Page 149 - Mechanism and Theory in Organic Chemistry

commonly used than those described here, have been reviewed by Arnett.40 The
reader is referred to that article for further information.
Heats of Protonation
Arnett has summarized the difficulties inherent in the currently available methods
of dealing with weak bases in solution.41 He notes, for example, that the pKa
values given in the literature for ketones, a very important class of compounds
that undergo a variety of acid-catalyzed reactions, vary over an unacceptably
wide range. The variations arise not only from the activity coefficient problems
mentioned above, but also from such practical problems as the effect of differing
media on position of the absorption peaks in the ultraviolet spectrum. Arnett has
proposed an alternative to the acidity function method for finding pKa values for
weak bases.42 He has measured the heats of protonation of a number of weak
bases in FS03H, in which most of the bases of interest are known from freezing-
point depression, electrical conductivity, ultraviolet spectroscopy, and nuclear
magnetic resonance measurements to be completely protonated. He finds a good
correlation of these heats of protonation with recorded pKa values for series like
the original Hammett nitroaniline indicators that are well 'behaved in acidity
function experiments. The heat of protonation method has the advantage over
the acidity function procedure that all measurements are made in the same
solvent; Arnett proposes that the pKa values obtained for ketones by the heat of
protonation procedure are more reliable than the older acidity function data.

3.3 STRENGTHS OF WEAK BRONSTED ACIDS43
The earliest attempts to evaluate quantitatively the acidity of very weak acids
were contemporaneous with Hammett's pioneering work with weak bases.
Conant and Wheland44 published the first investigations in this area in 1932,
and their results were extended and refined a few years later by M~Ewen.~~. 46
Since organometallic compounds of the alkali metals behave chemically like
carbanions, these investigators reasoned that if an organosodium or organo-
potassium compound, RIM, were mixed with a hydrocarbon, R,H, the equili-
brium constant for the resulting reaction, Equation 3.41, would be a measure
of the relative acidities of the two hydrocarbons RIH and R2H.

The equilibrium constant for Equation 3.41 does not measure directly the
pKa difference between RIH and R2H, because the pKa is defined in terms oiTiF

40 See note 21, p. 133.
41 (a) E. M. Arnett, R. P. Quirk, and J. J. Burke, J. Amer. Chem. SOC., 92, 1260 (1970); (b) E. M.
Arnett, R. P. Quirk, and J. W. Larsen, ibid., p. 3977.
42 See note 41.
43 For reviews see: (a) A. Streitwieser, Jr., and J. H. Hamrnons, Prog. Phys. Org. Chem., 3, 41 (1965);
(b) H. Fischer and D. Rewicki, Prog. in Org. Chem. (Cook and Carruthers, Eds.), 7, 116 (1968) ; (c)
J. R. Jones, The Ionisation of Carbon Acids, Academic Press, London, 1973.
44 J. B. Conant and G. W. Wheland, J. Amer. Chem. Soc., 54, 1212 (1932).
45 W. K. McEwen, J. Amer. Chem. SOC., 58, 1124 (1936).
46 See D. J. Cram, Fundamentals of Carbanion Chemistry, Academic Press, New York, 1965, for discussion
of these results.