Handbook of Solvents - George Wypych - ChemTech - Ventech!

510 Y. Y. Fialkov, V. L. Chumak
The following is an example of eq. [9.12]: In the mixture AsCl 3 - SnCl 4, stannous tetrachloride
is a base and arsenic trichloride is an acid:
( )
+ −2
2AsCl 3 + SnCl 4 ↔ AsCl 2 •SnCl
2
6
9.2.8.3 Amphoterism in systems H-acid-L-acid
Amphoterism phenomenon is the mechanism of acid-base interaction in systems formed by
H-acids or two L-acids in binary systems. For example, in stannous tetrachloridecarboxylic
acid system, acid-base interaction occurs with SnCl4 being an acid:
[ ( ) ]
SnCl + 2RCOOH
↔ SnCl RCOO H
4 4 2 2
The product of this interaction is such a strong hydrogen acid that it is neutralized by
the excess of RCOOH:
[ ( ) ] ( )
[ ]
SnCl RCOO H + RCOOH ↔ SnCl RCOO H + RCOOH
− +
4 2 2 4 2 2
The mechanism of acid-base interaction in binary solvents of the mentioned type depends
often on component relation. In systems such as stannous pentachloride-acetic acid,
diluted by SbCl 5 solutions, interaction proceeds according to the scheme:
2SbCl + 4HAc ↔ SbCl Ac + SbCl + Cl + 2HAc
− − +
5 3 2 6 2
In solutions of moderate concentrations, the following scheme is correct:
2SbCl + 2HAc
↔ SbCl Ac + Cl + H Ac
− +
5 4 2
9.2.8.4 Amphoterism in binary solutions amine-amine
If amines differ substantially in their energies of proton affinity, one of them has a proton-donor
function in binary mixtures of these amines, so that it acts as an acid:
| | | | | |
−
NH + N −↔N + −
•HN↔ N
+
+ H N −
[9.13]
| | | | | |
In the overwhelming majority of cases, this acid-base interaction limits itself only to
the first stage - formation of heteromolecular adduct. Interactions occurring in all stages of
the scheme [9.13] are typical of binary solvents such as triethylene amine-pyridine or
N-diethylaniline. Proton-donor function towards the amine component reveals itself distinctly
in the case of diphenylamine.
To conclude the section discussing amphoterism in binary solvents, we have constructed
a series of chemical equilibria that help to visualize the meaning of amphoterism as
a common property of chemical compounds: