quantitative determinations of pK's of some hydrocarbon acids, but no other quantitative work in this area has been reported. We report herein the results of more precise equilibrium acidity measurements in the pK range 18-34. Equilibrium constants were determined in cyclohexylamine for reactions (1) in which AH and BH are
for a constant solvent system and
A-M+
for a changing solvent system, where K, = thermodynamic ionization constant of indicator n, 'K,' = concentration ionization constant of indicator n in solvent 2, and T n = activity coefficient ratio for indicator II in solvent s (DMSO). In order for the same relative acidity to be obtained in the two types of solvent systems, it is necessary that
sr, - _ srl
2r3. . . . ~- rn-l lrl 2r23r3 'rn-1 srn Ir2
-
-
S-1
.
+ BH
AH
+ B-M+
(1)
hydrocarbons whose corresponding carbanions show usable spectral differences and M is lithium or cesium, The solutions were made up on a vacuum line with careful exclusion of air and moisture using known amounts of hydrocarbons and solvent and amounts of lithium cyclohexylamide or cesium cyclohexylamide such that measurable amounts of all four components were present. The concentrations of the two anions at equilibrium were determined from visible spectra of the solutions and the spectral data of Streitwieser and Brauman4 plus the additional results in Table I. The organocesium compounds studied obeyed Beer's law over a concentration range of at least 10-fold, showing that reaction 2 goes to completion. The CaHiiNH-Cs+ + R H +R-Cs+ + CcHiiNH2 (2)
It seems very improbable that this relationship would hold if the individual ratios changed markedly, and this suggests that the ratios are nearly constant and equal to unity by definition. This in turn suggests that an H- scale based on the indicators will be valid in these lithium salts of all compounds more acidic than p solvents of high dielectric constants. biphenylyldiphenylmethane also obeyed Beer's law, Although the indicators appear to behave ideally in but Beer's law correlations could not be obtained for these solvents, it is unlikely that this will be generally solutions of p-biphenylyldiphenylmethane, triphenyltrue. Due to the highly delocalized charge on the methane, or diphenylmethane in lithium cyclohexylindicator anions and the high dielectric constant of, the These compounds are clearly solvents, the salts are probably highly d i s s ~ c i a t e d . ~ ~amide-cyclohexylamine. ~ not much more acidic than cyclohexylamine and are There should also be a minimum of difference in specific not completely converted to the lithium salts. I t is solvation effects on the anion and neutral molecule. clear from the differences between the cesium and In solvents of low dielectric constant, ion pairing is lithium salts of these hydrocarbons that the acidity of important and could cause large deviations from idecyclohexylamine relative to hydrocarbons depends ality. This effect probably accounts for the different on the metal used. This result is undoubtedly a manirelative acidities found by Streitwieser, et festation of the concentrated charge in cyclohexylamide for some of the indicators studied here. ion compared to the charge delocalization in the carb(8) K . Ziegler and H. Wollschitt, Ann. 479, 123 (1930). anion. (9) Preliminary conductivity measurements made in this laboratory The absorbances of the equilibrium solutions deby Dr. G . J. McDonald support this statement. ( I O ) A. Streitwieser, Jr., J. I. Brauman, J . H. Hammons, and A. H. creased slowly with time, presumably from reaction of Pudjaatmaka, J . Am. Chem. SOC.,87, 384 (1965). base with water in the glass or with the silicone stopcock grease, but the calculated equilibrium constant Edwin C. Steiner, Joanne M. Gilbert for any pair of hydrocarbons did not vary significantly Edgar C . Britton Research Laboratory The Dow Chemical Company, Midland, Michigan over the observed range of anion concentrations. Our Received September 8, 1964. procedure establishes relative equilibrium acidities; however, it is convenient to record the results as absolute pK values. This was done by arbitrary reference to 9Equilibrium Acidities of Hydrocarbon phenylfluorene, which Langford and Burwells found to Acids in Cyclohexylamine' have pK = 18.5 & 0.1 in aqueous sulfolane. The results are summarized in Table 11. Although we plan Sir: to extend the present methods to additional hydroThe importance of acidity measurements on proton Earbons of interest and to provide cross-checks within acids in the elucidation of molecular properties has the present series, most of the important equilibria long been recognized. Nevertheless, experimental work in Table I1 have been reproduced by two independent on equilibrium acidities of hydrocarbon acids too weak workers6 and we do not expect the present numbers to to be studied in hydroxylic solvents (Le., pK's greater change by more than -0.2 pK unit. than about 20) has been almost nonexistent. In the One of the important aspects of the present results is 1930's, Conant and Wheland2 and McEwen3made semitheir confirmation of McEwen's approximate assignments. Our earlier suggestion' that the McEwen scale ( I ) Acidity of Hydrocarbons. XVI. Part XV: A. Streitwieser, Jr., R . A. Caldwell, and M . R. Granger, J . A m . Chem. SOC.,86,3578 (1964). This research was supported in part by grants from the Air Force Office of Scientific Research and the Petroleum Research Fund of the American Chemical Society. ( 2 ) J . B. Conant and G. W. Wheland, ibid., 54, I212 (1932). ( 3 ) W . I
