Page 599 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

When the acidities of hydrocarbons are compared in terms of the relative stabilities 581
of neutral and anionic forms, the appropriate data are equilibrium acidity measure-
ments, which relate directly to the relative stability of the neutral and anionic species. SECTION 6.1
For compounds with pK> ∼35, it is difficult to obtain equilibrium data. In such Acidity of Hydrocarbons
cases, it may be possible to compare the rates of deprotonation, i.e., the kinetic acidity.
These comparisons can be made between different protons in the same compound or
between two different compounds by following an isotopic exchange. In the presence
of a deuterated solvent, the rate of incorporation of deuterium is a measure of the rate
3
3
of carbanion formation. Tritium ( H)-NMR spectroscopy is also a sensitive method
for direct measurement of kinetic acidity. 4
RH + B – R – +BH
R – + SD RD + S –
S – +BH SH + B –

It has been found that there is often a correlation between the rate of proton abstraction
(kinetic acidity) and the thermodynamic stability of the carbanion (thermodynamic
acidity). Owing to this relationship, kinetic measurements can be used to extend
scales of hydrocarbon acidities. These kinetic measurements have the advantage of
not requiring the presence of a measurable concentration of the carbanion; instead, the
relative ease of carbanion formation is judged by the rate at which exchange occurs.
This method is applicable to weakly acidic hydrocarbons for which no suitable base
will generate a measurable carbanion concentration.
The kinetic method of determining relative acidity suffers from one serious
complication, however, which has to do with the fate of the ion pair that is formed
5
immediately on abstraction of the proton. If the ion pair separates and diffuses rapidly
into the solution, so that each deprotonation results in exchange, the exchange rate is
an accurate measure of the rate of deprotonation. Under many conditions of solvent
and base, however, an ion pair may return to reactants at a rate exceeding protonation
of the carbanion by the solvent, a phenomenon known as internal return.

ionization dissociation
+
+ –
3 –
R C H + M B [ R C M + BH ] R C – + M + + BH
3
3
internal
return
SD exchange
R CD + S –
3
When there is internal return, a deprotonation event escapes detection because exchange
does not occur. One experimental test for the occurrence of internal return is racem-
ization at chiral carbanionic sites that takes place without exchange. Even racemization
cannot be regarded as an absolute measure of the deprotonation rate because, under
some conditions, hydrogen-deuterium exchange has been shown to occur with retention
of configuration. Owing to these uncertainties about the fate of ion pairs, it is important
3
A. I. Shatenshtein, Adv. Phys. Org. Chem., 1, 155 (1963).
4 R. E. Dixon, P. G. Williams, M. Saljoughian, M. A. Long, and A. Streitwieser, Magn. Res. Chem., 29, 509
(1991); A. Streitwieser, L. Xie, P. Speers, and P. G. Williams, Magn. Res. Chem., 36, S 209 (1998).
5
W. T. Ford, E. W. Graham, and D. J. Cram, J. Am. Chem. Soc., 89, 4661 (1967); D. J. Cram,
C. A. Kingsbury, and B. Rickborn, J. Am. Chem. Soc., 83, 3688 (1961).