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Dissociation Constants of Bicyclo [ 2.2.21oct-2-ene-1-carboxylic Acids, Dibenzobicyclo[2.2.21octa-2,5-diene-1-carboxylic Acids, and Cubanecarboxylic Acids' Frank W. Baker, Roger C . Parish, and Leon M. Stock
Contribution f r o m the George Herbert Jones Laboratory, Department of Chemistry, Unioersity of Chicago, Chicago, Illinois 60637. Received June 19, 1967 Abstract: Thermodynamic dissociation constants are reported for 4-substituted bicyclo[2.2.2]oct-2-ene-l-carboxylic acids, 4.-substituted dibenzobicyclo[2.2.2]octa-2,5-diene-l-carboxylicacids, and cubanedicarboxylic acid in 50z (weight) ethanol-water at 25". The experimental results indicate that variations in the hybridization of the carbon-carbon bonds between the substituent and the reaction site and a variation in the number of paths between the substituent and the reaction site have a negligible influence on the efficiency of propagation of the polar effect. These observations are compatible with the field effect model and incompatible with the inductive model for the propagation mechanism. The Kirkwood-Westheimer model yields reasonable estimates of the substituent effects. In addition, the origin of the polar effects of the methyl and trifluoromethyl groups are discussed.
T
wo distinct models, the field effect and the inductive effect, have been considered in recent discussions of the mechanism of propagation of the polar e f f e ~ t . ~ -In ~ brief, the inductive model presumes that substituent effects are propagated by the successive polarization of the bonds between the substituent and the reaction site. The net polar effect, according to this model, depends on the number and nature of the paths between the substituent and the reaction site. Substituent effects are transmitted through both the u-bond network (a-inductive effect) and, when present, the 7r-bond network (7r-inductive e f f e ~ t ) . ~There ,~ is no general agreement concerning either the absolute or relative importance of these contributions to the observed polar effect. However, theoretical considerations require that substituent effects be transmitted more effectively through the more polarizable 7r bonds than through a bonds. The transmission efficiency is measured empirically by E , the transmission coefficient, and many values have been proposed for this parameter.3-8 Bowden's suggestion^,^ E (sp3 carbon) = 0.48, t (spz carbon) = 0.67, E (aryl carbon) = 0.60, reflect the viewpoint that a large value is necessary for the accommodation of the experimental facts3-j and that polar effects are propagated more effectively through 7r bonds. 3-5.7 The field effect model presumes that the polar effect originates in bond dipole moments and that the effect is propagated according t o the classical laws of electrostatics. 3-7 In the Kirkwood-Westheimer formulation of this t h e ~ r y , ~ - lthe l molecule is treated as a cavity (1) Chemistry of the Bicyclo[2.2.2]octanes. V. This research was supported by a grant, GP4398, from the National Science Foundation. (2) The term, polar effect, is used to characterize the influence of unconjugated, sterically remote substituents on equilibrium or rate processes. This designation permits the use of the terms, inductive effectand field effect,for the description of the transmission mechanism. (3) M . J. S. Dewar and P. J . Grisdale, J . Am. Chem. SOC.,84, 3539, 3548 (1962). (4) K. Bowden, Can. J . Chem., 41,2781 (1963). ( 5 ) S. Ehrenson, Progr. Phj's. Org. Chem., 2, 195 (1964). (6) C. D. Ritchie and W. F. Sager, ibid., 2 , 323 (1964). (7) J . N. Murrell, S. F. A . Kettle, and J. M. Tedder, "Valence Theory," John Wiley and Sons, Ltd., London, 1965, Chapter 16. (8) J. C. McGowan, J . Appl. Chem. (London), 10, 312 (1960). (9) J. G. Kirkwood and F. H. Westheimer, J . Chem. Phys., 6, 506 ( 1 938).
of low dielectric constant submerged in a solvent continuum. Kirkwood and Westheimer suggested that the cavity dielectric constant was about 2.9,1n In the absence of a method to assess the constant, subsequent workers have used this value for a variety of compounds. This approach is not unreasonable because, as Ehrenson points out, the contribution of the niolecular framework to the effective dielectric constant must be relatively insensitive to the character of the chemical bonds within the ~ a v i t y . ~Thus, the two limiting models differ with respect to their predictions concerning the influence of changes in the character of the cavity on the efficiency of propagation. We have examined this point to establish whether, or not, one of the models was more satisfactory. The influences of unsaturation and the number of paths between the substituent and the reaction site are treated in this report. The importance of unsaturation was studied by the investigation of 4-substituted bicyclo[2.2.2]oct-2-ene- 1carboxylic acids and 4-substituted dibenzobicyclo[2.2.2]octa-2,5-diene-l-carboxylic acids. Substituent
3" R
@@ R
effects in these two series of acids may be compared directly with the results for the 4-substituted bicyclo[2.2.2]octane- I-carboxylic ,acids, examined prev i o u ~ l y , ~because ~ - ~ ~ the distance and number of paths between the substituent and the reaction site remain the same. Accordingly, the results for these acids reflect the relative efficiency of unconjugated, unsaturated B bonds compared to saturated linkages for the propagation of the polar effect. In addition, cubane-l(10) (11) (12) (13)
F. H. Westheimer and J. G. Kirkwood, ibid., 6,513 (1938). C. Tanford, J . Am. Chem. SOC.,79,5348 (1957). J. D. Roberts and W. T. Moreland, ibid., 75, 2167 (1953). H. D. Holtz and L. M. Stock, ibid., 86, 5188 (1964). (14) C. F. Wilcox and J. S. McIntyre, J . Org. Chem., 30, 777 (1965).
Baker, Parish, Stock
Polar Efect in Carboxylic Acids
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Table I. Thermodynamic Dissociation Constants for Some Rigid Carboxylic Acids in 50
(weight) Ethanol-Water at 25
106K 4-Substituent
H CHI OCH3 F CI Br COzCzHj CONHi CF3 CN NOn COiH Stat cor
con-
Stat cor N(CHa)3+C1-
Octane acids
Octene acids
0.134 f 0.015
7
Dibenzo acids
0.286 f 0.008 0.314 f 0.007
1.78 1.67 3.14 5.52 6.15 6.12
1.92 f 0.11
0.568 f 0.005
0.993 1.09 1.64 3.20
f f f f
2.09 1.05 0.097 0.194 14.6
i 0.08 f 0.04 f 0.0015 f 0.003 f 0.4
f 0.06 i 0.09 f 0.05 f 0.17 i 0.09 i 0.59
H CH3 CnHj CHzOH COyCzHj F
c1
Br OH OCH3 CFa CONHz CN NOz
COnH
con-
N(CH&+
1.11 f 0 . 0 1
0.006 0.02 0.01 0.06 11.5 i 0 . 6
0.787 0.398 0.0337 0.0674
f 0.013 f 0.006 f 0.0003 f 0.0006
Table 11. Log (K/KH) for 4-Substituted Bicyclic Carboxylic Acids in 50% (weight) Ethanol-Water at 25” 4-Substituent
Cubane acids
Log (K/KH) Octene acids*
Octane acids.
0.000 -0.013 -0,020 0.074 0.473 0.739 0.736 0.370 0.472 0.627
O.OO0 f 0.032 i 0.030 =k 0.031 f 0.035
+0.041 f 0.015
f 0.039
0.827 f 0.028
f 0.044 f 0.032 f 0.029b
a Reference 13. * This study. Reference 12. (weight) ethanol-water. e Statistically corrected.
f
0.05 0.03 0.002 0.004
-
Dibenzo acids*
0.000 -0,027 f 0.039
0.491 i 0.029 0.539 i 0.021 0.537 f 0.054 0.246 f 0,011
0.758 i 0.012 0.581 i- 0.015 1.049 + 0.015
0.810 i 0.036 0.565 & 0.021e -0.169 i 0.014e 1.707 i 0.017
The value reported for 50% (volume) ethanol-water has been extrapolated to 50% Reference 14.
carboxylic acid and cubane-l,4-dicarboxylic acid l 5 were studied to examine the proposition that additional paths between the substituent and the reaction site enhance polar effects. The results for these acids may be compared with the data for bicyclo[2.2.2]octane1,4-dicarboxylic acid because the C1C4distance differs by only 0.1 A. Results The thermodynamic dissociation constants of recrystallized and sublimed samples of the acidsI6 were determined by the differential potentiometric method” or by potentiometric titration. Independently prepared solutions, different batches of solvent, and different electrodes were used in this work. In addition, the dissociation constant of benzoic acid was frequently determined to ensure the reliability of the data. The (15) P. E. Eaton and T. W. Cole, J . A m . Chem. Soc., 86, 962, 3157 (1964). The cubane acids were generously supplied by Professor P. E. Eaton and Dr. Cole. (16) The methods used for the preparation of the 4-substituted bicyclo[2.2.2]oct-2-ene-l-carboxylic acids are described by F. W. Baker and L. M. Stock, J . Org. Chem., in press. The 4-substituted dibenzobicyclo[2.2.2]octa-2,5-diene-l-carboxylicacids were prepared as described in the Experimental Section, (17) A. L. Bacarella, E. Grunwald, H. P. Marshall, and E. L. Purlee, J . Org. Chem., 20, 747 (1955). (18) J. C. Speakman,J. Chem. Soc., 855 (1940).
Journal of the American Chemical Society
f i f f
0.541 i 0.012
f 0.032
0.93Cmd 1.058 f 0.032 0.468 i 0.030*,e -0.298 f 0.026bse 1. 5Od,f
7.49 3.79 0.298 0.596
pK for benzoic acid measured in this study, 5.74 f 0.01, is in good agreement with previous determindtions, 5.76 f 0.0219 and 5.74 f 0.01.13 The measured constant of bicyclo[2.2.2]octane-l-carboxylic acid was also in excellent agreement with the earlier result.13 The results are summarized in Table I. For convenience, the bicyclo[2.2.2]octane-1-carboxylic acids are referred to as the octane acids, the bicyclo[2.2.2]oct-2-ene-lcarboxylic acids as the octene acids, and the dibenzobicyclo[2.2.2]octa-2,5-diene-l-carboxylicacids as the dibenzo acids in Table I and in the subsequent discussion. Discussion
The dissociation constants for the octane, octene, and dibenzo acids are summarized in Table 11. The results for the three series of acids are correlated by the uI constants based on chemical reactivity.20,21 (19) E . Grunwald and B. J. Berkowitz, J . Am. Chem. Soc., 73, 4939 (1951). (20) (a) R. W. Taft, “Steric Effects in Organic Chemistry,” M. S. Newman, Ed., John Wiley and Sons, Inc., New York, N. Y . , 1956, Chapter 13; (b) R. W. Taft, J . Phys. Chem., 64, 1805 (1960); ( c ) R. W . Taft, E. Price, I. R. Fox, I. C. Lewis, I
