Handbook of Solvents - George Wypych - ChemTech - Ventech!

606 Javier Catalán
Figure 10.3.1. Plot of Kosower’s Z values vs. the predicted
Z values according to eq. [10.3.16].
Scheme V.
Figure 10.3.2. Plot of Brooker’s χ R values vs. the predicted
χ R values according to [eq. 10.3.17].
It should be noted that the three solvent effects (polarity, acidity and basicity) increase
Z; also, as can be seen from Figure 10.3.1, the greatest imprecision in this parameter corresponds
to solvents with Z values below 70 kcal mol -1 .
The 28 solvents in Table 10.3.1 with known values of χ R reveal that this parameter reflects
solvent polarity and, to a lesser extent, also solvent acidity:
χ R = -15.81(±1.19)SPP - 4.74(±0.90)SA + 58.84(±0.94) [10.3.17]
with r = 0.958 and sd = 0.78 kcal mol -1 .
It should be noted that the two solvent effects involved (polarity and acidity) contribute
to decreasing χ R. As can be seen from Figure 10.3.2, the precision in χ R is similarly good
throughout the studied range. The structure of the probe used to construct this scale (5) reveals
that its electron-donor site (an amino group) is not accessible for the solvent, so the
charge transfer involved in the electronic excitation does not alter its solvation; rather, it increases
its polarity and localization of the charge on the acceptor sites of the compound (carbonyl
groups), see scheme V, so increasing solvent polarity and acidity will result in a
bathochromically shifted electronic transition by effect of the increased stabilization of the
excited state through general and specific interactions.
The results of the multi-parameter analysis of P y suggests that its values are highly imprecise;
even if the value for the gas phase is removed, this parameter only reflects -in an im-