Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.2 Chemical interaction between components 509
9.2.7 EXCHANGE INTERACTION
Exchange interaction
mA + nB ↔ pC + qD
[9.10]
is not rare in the practice of application of mixed solvents. Carbonic acid-alcohol is an example
of the etherification reaction. Carbonic acid anhydride-amine exemplifies acylation
process (CH 3CO) 2O+C 6H 5NH 2 ↔ C 6H 5NHCOCH 3 +CH 3COOH; carbonic acid-another
anhydride participate in acylic exchange: RCOOH + (R 1CO) 2O ↔ RCOOCOR 1 +
R 1COOH. The study of the exchange interaction mechanism gives forcible argument to believe
that in most cases the process goes through formation of intermediate - “associate”:
[ ]
mA + nB ↔ A B ↔ pC + qD
[9.11]
m n
9.2.8 AMPHOTERISM OF MIXED SOLVENT COMPONENTS
The monograph of one of the authors of this chapter 7 contains a detailed bibliography of
works related to this paragraph.
9.2.8.1 Amphoterism of hydrogen acids
In the first decades of the 20 th century, Hanch ascertained that compounds which are typical
acids show distinctly manifested amphoterism in water solutions while they interact with
one another in the absence of solvent (water in this case). Thus, in binary solvents
H2SO4-CH3COOH and HClO4-CH3COOH-acetic acid accepts proton forming acylonium,
CH3COOH 2+ , cation. At present time, many systems formed by two hydrogen acids are
being studied. In respect to one of the strongest mineral acids, CF3SO3H, all mineral acids
are bases. Thus, trifluoromethane sulfuric acid imposes its proton even on sulfuric acid,
forcing it to act as a base: 8
CF SO H + H SO ↔CF SO •HSO
− +
3 3 2 4 3 3 3 4
Sulphuric acid, with the exception of the mentioned case, and in the mixtures with
perchloric acid (no proton transfer), shows acidic function towards all other hydrogen acids.
The well-known behavior of acidic function of absolute nitric acid towards absolute
sulphuric acid is a circumstance widely used for the nitration of organic compounds.
Trifluoroacetic acid reveals proton-acceptor function towards all strong mineral acids,
but, at the same time, it acts as an acid towards acetic and monochloroacetic acids.
In mixed solvents formed by two aliphatic carbonic acids, proton transfer does not
take place and interaction is usually limited to a mixed associate formation, according to the
equilibrium [9.2].
9.2.8.2 Amphoterism of L-acids
In binary liquid mixtures of the so-called aprotic acids (Lewis acids or L-acids) formed by
halogenides of metals of III-IV groups of the periodic system, there are often cases when
one of the components is an anion-donor (base) and the second component is an anion-acceptor
(acid):
+
MX + M X ↔ MX + M X
−
I m II n I m−1 II n+
1 [9.12]