Page 145 - Mechanism and Theory in Organic Chemistry

indicator bases, found an acidity scale, designated H, which is different from
Ho. A group of cyclic amines, indoles of general structure 1, were investigated by
Hinman and Lang:z4

these indicators gave still another acidity scale, denoted HI, which differed slightly
from the H scale. Another scale, HA, was established by Yates, Stevens, and
Katritzkyz5 with a series of amides as indicators. Still another function, HE, is
based on the behavior of triarylcarbinols. These substances, studied by Deno,
Jaruzelski, and Schrie~heirn,~~ typically react according to Equation 3.39 to form
water (which is converted partly to oxonium ion) and a carbocation. The HR
function thus includes the activity of water in addition to the quantities of

Equation 3.38. A slightly different function, HA, is derived from HE by subtrac-
tion of the logarithm of water activity (Equation 3.40).

Hydrocarbons containing carbon-carbon double bonds can be protonated in
strong acid media,z7 and a scale designated H, appropriate to these substances
has been establi~hed.~~
Whereas sulfuric acid has been the most frequently used acid, acidity
function scales have been set up for other strong acid mixtures. Of particular
interest have been the superacid media usually prepared from mixtures of fluoro-
sulfuric acid, HSO,F, with various Lewis acids such as SO, or SbF,. These
media, the most acidic known, have made possible direct observation of highly
reactive carbocations (see Section 5.4), and the protonation of extremely weak
bases.z9 Figure 3.1 shows the behavior of some of the different acidity functions
in sulfuric acid-water mixtures, Figure 3.2 gives the behavior of Ho in mixtures
of water with various strong acids, and Figure 3.3 presents data for mixtures of
HS03F with SbF5.30.31
The proliferation of acidity scales, each with an equally sound basis and no
one of which can claim to be any more fundamental or correct than any other, is
a rather disappointing development. It nevertheless illustrates the point discussed
24 R. L. Hinman and J. Lang, J. Amer. Chem. Soc., 86, 3796 (1964).
25 K. Yates, J. B. Stevens, and A. R. Katritzky, Can. J. Chem., 42, 1957 (1964).
28 N. C. Deno, J. J. Jaruzelski, and A. Schriesheim, J. Amer. Chem. Soc., 77, 3044 (1955).
27 (a) H. H. Perkampus, Adu. Phys. Org. Chem., 4, 195 (1966); (b) D. M. Brouwer, E. L. Mackor, and
C. MacLean, in Carbonium Ions, Vol. 11, G. A. Olah and P. v. R. Schleyer, Eds., Wiley-Interscience,
New York, 1970, p. 848.
28 M. T. Reagan, J. Amer. Chem. Soc., 91, 5506 (1969).
29 G. A. Olah and J. Shen, J. Amer. Chem. Soc., 95, 3582 (1973).
30 Superacid mixtures: R. J. Gillespie and T. E. Peel, J. Amer. Chem. Soc., 95, 5173 (1973).
31 For a more complete discussion of the various acidity functions, see Hamrnett, Physical Organic
Chemistry, chapter 9.