Chemistry Reference
In-Depth Information
FIGURE 1.16.
The correlation of log
k
p
with Swain's acidity parameters. The acidity
parameters for formic acid, acetic acid, methanol, ethanol, ethylene glycol and formamide
(solid circles) are obtained from Swain's paper.
89
The acidity parameters for 2,2,2-trifluor-
oethanol and propionic acid (open circles) are calculated from KAT's parameters.
90
The
dashed line is the best-fit linear correlation of the data for all eight solvents of this study
(
R
¼
0.97). The solid line is the linear correlation of the data for the six solvents excluding
2,2,2-trifluoroethanol and propionic acid (
R
¼
0.99).
Source
: Reprinted with permission from
Ref. 41.
properties, taken from the literature, such as reaction rate constants, equilibrium
constants, product ratios, and UV-Vis, IR, and NMR spectra. They found that A and
B can describe solvent hydrogen-bond donating and accepting abilities and proposed
that these parameters could be used to evaluate
bulk
solvent acidity and basicity. To a
first-order approximation,
k
H
þ
[H
þ
][
1
FlN]
¼
k
p
[
1
FlN], where
k
H
þ
is the bimolecular rate constant for the reaction between
1
FlN and protons, [H
þ
] and [
1
FlN] are the proton and 2-fluorenyl nitrene concen-
trations in the solvent. It is of course possible that the nitrene reacts with the solvent
molecule directly to achieve proton transfer from solvent to solute. This measure of
proton-donating ability also correlates with acidity values. Thus, the nitrene pro-
tonation rate can, in principle, correlate with the acidity of bulk solvent, as quantified
by the acidity scale.
Excluding TFE and propionic acid, six of the eight solvents used in this study have
known Swain acidity parameters.
89
The correlation of log
k
p
with the six solvent
acidities is excellent (
R
the rate of nitrene protonation
¼
0.99, the solid line of Fig. 1.16). Stimulated by the
similarity between the KAT and Swain's parameters, Marcus found a relationship
between these two sets of solvent empirical parameters,
90
which is that acidity
¼
¼
p
and basicity
p
(exclude acetic
0.03
þ
0.64
aþ
0.25
¼
0.04
þ
0.035
bþ
0.94
