Handbook of Solvents - George Wypych - ChemTech - Ventech!

508 Y. Y. Fialkov, V. L. Chumak
In many cases, ionic associate undergoes the process of ionic dissociation (see paragraph
9.2.5). If permittivity of mixture is not high, interaction can limit itself to formation of
non-ionogenic ionic associate, as exemplified by the binary solvent anisol-m-cresol, where
the following equilibrium is established:
[ ]
CHOCH+ CHCHOH↔CHOCH•CHCHOH↔ CHOCH•H•CHCHO 6 5 3 3 6 4 6 5 3 3 6 4 6 5 3
+
3 6 4
9.2.5 ELECTROLYTIC DISSOCIATION (IONIC ASSOCIATION)
In solvents having high permittivity, the ionic associate decomposes into ions (mostly associated)
to a variable degree:
q+
p−
q+
p− q+ p− q+ p−
K p Lq ↔ K p−1L+
K + L ↔� ↔ pK + qL
[9.7]
1
q−
Electrolyte solution is formed according to eq. [9.7]. Conductivity is a distinctive feature
of any material. The practice of application of individual and mixed solvents refers to
electrolyte solutions, having conductivities > 10 -3 Cm/m. Binary systems, such as carbonic
acids-amines, cresol-amines, DMSO-carbonic acids, hexamethyl phosphorous triamide acids,
and all the systems acids-water, are the examples of such solvents.
The common scheme of equilibrium in binary solvents, where components interaction
proceeds until ions formation, may be represented by the common scheme:
−
[9.8]
where:
Km,Kn the constants of homomolecular association; respectively
Kadd the constant of the process formation of heteromolecular adduct
Ki the ionization constant
Ka the association constant.
The system acetic acid-pyridine 4 may serve as an example of binary solvent whose equilibrium
constants of all stages of the scheme [9.8] have been estimated.
9.2.6 REACTIONS OF COMPOSITION
As a result of interaction between components of mixed solvents, profound rearrangement
of bonds takes place. The process is determined by such high equilibrium constant that it is
possible to consider the process as practically irreversible:
mA + nB → pC
[9.9]
where:
m, n, p stoichiometric coefficients
C compound formed from A and B
Carbonic acid anhydrides-water (e.g., (CH3CO) 2O+H2O→2CH3COOH), systems
isothiocyanates-amines (e.g., C3H5NCS+C2H5NH2 → C2H5NHCNSC3H5) are examples of
such binary solvents.
It is pertinent that if solvents are mixed at stoichiometric ratio (m/n), they form individual
liquid.