Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 587
tion, as shown below, involves sterically hindering the solvent approach by protecting the
basic site of the probe with bulky alkyl (e.g., tert-butyl) groups at adjacent positions. 24
One can therefore conclude that constructing an appropriate general or specific solvent
scale entails finding a suitable molecular probe to preferentially assess the effect considered
and a suitable homomorph to subtract any side effects extraneous to the interaction
of interest. Any other type of semi-empirical evaluation will inevitably lead to scales contaminated
with other solvent effects, as shown later on. Accordingly, one will hardly be able
to provide a global description of solvent effects on the basis of a single-parameter scale.
10.3.3 SINGLE-PARAMETER SOLVENT SCALES: THE Y, G, ET(30), Py ,Z,χR, Φ, AND S� SCALES
Below are briefly described the most widely used single-parameter solvent scales, with special
emphasis on their foundation and use.
10.3.3.1 The solvent ionizing power scale or Y scale
In 1948, Grunwald and Winstein 25 introduced
the concept of “ionizing power of the
solvent”, Y, based on the strong influence
Scheme II.
of the solvent on the solvolysis rate of alkyl
halides in general and tert-butyl chloride in
particular (see Scheme II). Y is calculated from the following equation:
tBuCl tBuCl
Y = logk −logk0
[10.3.1]
where k tBuCl and k 0 tBuCl are the solvolysis rate constants at 25 o C for tert-butyl chloride in the
solvent concerned and in an 80% v/v ethanol/water mixture - the latter constant is used as
reference for the process.
The strong solvent dependence of the solvolysis rate of tert-butyl chloride was examined
by Grunwald and Winstein 25,26 in the light of the Brönsted equation. They found the
logarithmic coefficient of activity for the reactant and transition state to vary linearly in a series
of mixtures and the variation to be largely the result of changes in coefficient of activity
for the reactant. By contrast, in the more poorly ionizing solvents, changes in k were found
to be primarily due to changes in coefficient of activity for the transition state.
In a series of papers, 25-32 Grunwald and Winstein showed that the solvolysis rate constants
for organic halides which exhibit values differing by more than 6 orders of magnitude
in this parameter can generally be accurately described by the following equation:
logk = mY + logk0
[10.3.2]
where k and k 0 are the rate constants in the solvent concerned and in the 80:20 v/v ethanol/water
mixture, and coefficient m denotes the ease of solvolysis of the halide concerned
relative to tert-butyl chloride. By grouping logarithms in eq. [10.3.2], one obtains
( )
log k / k = mY
[10.3.3]
0
which is analogous to the Hammett equation: 33
log ( k / k0 ) =ρσ [10.3.4]