent (ε=8.3).
Formation of the internal-molecular H-bonding is the cause of the increasing conformer
stability, often of gosh-type. Therefore solvents able to form sufficiently strong external
H-bonds destroy the internal-molecular H-bonding. It leads to change of conformer
occupation and also to decrease of the internal rotation barrier.
Due to sufficiently high donority, the benzene has a comparatively high degree of conformer
specific solvation (“benzene effect”). This leads to often stronger stabilization of
conformer with higher dipole moment in benzene than in substantially more polar
acetonitrile.
In a number of cases, the well-fulfilled linear correlation between conformer transformation
constants and the parameter ET of mixed solvent exists (see Section 9.3.4.3).
It is often the solvent effect that is the only method of radical change of relative contents
of different conformer forms. Thus, with the help of the isochore equation of chemical
reaction, the data on equilibrium constants and enthalpies of dichloroacetaldehyde conformer
transformation allow us to calculate that, to reach the equilibrium constant of axial
rotamer formation in cyclohexane as solvent (it is equal to 0.79) to magnitude K=0.075 (as
it is reached in DMSO as solvent), it is necessary to cool the cyclohexane solution to 64K
(-209 o C). At the same time, it is not possible because cyclohexane freezing point is +6.5 o C.
By analogy, to reach the “dimethylsulfoxide” constant to value of “cyclohexane”, DMSO
solution must be heated to 435K (162 o C).
9.4.4 SOLVENT EFFECT ON THE PROCESS OF HETEROMOLECULAR
ASSOCIATION
Solvent effect on the process of heteromolecular association
mE + nF ↔ E mFn [9.64]
has been studied in detail. 22,23 In spite of this, monographs are mainly devoted to individual
solvents.
In universal media formed by two solvate-inert solvents according to equation [9.53a],
equilibrium constants of the process of heteromolecular adduct formation depend exponentially
on reciprocal permittivity; thus
ln K = a + b / ε [9.65]
add
The process of adduct formation of acetic acid (HAc) with tributyl phosphate (TBP)
nHAc•TBP in the mixed solvent n-hexane-nitrobenzene 7 can serve as an example of the
[9.65] dependence validity. Equilibrium constants of the mentioned above adducts in individual
and some binary mixed solvents are presented in Table 9.2.
The concentration of adduct EF cM is related to constant Kadd and initial concentration
o
of the components c M by equation
o ( add M )
o
12 /
cM= ⎡2Kadd
cM+ 1− 4K c + 1 ⎤ / 2Kadd
[9.66]
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