Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.2 Chemical interaction between components 507
The scheme [9.3] includes the following stages occurring in sequence:
I interaction of the first tautomeric form, A 1H, with the solvent that leads to
formation of a addition product, A 1HS, (the solvate composition may be
more complicated)
- +
II ionization of solvate with formation of ionic couple, AHS 1
III ionic couple may decompose into ions (stage III a), transfer of ionized
complex into a product of solvent addition already in the second
tautomeric form
IV formation of the free tautomeric form, A2HA2H. The nature of the chemical type of influences on keto-enol equilibrium is obvious: the
more basic the solvent, the higher the degree of keto-enol transformation into enol form. If
solvent is indifferent with enough degrees of approximation, the well-defined dependence
of the constants of keto-enol equilibrium on solvent permittivity is encountered. The difference
in dipole moments of tautomeric forms is the basis of these relationships.
In mixed solvents such as ACR-B, the first component may undergo rearrangement
reactions:
ACR ↔ ARC
[9.4]
These occur under the solvent influence or even with its direct participation:
+ B + B
ACR ←⎯→� ←⎯→ARC [9.4a]
9.2.3 HETEROMOLECULAR ASSOCIATION
When components of the binary solvent A-B are solvent-active to one another, the sum of
chemical equilibrium is established:
mA + nB ↔ AmBn [9.5]
In most scenarios, the interaction proceeds in steps, i.e., equilibrium solution represents
the mixture of heteroassociates of different stoichiometry: AB, AB2,A2B, etc. Systems
formed by O-, S-, N-, P-bases with various H-donors (e.g., amines-carboxylic acids,
esters-carboxylic acids (phenols), dimethylsulfoxide-carboxylic acid) refer to this type of
interaction.
The systems having components which interact by means of donor-acceptor bond
(without proton transfer) belong to the same type of solvents (e.g., pyridine-chloracetyl,
dimethylsulfoxide-tetrachloroethylene, etc.). Components of mixed solvents of such type
are more or less associated in their individual states. Therefore, processes of
heteromolecular association in such solvents occur along with processes of homomolecular
association, which tend to decrease heteromolecular associations.
9.2.4 HETEROMOLECULAR ASSOCIATE IONIZATION
In some cases, rearrangement of bonds leads to formation of electro-neutral ionic associate
in binary solvents where heteromolecular associates are formed:
q
A B ↔ K L
+ p−
m n p q
[9.6]