Handbook of Solvents - George Wypych - ChemTech - Ventech!

738 Roland Schmid
Figure 13.1.1. Relationship between second-order rate constants at
50°C of the reaction of p-nitrofluorobenzene and piperidine and the
solvent dielectric properties [from ref. 21].
The first solvent property
applied to correlate reactivity data
was the static dielectric constant ε
(also termed ε s) in the form of dielectric
functions as suggested
from elementary electrostatic theories
as those by Born (1/ε),
Kirkwood (ε-1)/(2ε+1), Clausius-
Mosotti (ε-1)/(ε+2), and
(ε-1)/(ε+1). A successful correlation
is shown in Figure 13.1.1 for
the rate of the S N2 reaction of
p-nitrofluorobenzene with
piperidine. 3 The classical dielectric
functions predict that reactivity
changes level out for dielectric
constants say above 30. For instance,
the Kirkwood function has
an upper limiting value of 0.5, with the value of 0.47 reached at ε = 25. The insert in Figure
13.1.1 illustrates this point. Therefore, since it has no limiting value, the log ε function may
be preferred. A theoretical justification can be given in the framework of the dielectric saturation
model of Block and Walker. 4
Picturing the solvent as a homogeneous dielectric continuum means in essence that the
solvent molecules have zero size and that the molecules cannot move. The most adequate
physical realization would be a lattice of permanent point dipoles that can rotate but cannot
translate.
13.1.3 THE CHEMICAL APPROACH
Because of the often-observed inadequacies of the dielectric approach, that is, using the dielectric
constant to order reactivity changes, the problem of correlating solvent effects was
next tackled by the use of empirical solvent parameters measuring some solvent-sensitive
physical property of a solute chosen as the model compound. Of these, spectral properties
such as solvatochromic and NMR shifts have made a spectacular contribution. Other important
scales are based on enthalpy data, with the best-known example being the donor number
(DN) measuring solvent’s Lewis basicity.
In the intervening years there is a proliferation of solvent scales that is really alarming.
It was the merit particularly of Gutmann and his group to disentangle the great body of empirical
parameters on the basis of the famous donor-acceptor concept or the coordination-chemical
approach. 5 This concept has its roots in the ideas of Lewis going back to
1923, with the terms donor and acceptor introduced by Sidgwick. 6 In this framework, the
two outstanding properties of a solvent are its donor (nucleophilic, basic, cation-solvating)
and acceptor (electrophilic, acidic, anion-solvating) abilities, and solute-solvent interactions
are considered as acid-base reactions in the Lewis’ sense.
Actually, many empirical parameters can be lumped into two broad classes, as judged
from the rough interrelationships found between various scales. 7 The one class is more concerned
with cation (or positive dipole’s end) solvation, with the most popular solvent basic-