Page 155 - Mechanism and Theory in Organic Chemistry

Thus it is not possible to use the method to determine pK's without first making
measurements on a number of compounds of known acid dissociation constant.
Pearson and Dil10n~~ collected data for exchange rates and equilibrium constants
of a number of carbon acids in the pKa range 4-20. These compounds all con-
tained electron-withdrawing groups (carbonyl, nitro, cyano, trifluoromethyl,
etc.) ; the correlation of rate and pKa was only rough. The problem is presumably
one of differences in behavior arising from variations in structure. Shatenshtein
measured exchange rates of a number of carbon acids in liquid ammonia;68 his
work demonstrated clearly the acidic properties of even saturated hydrocarbons,
and allowed a qualitative measure of relative acidity of various types of carbon-
hydrogen bonds.
Streitwieser and co-workers have extended their measurements of equili-
brium acidities in cyclohexylamine to determination of exchange rates.69 They
have made quantitative correlations between exchange rate and the pK,'s
determined by equilibrium methods for various aromatic compounds and have
thus been able to verif) that the Brernsted relation holds for these substances and
to find Brernsted coefficients a for various types of compounds. A third method
for evaluating pKa of weak acids, which has been used by Applequist70 and by
De~sy,~l involves the study of exchange reactions of organometallic compounds
(Equations 3.54 and 3.55).

The MSAD Scale
In 1965, Cram established a scale of acidities reaching to the very weak carbon
acids by combining data from the various methods.72 The basis of the scale is the
value of pKa = 18.5 found by Langford and Burwell for 9-phenylfluorene (2),73
and it includes the equilibrium measurements of Streitwieser and others up to
pKa 33. Table 3.1 records some selected equilibrium values in this range. Beyond
pKa 33, direct equilibrium methods fail and only the kinetic and organometallic
techniques can be used. Cram compared Streitwieser's exchange-rate measure-
ments for triphenylmethane and for cumene (4),74 -

87 See note 63, p. 143.
68 See note 64, p. 143.
69 See note 65, p. 143.
D. E. Applequist and D. F. O'Brien, J. Amer. Chem. Sod., 85, 743 (1963).
(a) R. M. Salinger and R. E. Dessy, Tetrahedron Lett., 729 (1963); (b) R. E. Dessy, W. Kitching,
T. Psarras, R. Salinger, A. Chen, and T. Chivers, J. Amer. Chem. Soc., 88, 460 (1966).
72 1). J. Cram, Fundamentals ofCarbanion Chemisty, Academic Press, New York, 1965, p. 19.
73 See note 51, p. 139.
74 See note 65 (d), p. 143.