Page 148 - Mechanism and Theory in Organic Chemistry

Strengths of Weak Brransted Bases 137

acidic and less basic than water. The convenient glass electrode and pH meter
can often be used successfully in nonaqueous media as long as the reference
solution used for standardization of the meter employs the same solvent.32 The pH
values determined, however, will be characteristic of the particular solvent
system and will not be directly transferable to the water scale. Acetic acid has
been used as a solvent for determining relative acidities of strong acids. The
mineral acids HClO,, HBr, and HC1, which all behave as strong acids in water,
are found to differ significantly in acidity.33 The use of a single solvent avoids
the difficulties inherent in making comparisons between different solvent systems,
as is done in work with acid-water mixtures, but at the same time the range of
acidities that can be considered is more limited. Furthermore, complications arise
if the dielectric constant is low (acetic acid E = 6.2 compared with E = 78.5 for
water) ;34 there is then extensive ion pairing.35
Acid-base reactions have been studied in other nonaqueous solvents, such
as acetonitrile, methanol, ethanol, dimethylformamide, and dimethyls~lfoxide.~~
When acids whose strength can also be measured in water are studied in these
solvents, the constants obtained are generally quite different, as would be
expected from the widely different dielectric constants and varying solvating
power of the different solvents. However, if the relative acidities of two compounds
in water and another solvent are compared, the difference in pKa between the
two acids is usually approximately independent of solvent (within about one pK
unit) as long as the acids being compared are of the same charge type and are very similar
in sttu~ture.~~ For example, two particular substituted carboxylic acids may be
expected to differ in acidity by roughly the same amount in dimethylformamide
as they do in water, even though the values of pKa found in the two solvents will
be quite different;38 but no such correlation would be expected if the comparison
were between a carboxylic acid and an anilinium ion (different charge type) or
between a carboxylic acid and a phenol (same charge type but different structural
type). It should be noted, however, that there appear to be exceptions even to
this rough rule-~f-thumb.~~
Other methods of making quantitative measurements on weak bases, less

'' (a) Hammett, Physical Or,ganic Chemistry, p. 265; (b) J. F. Coetzee, Prog. Phys. Org. Chem., 4, 64
(1967); (c) I. M. Kolthoff and T. B. Reddy, Inorg. Chem., 1, 189 (1962); (d) C. D. Ritchie and R. E.
Uschold, J. Am. Chem. Soc., 89, 1721, 2752 (1967).
'' R. P. Bell, The Proton in Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1973, p. 46.
34 See Table 2.1 1 (Section 2.4).
35 In acetic acid it is possible to measure separately the equilibrium constant of proton transfer to
form an ion pair and the constant for dissociation of ion pairs to form free ions. [I. M. Koltho@ and
S. Bruckenstein, J. Amer. Chem. Soc., 78, 1 (1956); S. Bruckenstein and I. M. Kolthoff, J. Amer.
Chem. Soc., 78, 10 (1955)l. G. W. Ceska and E. Grunwald, J. Amer. Chem. Soc., 89, 1371, 1377 (1967)
applied this technique to a number of substituted anilines and concluded that the equilibrium
constant of the ionization step, rather than the overall acid dissociation constant, is the quantity
that should be considered in discussions of effects of structural changes on acidity.
'' (a) M. M. Davis, Acid-Base Behavior in Aprotic Organic Solvents, Nat. Bur. Stds. Monograph 105,
1968; (b) I. M. Kolthoff, M. K. Chantooni, Jr., and S. Bhowmik, J. Amer. Chem. Soc., 90, 23 (1968);
(c) J. F. Coetzee and G. R. Padmanabhan, J. Amer. Chem. Soc., 87, 5005 (1965); (d) B. W. Clare,
D. Cook, E. C. F. KO, Y. C. Mac, and A. J. Parker, J. Amer. Chem. Soc., 88, 191 1 (1966); (e) C. D.
Ritchie and G. H. Megerle, J. Amer. Chem. Soc., 89, 1447, 1452 (1967).
" Bell, The Proton in Chemistry, p. 56; see also footnotes 36 (c) and 36 (d).
See note 36 (d).
'' See notes 36 (c) and 36 (d).