Page 159 - Mechanism and Theory in Organic Chemistry

assume that the line continues with slope a = 0.3, we find pKa 48 for cumene,
over ten units higher than the MSAD value. The MSAD scale, based on what is
evidently much too high a value for a above pKa 31, is therefore in error above
that point.
If we accept Streitwieser's results, we are faced with a dilemma for acids
weaker than cumene. There is an indication from results of exchange rates in
fluorobenzenes that when the anion is not delocalized, a will be roughly 0.9;
these measurements yield pKa = 43 for benzene.81 If we assume that a will
continue to be 0.9 for the weaker saturated carbon acids, which also yield non-
delocalized anions, we can revise the MSAD scale for these substances. In Table
3.1 we list equilibrium pKa values for some selected compounds. These numbers
correspond closely to Cram's scale below pK 3 1, with some modifications to take
account of more recent data ; they should give reasonably reliable relative acidities
up to toluene. One must, nevertheless, always remember that they are tied to the
water pK scale through the value listed for 9-phenylfluorene. This pK is deter-
mined by acidity function methods and is subject to all the uncertainties attend-
ant on those measurement^.^^ Beyond pKa 41, the scale is based on pKa = 43
for benzene, assuming a = 0.9. The results of Dessy and of Applequist do not
correlate as well with the revised scale as they did with MSAD, and no attempt
has been made to include them. It must be emphasized strongly that the values
listed in the high pKa range are approximate estimates only, and likely to be
changed, perhaps drastically, by the results of further experimental work. It is
also important to realize that even at the low end of the scale, values represent
dissociation to ion pairs and depend on the cation and on solution phenomena.
Relative acidities peported in different solvent systems may differ s~bstantially.~~

A. Streitwieser, Jr., P. J. Scannon, and H. M. Niemeyer, J. Amer. Chem. Soc., 94, 7936 (1972).
It should be noted that measurements reported by Ritchie and Uschold (notes 56 (a), 56 (b),
p. 140) yielded a value of 16.4for the pK, of 9-phenylfluorene; if this value is used instead of the earlier
one of Langford and Burwell, the entire scale of Table 3.1 is lowered by two pK units. The revision
has not been made here, as it seems likely that future work will result in further changes. See also
note 55, p. 140.
83 (a) F. G. Bordwell and W. S. Matthews, J. Amer. Chem. Soc., 96, 1214 (1974); (b) F. G. Bordwell,
W. S. Matthews, and N. R. Vanier, J. Amer. Chem. Soc., 97, 442 (1975).

S,: A. Streitwieser, Jr., W. R. Young, and R. A. Caldwell, J. Amer. Chem. Soc., 91, 527 (1969).
S4: A. Streitwieser, Jr., R. A. Caldwell, and W. R. Young, J. Amer. Chem. Soc., 91, 529 (1969).
Ss: A. Streitwieser, Jr., and G. R. Ziegler, J. Amer. Chem. Soc., 91, 5081 (1969).
Sg: A. Streitwieser, Jr., W. B. Hollyhead, G. Sonnichsen, A. H. Pudjaatmaka, C. J. Chang,'
and T. L. Kruger, J. Amer. Chem. Soc., 93, 5096 (1971).
S,: A. Streitwieser, Jr., and D. M. E. Reuben, J. Amn. Chem. Soc., 93, 1794 (1971).
Se: A. Streitwieser, Jr., M. R. Granger, F. Mares, and R. A. Wolf, J. Amn. Chem. Soc., 95, 4257
(1973).
Ss: A. Streitwieser, Jr., P. J. Scannon, and H. M. Niemeyer, J. Amn. Chem. Soc., 94, 7936 (1972).
Slo: M. J. Maskornick and A. Streitwieser, Jr., Tetrahedron ktt., 1625 (1972).
S,,: A. Streitwieser, Jr., and D. R. Taylor, J. Chem. 6c. D, 1248 (1970).
a Calculated from data of Ref. D assuming pK of indene =: 20 and of fluorene =: 23.
The value given is that found in cyclohexylamine. F. G. Bordwell and W. S. Matthews, J. Amer.
Chem. Soc., 96, 1214 (1974), report 29 (corrected to the present scale) in dimethylsulfoxide.
In liquid NH,; corrected to triphenylmethane = 31.
Reported to be between diphenylmethane and toluene.
Estimated by B&B from electrochemical data.