Page 156 - Mechanism and Theory in Organic Chemistry

Strengths of Weak Brsnsted Acids 145

with measurements of McEwen on organometallic eq~ilibria.~~ McEwen's pKa
values for the two compounds differ by about 4.5 units, while Streitwieser's
exchange rates differ by about five powers of ten. The Brcansted a calculated from
these results is 1.1. Benzene has an exchange rate close to that of cumene, and
toluene exchanges somewhat more rapidly; these compounds would fit into the
scale at about pKa 36 and 34, respectively. Cram then extended the scale to still
weaker acids for which exchange rates were known from Streitwieser's
and concluded that cyclopentane and cyclohexane had pK,'s of about 43 and 44
(relative to triphenylmethane = 31.5). Cram then noted that the equilibrium
measurements of Applequist on reaction 3.54 and of Dessy on Reaction 3.55
correlated well with the scale, and was therefore able to assign pKa's to several
other compounds. The final list of acidities Cram named the MSAD scale; it
served for a number of years as the best available guide to pKaYs of the very weak
acids.77

The breakdown of the MSAD scale In 1971, Streitwieser and his
collaborators reported new Brcansted catalysis law correlations for exchange rates
of substituted fluorenes and of polyarylmethanes in methanol-sodium methoxide.
These compounds all have pKaYs in the range that can be determined by equili-
brium methods; the values of a were 0.37 for the fluorenes and 0.58 for the poly-
a~ylmethanes.~~ Both correlations are accurately linear over ranges of nearly
ten pK units, and are clearly not two parts of a single line of varying slope. The
authors proposed that the difference in slope results from the different position of
the transition state along the reaction coordinate for proton transfer in the two
series. The fluorenes, which have the extra anion-stabilizing influence of a
cyclopentadienide ring (Section 1.5, p. 39), reach the transition state relatively
early, and the polyarylmethanes, somewhat less effective in anion stabilization,
reach the transition state only when proton transfer is further advanced.
Then, in 1973, Streitwieser reported that the polyarylmethane exchange
rates measured in cyclohexylamine-cyclohexylamide correlate with equilibrium
pK values with a = 0.31 .79 Apparently, when the proton is removed by cyclo-
hexylamide, the polyarylmethanes have early transition states, just as the
fluorenes did for proton removal by the weaker base metho~ide.~~ A short extra-
polation of the Brcansted correlation led to a pK, for toluene of 40.9, about seven
units higher than the value assigned in the MSAD scale. Furthermore, if we

75 See note 45, p. 138.
76 (a) See note 65 (d), p. 143; (b) A. Streitwieser, Jr., W. R. Young, and R. A. Caldwell, J. Amer.
Chem. Soc., 91, 527 (1969); (c) A. Streitwieser, Jr., R. A. Caldwell, and W. R. Young, J. Amer. Chem.
Soc., 91, 529 (1969).
77 MSAD stands for McEwen, Streitwieser, Applequist, and Dessy.
(a) See note 65 (b), p. 143; (b) A. Streitwieser, Jr., W. B. Hollyhead, A. H. Pudjaatmaka, P. H.
Owens, T. L. Kruger, P. A. Rubenstein, R. A. MacQuarrie, M. L. Brokaw, W. K. C. Chu, and
H. M. Niemeyer, J. Amer. Chem. Soc., 93, 5088 (1971).
79 A. Streitwieser, Jr., M. R. Granger, F. Mares, and R. A. Wolf, J. Amer. Chem. Soc., 95,4257 (1973).
Bo The situation is complicated by the finding of a large isotope effect, kR/k, = 11, indicating a
transition state symmetrical with respect to the position of the proton between the acid and the
base. Streitwieser proposed that at the transition state the inicpient anion is still pyramidal, so that
little charge can be delocalized onto the aryl rings. At the transition state, most of the negative charge
therefore still resides on the base, hence a is low even though proton transfer is further advanced.