Page 600 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

582 that a linear relationship between exchange rates and equilibrium acidity be established
for representative examples of the compounds under study. A satisfactory correlation
CHAPTER 6 provides a basis for using kinetic acidity data for compounds of that structural type.
Carbanions and Other The nature of the solvent in which the extent or rate of deprotonation is determined
Carbon Nucleophiles
has a significant effect on the apparent acidity of the hydrocarbon. In general, the
extent of ion aggregation is primarily a function of the ability of the solvent to solvate
the ionic species. In THF, DME, and other ethers, there is usually extensive ion
aggregation. In dipolar aprotic solvents, especially dimethyl sulfoxide, ion pairing is
6
less significant. The identity of the cation also has a significant effect on the extent
of ion pairing. Hard cations promote ion pairing and aggregation. Because of these
factors, the numerical pK values are not absolute and are specific to the solvent
and cation. Nevertheless, they provide a useful measure of relative acidity. The two
solvents that have been used for most quantitative measurements on hydrocarbons are
dimethyl sulfoxide and cyclohexylamine.
A series of hydrocarbons has been studied in cyclohexylamine, using cesium
cyclohexylamide as base. For many of the compounds studied, spectroscopic measure-
ments were used to determine the relative extent of deprotonation of two hydrocarbons
7
and thus establish relative acidity. For other hydrocarbons, the acidity was derived
by kinetic measurements. It was shown that the rate of tritium exchange for a series
of related hydrocarbons is linearly related to the equilibrium acidities of these hydro-
carbons in the solvent system. This method was used to extend the scale to hydro-
carbons such as toluene for which the exchange rate, but not equilibrium data, can
8
be obtained. Representative values of some hydrocarbons with pK values ranging
from 16 to above 40 are given in Table 6.2. The pK values of a wide variety of
9
organic compounds have been determined in DMSO, and some of these values are
listed in Table 6.2 as well. It is not expected that these values will be numerically
identical with those in other solvents, but for most compounds the same relative order
of acidity is observed. For synthetic purposes, carbanions are usually generated in
ether solvents, often THF or DME. There are relatively few quantitative data available
on hydrocarbon acidity in such solvents. Table 6.2 contains a few entries for Cs salts.
+
The numerical values are scaled with reference to the pK of 9-phenylfluorene. 10 The
acidity trends are similar to those in cyclohexylamine and DMSO.
Some of the relative acidities in Table 6.2 can be easily understood. The order of
decreasing acidity Ph CH > Ph CH > PhCH , for example, reflects the ability of each
3
2
2
3
successive phenyl group to stabilize the negative charge on carbon. This stabilization is
a combination of both resonance and the polar EWG effect of the phenyl groups. The
much greater acidity of fluorene relative to dibenzocycloheptatriene (Entries 5 and 6)
is the result of the aromaticity of the cyclopentadienide ring in the anion of fluorene.
Cyclopentadiene (Entry 9) is an exceptionally acidic hydrocarbon, comparable in
acidity to simple alcohols, owing to the aromatic stabilization of the anion. Some more
subtle effects are seen as well. Note that fusion of a benzene ring decreases the acidity
6
E. M. Arnett, T. C. Moriarity, L. E. Small, J. P. Rudolph, and R. P. Quirk, J. Am. Chem. Soc., 95, 1492
(1973); T. E. Hogen-Esch and J. Smid, J. Am. Chem. Soc., 88, 307 (1966).
7 A. Streitwieser, Jr., J. R. Murdoch, G. Hafelinger, and C. J. Chang, J. Am. Chem. Soc., 95, 4248 (1973);
A. Streitwieser, Jr., E. Ciuffarin, and J. H. Hammons, J. Am. Chem. Soc., 89, 63 (1967); A. Streitwieser,
Jr., E. Juaristi, and L. L. Nebenzahl, in Comprehensive Carbanion Chemistry, Part A, E. Buncel and
T. Durst, ed., Elsevier, New York, 1980, Chap. 7.
8
A. Streitwieser, Jr., M. R. Granger, F. Mares, and R. A. Wolf, J. Am. Chem. Soc., 95, 4257 (1973).
9 F. G. Bordwell, Acc. Chem. Res., 21, 456 (1988).
10
D. A. Bors, M. J. Kaufman, and A. Streitwieser, Jr., J. Am. Chem. Soc., 107, 6975 (1985).