Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 605
10.3.7 APPLICATIONS OF THE PURE SPP, SA AND SB SCALES
The SPP general solvent scale, and the SA and SB specific solvent scales, are orthogonal to
one another, as can be inferred from the small correlation coefficients obtained in mutual
fittings involving the 200 solvents listed in Table 10.3.1 [r 2 (SPP vs. SA) = 0.13, r 2 (SPP vs.
SB) = 0.10 and r 2 (SA vs. SB) = 0.01]. These results support the use of these scales for the
multi-parameter analysis of other solvent scales or data sets sensitive to the solvent effect on
the basis of the following equation:
P= aSPP+bSA+cSB+P 0
[10.3.15]
where P is the quantity to be described in a given solvent; SPP, SA and SB are the corresponding
polarity/polarizability, acidity and basicity values for such a solvent; coefficients
a, b and c denote the sensitivity of P to such effects; and P0 is the P value in the absence of
solvent (i.e., the gas phase, which is given a zero value in our scales).
Each of these scales (SPP, SB and SA) has previously been compared with other reported
solvent scales and found to be pure scales for the respective effects. However, eq.
[10.3.15] is used below to perform a multi-parameter analysis of various reported scales and
experimental data sets of interest.
10.3.7.1 Other reported solvents scales
Reported scales for describing solvent polarity [f(ε,n), 21,79 π *80 and S� 60 ], basicity (DN 20 and
β 18 ) and acidity (AN 81 and α 18 ) were previously analyzed against our SPP, SA and SB scales
in the originating references, so no further comment is made here.
Rather, this section analyses the behavior of reported single-parameter scales developed
to describe the global behavior of solvents [viz. the Z, χR,Py,Φ, G and ET(30) scales]
against the SPP, SB and SA scales on the basis of the 200 solvents listed in Table 10.3.1.
The Y scale, established to describe the behavior of solvolysis-like kinetics, is dealt with in
the section devoted to the description of kinetic data. The data used in this analysis were
taken from the following sources: those for the Z scale from the recent review paper by
Marcus; 56 those for the χR, Φ and G scales from the compilation in Table 7.2 of Reichardt’s
book; 1 those for parameter Py from the paper by Dong and Winnik; 50 and those for ET(30) from the recent review by Reichardt 13 or Table 7.3 in his book. 1
The Z value for the 51 solvents in Table 10.3.1 reported by Marcus allow one to establish
the following equation:
Z = 22.37(±4.85)SPP + 7.68(±1.64)SB + 31.27(±1.81)SA + 41.03(±4.15) [10.3.16]
with r = 0.944 and sd = 3.03 kcal mol -1 .
This equation reveals that Z is largely the result of solvent acidity and polarity, and
also, to a lesser extent, of solvent basicity. The strong delocalization of charge in the structure
of the probe (4) upon electronic excitation, see scheme IV, accounts for the fact that the
electronic transition of this probe is hypsochromically shifted by solvent polarity and acidity.
The shift is a result of the increased stabilizing effect of the solvent -exerted via general
and specific interactions- in the electronic ground state being partly lost upon electronic excitation
through delocalization of the charge in the electronic excited state, which gives rise
to decreased polarity and specific solvation. On the other hand, the small contribution of
solvent basicity appears to have no immediate explanation.