EFFECT OF SUBSTITUENTS ON ACID STRENGTH
485
DIPPY,J. F. J., AND LEWIS,R. 11.: J. Chem. Soc. 1936, 644. DIPPY,J. F. J., ASD LEWIS,R. H . : J. Chem. Soc. 1937, 1426. DIPPY,J. F. J., A N D WILLIAMS, F. R . : J. Chem. SOC. 1934, 1888. DIPPY,J. F. J., WILLIAMS, F. R., A Z D LEWIS,R. H.: J. Chem. SOC.1936, 343. ELLIOTT, J. H., ASD KILPATRICK, M.: J. Phys. Chem. 4G, 454 (1941). ELLIOTT, J. H., A X D KILPATRICK, 11.:J. Phys. Chem. 46, 466 (1941). FALKENHAGEX, H.: Electrolytes. Translated from the German by R. P. Bell. Oxford University Press, Oxford (1934). (13) HAMMETT, L. P.: J. Chem. Phys. 4, 613 (1936). (14) HAhlMETT, L. P . : J. .4m. Chem. SOC. 69, 96 (1937). (15) JENKINS, H. 0.: J. Chem. SOC.1939, 640. H. 0 . : J. Chem. SOC.1939, 1137. (16) JEXKINS, M., AND XIEARS,W.H . : J. Am. Chem. SOC.62, 3051 (1940). (17) KILPATRICK, (18) KILPATRICK, M.:Trans Electrochem. SOC. 72, 95 (1937). (19) KIRKWOOD, J. G., A N D WESTHEIhlER, F. H. : J. Chem. Phys. 6, 506 (1938). (20) LAWRIE, J. W.: Glycerol and the Glycols. The Chemical Catalog Company, Inc., New Tork (1928). (21) MASON,R. B., AND KILPATRICK, M.: J. Am. Chem. SOC. 69, 572 (1937). (22) MIINNICK, L. J., AND KILPATRICK, M.J . : Phys. Chem. 43, 259 (1939). E.: Trans. Electrochem. SOC.64, 209 (1933). (23) NEWBERRY, B., AND MEIER,H. F.: J. 4 m . Chem. SOC.56, 1918 (1934). (24) SAXTON, (25) SCHWARZENBhCH, G., AND EGLI,H.: Helv. Chim. Acta 17, 1183 (1934). F. H . : J. Am. Chem. SOC.61, 1977 (1939). (26) WESTHEIMER, F. H., AND KIRKWOOD, J. G . : J. Chem. Phys. 6, 513 (1938). (27) WESTHEIMER, L. A , , AND HAMMETT, L. P.: J. Am. Chem. SOC.57, 2289 (193.5). (28) WOOTEN, W. F. K.: Proc. Roy. SOC. (London) A l a , 440 (1933). (29) WYNNE-JONES,
(6) (7) (8) (9) (10) (11) (12)
THE EFFECT OF SUBSTITUEXTS OK THE ACID STREKGTH OF BENZOIC ACID. IV1
IN DIOXANE-WATER JOHK H. ELLIOTT
AND
MARTIS KILPATRICK
Department o j Chemistry and Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania Received August 20, 1940
This paper extends to mixed solvents the study of the effect of solvent on relative acid strengths. Dioxane-water mixtures permit a study over a wide range of dielectric constants (1). Minnick and Kilpatrick (10) 1 This paper was abstracted from the dissertation presented by John Habersham Elliott t o the Faculty of the Graduate School of the University of Pennsylvania in partial fulfilment of the requirements for the degree of Doctor of Philosophy, April, 1940. It was read a t the Sinety-ninth Meeting of the American Chemical Society, which was held in Cincinnati, Ohio, April, 1940.
.
486
JOHN H ELLIOTT AND MARTIN KILPATRICK TABLE 1
The dissociation constant of benzoic acid i n diozane-water miztures conlainina
Dioxane. weight per cent . . . . . . . . . 0 . Do . . . . . . . . . . . . . . . . 78.5 pK, (p = 0.05) ..... 4.039 pK(p = 0) ......... 4.200 (2)
26.5 55 4.820
61 .0 25 6.158 7.007
43.5 40
5.469
73.5 15 7.029 (10)
TABLE 2 Log
KA.B~ i n diozane-water miztures containing lithium chloride
Weight per cent of
dioxane ............... Do ......................
26.1
1
T = 25.C
.
43.3 40
61.0 26 LOO
L)UBETF?UINT
I
73.5 15
K A ~ B ~
Ortho-substituted benzoic acids NO, . . . . . . . . . . . . . . .
I.................. Br . . . . . . . . . . . . . . . . .
c1. . . . . . . . . . . . . . . . .
CHI . . . . . . . . . . . . . . . OCHI . . . . . . . . . . . . . . OH . . . . . . . . . . . . . . . .
1.782 1.003 1.131 1.085 0.035 -0.058
1.490
1.661 0.873
1.576 0.752
1.004 0.971 -0.079 -0.095 1.596
0.885
0.911 1.724
0.836 -0.214 -0.069 1.775
Meta-substituted benzoic acids
NO*. . . . . . . . . . . . . . I................. Br . . . . . . . . . . . . . . . . c1. . . . . . . . . . . . . . . . F. . . . . . . . . . . . . . . . . CHI . . . . . . . . . . . . . . OH . . . . . . . . . . . . . . .
0.889 0.372 0.433 0.441 0.411 -0.124 0.010
0.973 0.397 0.470 0.483 0.412 -0.142
0.514
O.OO0
1.060 0.456 0.527 0.532 0.488 -0.145 -0.022
Para-substituted benzoic acids NO*................
I.................. Br . . . . . . . . . . . . . . . . . c1. . . . . . . . . . . . . . . . . F. . . . . . . . . . . . . . . . . . CHI. . . . . . . . . . . . . . . OCHI . . . . . . . . . . . . . .
0.974
* *
*
0.150 -0.229 -0.332
1.100 0.378 0.375 0.380 0.220 -0.215 -0.325
1.180
0.413
0.375 0.427 0.390 0.254 -0.210 -0.361 -0.534
487
EFFECT OF SUBSTITUENTS ON ACID STRENGTH
found that the relative acid strengths of carboxylic acids in ethyl alcohol and dioxane-water mixtures of the same dielectric constant were the same. The measurements included only two substituted benzoic acids. The results are now extended to twenty acids in media having the dielectric constant 55, 40, and 15. TABLE 3 pKc i n dioxaneluater mixtures; p = 0.06, mostly lithium chloride T = 25OC. Weight per aent of dioxane. , . . . . . , .. . . .. Do . . . . . .., . . .. . . . , . . . . . .
!
..
LIUBsTlTUENT
3.038 3.817 3.689 3.735 4.785 4.878 3.330
NO*.. . . . . . . . . . . , . . I. . . . . . . . . . . . . . . . . .
Br... . . . . . . . . . . . . . , Cl.. . . . . . . . . . . . . . . . F..... . . , . . . .. . . ,.. CHa.. . . . . . . . . . . . , . OH... . .... . . . . . . , ,
3.931 4.448 4.387 4.379 4.409 4.944 4.810
NO*. . . . . . . . . . . . . . .
3.846
,
I. . . . . . . . . . , , , , . . , , Br . . . . . . . . . . . . . . . . . c1.. . . . , , , . . . , . . , . . F... . . . , . . .. , . , . . , . CHs.. . . . . . . . . . . , . , . OCHs.. . . . . . . . . . . . . O H . . . . . , , . , . , . . .. .
4.670 5.049 5.152 5.241
1
26 61.0
1
73.8 15
PKC
66
... . , ., , , I.., . . . . . . . , . . .. . . . Br . . . . . , . . . . . . . . , . . c1.. . . . . . . . . . . , . . . . CHI. . . . . . . . . . . . . . . OCHs. . . . . . . . . . . . . . OH . . . , . , . . , . , . , , . . N O z . .. . .
$5
26.5
4.596 4.465 4.498
5.247
i:g
~
1
4.434
3.873
~
,
4.496 5.072 5.057 5.611 5.469
5.969 6.573 6.502 7.497 6.541 7.174 7.051
4.369 5.091 5.094 5.089 5.249 5.684 5.794 5.942
5.849 6.654 6.602 6.639 6.775 7.239 7.390 7.563
5.644
1
5.453 6.277 6.144 6.193 7.243 7.098 5.254
5.745
The experimental method was described in an earlier paper (5),and the measure of the precision of the results was estimated at 0.01 unit in log K*J,~. In the experiments with 61 per cent and 73.5 per cent dioxane, the salt-bridge solutions were 0.20 and 0.15 molar in lithium chloride, respectively. I n the other cases the bridge solution was 1.0 molar in
488
JOHN H. ELLIOTT AND MARTIN KILPATRICK
lithium chloride. Dioxane was purified by refluxing with metallic sodium for 12 hr. or more, fractionating in a Snyder column, and collecting the middle fraction over a 0.1" range. The index of refraction was n:" = 1.4200, as compared with a literature value of ni5' = 1.4198 ( 7 ) . The freezing point was 11.71"C. (value given in the literature, 11.78OC. (7)). Since water is more basic than the alcohols, the dissociation constants of the carboxylic acids are appreciably higher and the acid-base ratio of the buffer solution must be corrected for the dissociation of the acid. This necessitates a knowledge of the dissociation constants of the acids in the medium employed. Table 1 summarizes the determination of the dissociation constant of benzoic acid by measurements against hydrochloric acid in a suitable concentration cell ( 5 ) . These values are a t p = 0.05 (mostly lithium chloride). The ratio to the thermodynamic constant is K,(0.05) ~- - 7.0 a t D = 25, as compared to 1.45 for the same ratio in water. K, That the ratio K J K , increases markedly for the same molarity of lithium chloride a t low dielectric constants of the mixture is evident from the results of Lynch and La Mer (8). Table 2 summarizes the results for the acid strengths relative to benzoic acid, and table 3 gives the negative logarithms of the dissociation constants a t p = 0.05. On the assumption that log KAzRo is independent of electrolyte concentration in these solutions, log KAzBois plotted against the reciprocal of the dielectric constants of the electrolyte-free media in figures 1 to 5. For comparison the best lines through the experimental data in the pure solvents are given (6). It is evident that the equation
is not applicable to the dioxane-water mixtures (10). That this lack of linearity of the relative acid strengths with the reciprocal of the dielectric constant is not confined to dioxane-water mixtures is indicated by figure 6. The data for this figure were obtained from the determinations of Michaelis and Mizutani (9), who determined
in buffer solutions 0.01 M in benzoic acid and benzoate and i.n salicylic acid-salicylate buffers of the same concentration. It was observed in the course of these experiments that the acids studied were far more soluble in dioxane than in water, while the reverse was true for their salts and lithium chloride. In fact, Lynch and La Mer (8) state that dioxane may be salted out of a dioxane-water mixture if the concentration of lithium chloride is sufficiently great. This indicates that
EFFECT O F SUBSTITUENTS ON ACID STRENGTH
489
490
JOHN H. ELLIOTT AND MARTIN KILPATRICK
@, Dioxane -#,O
Mixtures
FIQ.5. Effect of substituents on acid strength. Curve I, o-methoxybenzoic acid; curve 11, m-fluorobenzoic acid; curve 111, p-methoxybenaoic acid.
...
0.01
0.02
0.03
0.04
FIQ. 6. Log K A = Bfor ~ o-hydroxybenaoic acid in water-alcohol mixtures 491
492
JOHN H. ELLIOTT AND MARTIN KILPATRICK
in a solution of lithium chloride the water is preferentially oriented around the lithium and chloride ions and that in the immediate neighborhood of these ions the solvent does not have the same composition as in the body of the solution. It seems likely that there is a preferential orientation of dioxane about the acid molecules, since these are much more soluble in dioxane than in water. This would lead to a lower dielectric constant of the solvent in the neighborhood of the acid molecule. Bury (3) states that there is evidence of formation of clusters of butyric acid in aqueous solution, and Butler (4) has found negative entropies of hydration for alcohols in water. These observations tend to confirm the orientation of solvent molecules about solute molecules. There is the possibility that effects of this sort might seriously affect the assumptions of the experimental measurements, but it seems more likely that equation 1 cannot be applied when the dielectric constants of the mixed solvents are used. BUMMARY
1. The relative acid strengths of the substituted benzoic acids have been determined in dioxane-water solutions containing lithium chloride and having various dielectric constants. 2. The relative acid strengths are not a linear function of the reciprocal of the dielectric constant and, in general, are not the same as the values in alcohols a t the same dielectric constant. (1) (2) (3) (4) (5) (6)
(7) (8) (9) (10)
REFEREXCES SHORT, 0. 4.:J. Am. Chem. Sac. 68, 1241 (1936). BROCKMAN, F. G., AND K I L P A T R I C K , M . : J . .4m. Chem. sac. 66, 1483 (1934). BURY,C . R.: Trans. Faraday Sac. 33, 237 (1936). BUTLER, J. A. V.: Trans. Faraday Sac. 33, 229 (1936). ELLIOTT, J. H . , AND KILPATRICK, M.: J. Phys. Chem. 45, 451 (1941). ELLIOTT, J. H., AND KILPATRICK, M.: J. Phys. Chem. 45, 472 (1941). HOVORKA, F., SCHAEFFER, R. A , , A X D DREISBACH, D . : J. Am. Chem. Sac. 68, 2264 (1936). LYXCH,C. C . , A N D LA MER,V. IC.: J. Am. Chem. Sac. 60, 1252 (1938). MICHAELIS, L., AND M I Z U T A N I , L f . : 2. physik. Chem. 116, 1% (1925). ~ I I X X I CL. K ,J., A N D KILPATRICK, 31.: J. Phys. Chem. 43, 259 (1939).
AKERLOF, G., AND
