Page 150 - Mechanism and Theory in Organic Chemistry

Strengths of Weak Brransted Acids 139

dissociated ions (Equation 3.42). Ether and benzene were used as solvents, and
more recent evidence47 indicates that the organometallics probably exist almost

entirely as ion pairs in nonpolar solvents. Conant and Wheland were aware of
the dissociation problem; on the basis of some earlier conductivity measure-
they
ment~,~~ assumed that the dissociation constants for the various organo-
metallics would be about the same, and that equilibrium constants for Reaction
3.41 would give reasonably accurate measures of pKa differences. On the basis
of this assumption, McEwen determined pKa values for a number of carbon
acids; he was also able to relate the acidities of the carbon acids studied with
those of several weak oxygen acids49 by studying equilibria such as Equation 3.43.

Further experiments designed to elucidate acid-base relationships among
weak acids have been carried out more recently by Streitwieser and his co-
worker~.~~ They studied the equilibrium shown in Equation 3.44, with cyclo-
hexylamine as solvent and lithium or cesium cyclohexylamide as base. Using
spectrophotometric methods to evaluate the position of the equilibrium, they
were able to find relative pKa values for a number of hydrocarbons in which the
conjugate base is, in most cases, a conjugated aromatic anion. In order to attach

definite pKa values to the results, these authors took as a reference point the
value ofpKa = 18.5 reported by Langford and Burwellsl for 9-phenylfluorene (2).
This value was determined in a solvent consisting of a mixture of water and

sulfolane (3) using the indicator methods described below. When McEwenYs
results are placed on a scale with 9-phenylfluorene having a pKa of 18.5, the
agreement with Streitwieser's results is reasonably good up to pK, about 31
(triphenymethane) .

47 T. E. Hogen-Esch and J. Smid, J. Amer. Chem. Soc., 87, 669 (1965).
K. Ziegler and H. Wollschitt, Ann., 479, 123 (1930).
We denote acids in which the acidic proton is attached to carbon as carbon acids, those with the
proton attached to oxygen as oxygen acids, and so forth for acids of other types.
60 See, for example: (a) A. Streitwieser, Jr., J. I. Brauman, J. H. Hammons, and A. H. Pudjaatmaka,
J. Amer. Chem. Soc., 87, 384 (1965); (b) A. Streitwieser, Jr., J. H. Hammons, E. Ciuffarin, and J. I.
Brauman, J. Amer. Chem. Soc., 89, 59 (1967); (c) A. Streitwieser, Jr., E. Ciuffarin, and J. H. Ham-
mons, J. Amer. Chem. Soc., 89,63 (1967); (d) A. Streitwieser, Jr., and D. M. E. Reuben, J. Amer. Chem.
SDG., 93, 1794 (1971); (e) A. Streitwieser, Jr., J. R. Murdoch, G. Hafelinger, and C. J. Chang,
J. Amer. Chem. Soc., 95, 4248 (1973) ; (f) Acidities found by these methods are ion pair acidities and
do not represent dissociation to free ions. See A. Streitwieser, Jr., and S. P. Ewing, J. Amer. Chem.
SOC., 97, 190 (1975).
" C. H. Langford and R. L. Burwell, Jr., J. Amer. Chem. Soc., 82, 1503 (1960).