Handbook of Solvents - George Wypych - ChemTech - Ventech!

742 Roland Schmid
AN = -133.8 - 0.00933ν 0
n = 12, r = 0.980, s = 4.58
Figure 13.1.4. Simplified orbital scheme for the charge transfer transition
in Fe(phen) 2(CN) 2 varying with solvation. The diagram, not
drawn to scale, is adjusted so that π* is constant [adapted from ref.
18].
[13.1.4]
13.1.4 DIELECTRIC VS.
CHEMICAL
APPROACH
Although the success of the empirical
solvent parameters has
tended to downgrade the usefulness
of the dielectric approach,
there are correlations that have
succeeded as exemplified by Figure
13.1.1. It is commonly held
that the empirical solvent parameters
are superior to dielectric estimates
because they are sensitive
to short-range phenomena not
captured in dielectric measurements.
This statement may not be
generalized, however, since it depends
strongly on the chemical reaction
investigated and the choice
of solvents. For instance, the rate
of the Menschutkin reaction between
tripropylamine and methyl
iodide in select solvents correlates
better with the log ε function than
with the solvent acceptor number.
19
Thus the solution chemists
were puzzled for a long time over
the question about when and when
not the dielectric approach is adequate.
In the meantime, this issue
has been unraveled, in that dielectric
estimates have no relevance to
the solvation of positive (partial)
charge. Thus, there is no relationship
between the free energies of
transfer for cations and the dielectric constant. 7 Likewise, note the solvent-dependence of
the solubilities of sodium chloride (Table 13.1.1) taken from Mayer’s work. 19 For instance,
the pairs of solvents H2O/PC and DMF/MeCN have similar ε′ s but vastly different abilities
to dissolve NaCl. In similar terms, the inclusion of a donor number term improves somewhat
the correlation in Figure 13.1.1, as may be seen in Figure 13.1.5. This would suggest
that the hydrogen of piperidine in the activated complex becomes acidic and is attacked by
the strong donor solvents DMF, DMA and DMSO (Scheme 13.1.1).