Handbook of Solvents - George Wypych - ChemTech - Ventech!

506 Y. Y. Fialkov, V. L. Chumak
B
B B
mA ←⎯→2Am/ 2 ←⎯→� ←⎯→Am [9.1]
where:
m number of molecules A forming the homomolecular associate (in subscript - association
degree)
The systems alcohol-carbon tetrachloride or carbon acids-cyclohexane are examples
of mixed solvents of this type. Interesting examples of such mixed solvents are the systems
formed of liquid tetraammonium salts, R4NA, and various liquids that are solvate-inert towards
such compounds.
The values of homomolecular association constants of alcohols and acids are high,
typically in the range of 10 3 -10 4 . Therefore concentration of the component A is rather low,
even at high content of solvate-inert component B, and may be neglected.
The relative degree of the decomposition of heteromolecular associates into aggregates
of a smaller association degree increases with increased permittivity of the non-associated
solvate-inert component. In solutions of equal concentration of the two mixed
solvents, acetic acid-cyclohexane (ε=1,88) and acetic acid-chlorobenzene (ε=5,6), the relative
degree of acid heteromolecular association is higher in the first system than in the second
system.
In mixed solvents, formed of two associated components in individual states Am and
Bn that do not interact with each other into specific solvation, the chemical equilibrium of
mixed heteromolecular associates is established:
( )
A + B ↔ x A / B / [9.2]
m n m x n x
where:
x number of molecules of heteromolecular associate
Whether to consider these heteromolecular associates as real chemical adducts is more
terminology than a chemical problem. In most cases investigators truly assume that in systems
formed of two alcohols or of two carboxylic acids, specific interaction does not exist.
In the case of mixed solvents formed of two carbon acids, that is true only when components
have similar proton affinity, as is the case of a system such as acetic acid-propionic acid (see
further paragraph 9.2.8).
9.2.2 CONFORMIC AND TAUTOMERIC EQUILIBRIUM.
REACTIONS OF ISOMERIZATION
Various conformers of the same compound differ in their dipole moments. Thus, in the
mixed solvents A-B, where A is a liquid whose molecules give conformer equilibrium, and
B is solvate-inert towards A, the A conformer ratio changes on addition of B component.
In binary solvents A-B where component A can exist in two tautomeric forms A1 and
A2 the ratio of concentrations of these forms changes on addition of component B. Common
theory of this influence has been worked out by Kabachnic 4,5 who proposed the common
scheme of equilibrium in systems of such type:
[9.3]