Page 151 - Mechanism and Theory in Organic Chemistry

H- Acidity Functions
Others have approached the problem from a slightly different viewpoint. This
alternative method is an extension to basic media of the acidity function tech-
niques discussed in the previous section.52 Solvents containing dimethylsulfoxide
mixed with water, methanol, or ethanol and a base (potassium hydroxide,
methoxide, or ethoxide) have been most commonly used, although other sub-
stances, such as sulfolane, have also been employed. Spectrophotometric measure-
ments of concentrations of acid and conjugate base forms of an appropriate
series of indicators establish an acidity function, called H-, for mixtures contain-
ing varying proportions of the solvents. The inidcators are usually substituted
anilines, the same class of compound as serves to establish the Ho scale in acidic
mixtures; here, however, the anilines are acting as acids instead of as bases
(Equation 3.45).
ArNH, + S- ArNH- + SH (3.45)
Once established, the H- scale is used to find pKa values for weak acids. A
number of measurements have been made by various gro~ps.~~-~~ The results
obtained at first appeared to disagree with Streitwieser's, but revision of values
for some compounds on the basis of further measurements brought the results
of the two methods into fairly good agreement. At the same time, however, it
became clear that the problems discussed in the previous sections relating to the
different behavior of substances of different structural type also apply to the H-
scale The activity coefficient ratio evidently is not the same for carbon
acids as for the nitrogen acids used to establish the scale.59 Thus the pKa values
found by these methods, while probably internally consistent for similar com-
pounds, are not on a firm basis with respect to their absolute relationship to the
water scale.
Arnett and his collaborators have extended the heat of protonation concept
(Section 3.2) to weak acids by measuring heats of deprotonation, AHD, of weak
acids in dimethylsulfoxide containing the dimethylsulfoxide conjugate base,
H3CSOCH,-.60 The results correlate well with pK, values of the amines used

52 For reviews, see: (a) K. Bowden, Chem. Rev., 66, 119 (1966); (b) C. H. Rochester, Quart. Rev.
(London), 20, 511 (1966); (c) J. R. Jones, Prog. Phys. Org. Chem., 9, 241 (1972).
53 (a) See note 51, p. 139; (b) R. Stewart, J. P. O'Donnell, D. J. Cram, and B. Rickborn, Tetra-
hedron, 18, 917 (1962). --
54 E. C. Steiner and J. M. Gilbert, J. Amer. Chem. Soc., 87, 382 (1965). _---
55 E. C. Steiner and J. D. Starkey, J. Amer. Chem. Soc., 89, 2751 (1967).
5"a) C. D. Ritchie and R. E. Uschold, J. Amer. Chem. Soc., 89, 2752 (1967); (b) C. D. Ritchie and
R. E. Uschold, J. Amer. Chem. Soc., 90, 2821 (1968); (c) M. M. Kreevoy and E. H. Baughman, J.
Amer. Chem. Soc., 95, 8178 (1973).
5' R. Kuhn and D. Rewicki, Ann., 704, 9 (1967); 706, 250 (1967).
See note 55, 56.
s9 H- is defined by the equation

(The derivation is analogous to that given in Section 3.3 for Ho.) The method will be successful only
if the activity coefficient ratio-is the same for all acids investigated as it is for the acids used as
indicators. Since this requirement is evidently not met, it may not be possible to establish a unique
solution pK scale for weak acids by using H-. i
60 E. M. Arnett, T. Q. Moriarity, L. E. Small, J. P. Rudolph, and R. P. Quirk, J. Amer. Chem. Soc.,
95, 1492 (1973).