Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.2 Effect of polymer/solvent acid-base interactions 577
With the chemical hardness, η, and the electronegativity (or negative chemical potential),
one can easily apply the HSAB principle to acid-base interaction and calculate the
number of electron transferred.
10.2.1.2.5 Effect of amphotericity of acid-base interaction: Gutmann’s numbers
Solvatation, solvolysis and ionic dissociation phenomena, in both aqueous and nonaqueous
solutions are subsumed by the Lewis definitions. In addition to the previous discussion of
the dual polarity character of Lewis acids and bases, it should be noted that many of them
are amphoteric, by definition. Donor number, DN, was developed 19 in order to correlate the
behavior of a solute in a variety of donor solvents with a given basicity or donicity. A relative
measurement of the basicity of a solvent D is given by the enthalpy of its reaction with
an arbitrarily chosen reference acid (SbCl5 in the Gutmann’s scale). Latter Mayer 20 introduced
an acceptor number, AN, as the relative 31 P NMR shift induced by triethylphosphine,
and relative to acidic strength (AN=0 for hexane and 100 for SbCl5). In 1989, Riddle and
Fowkes 21 modify these AN numbers, to express them, AN*, in correct enthalpic unit
(kcal/mol). Table 10.2.3 gathers electron acceptor number AN and AN* and electron donor
number DN for amphoteric solvents.
Table 10.2.3. Acceptor number, AN, and donor number, DN, for common solvents
[After references 19, 21]
Amphoteric solvent AN AN*, kcal/mol DN, kcal/mol
Acetone 12.5 2.5 17.0
Diethyl ether 3.9 1.4 19.2
Formamide 39.8 9.3 26.6
Tetrahydrofuran 8.0 0.5 20.0
Pyridine 14.2 0.14 33.1
Ethyl acetate 1.5 17.1
Benzonitrile 15.5 0.06 11.9
Nitromethane 20.5 4.3 2.7
Water 54.8 15.1 18.0
The most important assumption of Gutmann’s approach is that the order of base
strengths established remains constant for all other acids (solutes), the value of the enthalpy
of formation of a given adduct is linearly related to the donor number of the base (solvent)
through the equation [10.2.8]:
where:
− ΔHAB = aADNB+ bB
[10.2.8]
ΔHAB DNB a A,b A
enthalpy of acid-base adduct formation
donor number of the base
constants characteristic of the acid