Handbook of Solvents - George Wypych - ChemTech - Ventech!

530 Y. Y. Fialkov, V. L. Chumak
lnK dim = -1.72 + 1817.1/T + 0.92/ε + 1553.4/εT
The dimer form concentration, c M (mol/l), is related to initial analytical concentration
of dissolved compound and dimerization constant in equation:
o ( M )
o
12 /
cM = ⎡4K
cM + − K c + ⎤
dim 1 8 dim 1 / 8Kdim
[9.59]
⎣⎢
⎦⎥
o
One can calculate, with the help of this equation, that in hexane at c M = 0.1, half of the dissolved
acetic acid is in dimer form (Kdim ≈ 1.5×10 3 ). In nitrobenzene solution (Kdim ≈ 10 2 ),
only a third of analytical concentration of acid is in dimer form. For phenol these values are
12% and 2%, respectively. Therefore the composition variation of universal mixed solvents,
for instance, using hexane-nitrobenzene binary solvent, is an effective method to
control the molecular composition of dissolved compounds able to undergo homomolecular
association, in particular, through H-bonding.
Typically, in specific solvents, the process of monomer formation [9.57] is characterized
by considerably lower dimerization constants, as compared with those in universal media.
In fact, acetic acid dimerization constant in water-dioxane binary solvent, the
components of which are solvation-active in respect to the acid, vary in the 0.05-1.2 range.
Replacing the solvent is often the only method to vary the molecular association state of dissolved
compound. To achieve dimer concentration in 0.1 M solution of phenol in n-hexane
equal to dimer concentration in nitrobenzene solution (50% at 25 o C), it would be necessary
to heat the solution to 480 o C, but it is impossible under ordinary experimental conditions.
9.4.3 MIXED SOLVENT INFLUENCE ON THE CONFORMER EQUILIBRIUM
Equilibrium took place in solutions
Conformer I ↔ Conformer II [9.60]
and the special case, such as
Cis-isomer ↔ Trans-isomer [9.61]
when investigated in detail. But the concentration-determining methods of equilibrium
[9.60, 9.61] members are often not very precise. Dielcometry, or in our measurement the
mean dipole moment, can be used only in the low-polar solvents. The NMR methods allowed
us to obtain the equilibrium constants of processes by the measurement of chemical
dislocation and constants of spin-spin interactions of nucleuses, which are quite precise but
only at low temperatures (that is why the numerous data, on conformer and isomerization
equilibrium, in comparison with other types of equilibria, are in solutions at low temperatures).
It is evident that the accuracy of the experimental definition of equilibrium constants
of the investigated process is insufficient. Thus, the summarization of experimental data on
solvent effect on [9.60, 9.61] equilibrium encounters several difficulties.
As a rule, the differences in conformer energies are not very high and change from one
tenth to 10-12 kJ/mol. This is one order value with dipole-dipole interaction and specific
solvation energies, even in low-active solvents. Moreover, the dipole moments of con-