EFFECT OF SUBSTITUENTS ON ACID STREKGTH OF BENZOIC ACID
221
latter value is in good agreement with the value of 112 found by Levy. The agreement with the value of 126 reported by Balson and Lawson is less satisfactory; apparently the value of 65 found by Tomiyama is in error. The reliability of the value of the present authors is estimated to be f 5 per cent. SUMMARY
A method is described amino acid-formaldehyde acids with formaldehyde. tion constant of the only formaldehyde.
for the determination of the constants of the equilibria by polarimetric titration of amino The value, 105 f 5, was found for the associacomplex formed between the proline anion and
REFERENCES LAWSON, A . : Biochem. J . SO, 1257 (1936). (2) DAVIS, W.A.: J. SOC.Chem. Ind. 18,502 (1897). (3) LEVY,M.:J. Biol. Chem. 99,767 (1932-33). (4) LEVY,M.,AND SILBERYAN, D . E . : J. Biol. Chem. 118, 723 (1937). (5) ROMIJN, G.Z.:Z. anal. Chem. 38.19 (1897). (6) TOMIYAMA, T.: J. Biol. Chem. 111, 51 (1935). (7) WADSWORTH,A , , AND PANGBORN, M. C . : J. Biol. Chem. 118.423 (1936).
(1) BALSON, E. W . ,
A'
AND
HB 0.005M LiB 0.005 M LiCl 0.045M Quinhydrone
LiCl 1.00 M
H X 0.005iZI LiX 0.005A1 Au LiCl 0.045M Quinhydrone
222
JOHN H. ELLIOTT
I
Au ~LiB u 0.005M ~ n
1
1
HB 0.025M Licl h ~ d i LiB ~ ~0.005 Jf AU l'OOM Quinhydrone I
223
EFFECT OF SUBSTITUEKTS ON ACID STRENGTH OF BEXZOIC ACID
after equal additions of lithium chloride to both arms. The theoretical E.M.F. is 41.35 millivolts. The observed values are given in table 1. TABLE 1 Test of the ezperimental method
...I
LiC1:LiB . . . . . . . . . . . . . . . . . . . . E . M . F. . . . . . . . . . . . . . . . . . . .
i
0
41.6
~
1
1.6 4.8 41.2 141.8
16.0 41.5
4;:;
~
40.0 41.1
TABLE 2
1
Effect of lithium chloride
LiC1:LiB. . . . . . . . . . . . . . . . . . . . . E.M.F. . . . . . . . . . . . . . . . . . . . . . . . . .
./
1
0 I 1.6 113.1 1107.8
4.8 107.3
I ~
16.0 40.0 8.0 106.7 1106.3 1106.1 ~
TABLE 3 Relative acid strengths i n n-propyl alcohol at 26°C. I
LOQ BUBBTTTUENT
Ortho
NO* ........................ Br.. . . . . . . . . . . . . . . . . . . . . . c1.. . . . . . . . . . . . . . . . . . . . F. . . . . . . . . . . . . . . . . . . . . . . . CHa. . . . . . . . . . . . . . . . . . . . OCHs . . . . . . . . . . . . . . . . . . . . ON.. . . . . . . . . . . . . . . . . . . . I
KA.B~ Para
Mets
1.804 1.096 1.162 1.086 0.960 0.041 0.255 1.569
1.140 0.450 0.432 0.389 0.213 -0.211 -0.363 -0.569
1.153 0.617 0.630
0.606 0.522 -0.095 -0.165
TABLE 4 The dissociation constants i n n-propyl alcohol containing 0.046 mole of lithium chloride per liter = 0.050;t = 25°C.;pK, = 8.603 for benzoic acid
-
PKO SCBBTITEENT
1
SO* . . . . . . . . . . . . . . . . . . . .
I .......................
'
Br.. . . . . . . . . . . . . . . . . . . . . .......................
c1
F......................... CHs ..... OCHa . . . . . . . . . . . . . . . . . . . .
OH......................
,
Ortho
6.799 7.507 7.441 7.517 7.643 8.562 8.348 7.034
I
Meta
I
7.450 7.986 7.973 7.997 8.081 8.698
1
8.768
1 I
1
I
Para
-l
1
' i ~
7.463 8.153 8.171 8.214 8.390 8.814 8.966 9.172
_-
224
JOHN H. ELLIOTT
Table 2 gives values for the cell.
HB 0.005M AulLiB Quinhydrone 0.005M!
FgM
HX 0.005M ILiX Quinhydrone 0.005M1.4~
where HX is o-nitrobenzoic acid, and it is seen that a relatively constant E.M.F. is obtained when a LiC1: LiB ratio of 8: 1 is reached. The precision measure of the values of log KAzBo is estimated as 0.010 from a consideration of the possible errors involved. The observed values of log K A ~ B ~ are reported in table 3. Table 4 gives values for pK, for the acids studied. These values are for a solution of ionic strength 0.05, and were determined by measuring a benzoic acid buffer against a suitable solution of hydrochloric acid in n-propyl alcohol. The value of 8.603 was found for benzoic acid in a solution of ionic strength 0.050and 0.045 &I in lithium chloride. The values of pK, for the substituted benzoic acids were calculated by the following equation p K (HX)
p K (HB)
- log KA~B,,
(3)
All measurements were made a t 25°C. DISCUSSION OF RESULTS
Wynne-Jones (14) predicted from theoretical considerations that log
KAsBo would vary linearly with the reciprocal of the dielectric constant of the medium, according to the following equation: log KAzBo = log Knnbo
+ L(1/D)
(4)
where log &bo is the intercept (1/D = 0) and L is the slope of the line. This has been found to be the case for the hydroxylic solvents water, methyl alcohol, ethyl alcohol, and for meta- and para-substituted acids only in the solvent ethylene glycol (6, 10, 12). It was further noted by Minnick and Kilpatrick (12) and Elliott and Kilpatrick (6) that the values found in butyl alcohol did not lie on the line defined by the values in water, methyl alcohol, and ethyl alcohol. This deviation was attributed to the more important rBle played by solvation effects, for example, dipole-dipole interactions in solvents of low dielectric constant (6). This makes the results in n-propyl alcohol of particular interest, as its dielectric constant, 20.1 (l), lies between that of ethyl alcohol, 24.2 (I), and that of butyl alcohol, 17.4 (13). I n order to see whether the values in n-propyl alcohol obey the predictions of the Wynne-Jones relationship, the values of log KA=%for n-propyl alcohol have been calculated, using values of L and log Ksrbodetermined by the method of least squares on the basis of data recently determined in
225
EFFECT OF SUBSTITUENTS ON ACID STRENGTH OF BENZOIC ACID
this laboratory (6,9, 10) and omitting values for ortho acids in the solvent ethylene glycol (see table 5). This was done because there is some difference between the values of L and log KBZbO calculated from colorimetric (10) and electrometric (6) results. TABLE 5 Deviations f r o m equation 4
-
n - P R o P n AwoEoL LOQ K*,.o
Loo
Yazbo
D
L
Ob-
served
0-1. . . . . . . . . . . . ,
......
o-Br . , . . . . . . . . . . . . . . . 0-c1.. , , . . . . . . . . . . . . . .
0-F. . . , . . , . . . , . . . , . , . , 0-CHS.. . . , . , . . . . . . . . , 0-OCHs. ..... 0-OH . . . , . . . . . . . . , . . .
i
I
D
20.1
Calcu-
lated
calou-
lated -ob-
served
~ - - ~ - _ _ 2.112-8.85 1.804 1.672-0.132 1.442 -8.03 1.096 1.042-0.054 1.419 -5.51 1.162 1.145 -0.017 1.342 -4.72 1.086 1.107 4-0.021 0.889 3.86 0.960 1.081 0.121 0.376 -8.66 0.041-0.055-0.096 0.056 3.95 0.255 0.253-0.002 0.982 17.1 1.569 1.833 0.264
-
n-BumL AmonOL
Ob-
nerved
KAzBp
17.4
Calculated
calcu-
lated -ob-
served
1.780 1.803-0.177 1.038 0.980-0.058 1.090 1.102 0.012 1.075 1.070 -0.005 0.926 1.111 0.185 0.002-0.122-0.124 0.281 0.283 0.002 1.505 1.964 0.459
m - N 0 2 . ., , . . , . , . . . , , .
0.491 17.4 1.153 1.357 0.204 1.100 1.491 0.391 0.225 10.0 0.617 0.723 0.106 0.566 0.800 0.234 m-Br . . , . . , . . . . . , . . . , 0.281 9.32 0.630 0.745 0.115 0.578 0.816 0.238 m-C1. , . . . . . . . . . . , . . . , 0.2801 8.60 0.606 0.708 0.102 0.585 0.774 0.189 0.237; 7.98 0.522 0.634 0.112 0.416 0.696 0.280 m-CHa.. . . . . . . . . . . . . . -0.0651 -0.37-0.095 -0.083 0.012 -0.095 -0.086 0.011 m-OH. . . , . . . . . , . , . . . , 0.219 -10.1 -0.165 -0.284 -0,119 -0.159 -0.361 -0.202 0.571 14.2
1.140 1.278 0.138 1.141 1.385 0.244
p - I . , . . . . . . . . . . . . . . . . 0.141 8.17, 0.450 0.5481 0.098 0.396 0.611 0.215 p-Br, . . . , , . . . . . . . . . . . 0.128 8.691 0.432' 0.560' 0.128 0.421. 0.627 0.206 p-Cl. . . . . . . . . . . . . , . . . . 0.104 8.541 0.389 0.529 0.140 0.394 0.595 0.201 p-F . . . . . . . . . . . . . . . . . . .0.0091 6,531 0.213, 0,3161 0.1031 0.215 0.366 0.151 p-CHs.. . . . , . . . , . , . . . . -0.164 -0.61 -0.211 0.194 0.0171-0.193 -0,199 -0.006 .O.m -3.53 -0.363 -0.403 -0.040 -0.360 -0.430 -0.070 p-OH. . . . . . . . . . . . . . . . .0.270/ -7.12 -0,569,-0,624-0,055 -0.566 -0.680 -0.114
Considering observed and calculated values of log K A I ~toobe in agreement if their difference is less than 0.025, it is seen that for the solvent propyl alcohol agreement with equation 4 occurs only in the case of the substituents o-Br, 0-C1, o-OCH3, m-CH3, and pCH3, while in eighteen other cases the disagreement is very pronounced. It is interesting to note that the same acids show good agreement with the calculated values in the solvent n-butyl alcohol. It seems safe to assume that these cases of agreement are exceptions and
226
JOHN H. ELLIOTT
that, in general, the values of log K A , B o for monosubstituted benzoic acids in n-propyl alcohol do not fall on the line determined by values in pure solvents of dielectric constant greater than 24.2. This is brought out to a more striking degree when the values of the difference between calculated and observed values are compared between the solvents n-propyl alcohol and n-butyl alcohol. Here it is seen that the deviations are in the same direction and are greater for n-butylalcohol, with only five exceptions. These exceptions occur for the same substituents that gave agreement in n-propyl alcohol: namely, o-Br, 0 4 1 , o-OCH3, m-CH3, and p-CHa. The values of log K A for~the ~latter ~ two cases are not sensitive to solvent change, and hence the fact that agreement is obtained between calculated and observed values is not particularly significant. I n the case of o-bromo, o-chloro-, and o-methoxy-benzoic acids, this behavior mould seem to be exceptional. I n eighteen out of the twenty-three cases tested, the differences between the calculated and observed values of log K A , B o are of the same sign and of greater magnitude in n-butyl alcohol than in n-propyl alcohol. This leads to the conclusion that log K A , B o ceases to be linear in 1/D at values of D less than 24.2 and that there is an increasing difference between observed and calculated values as D becomes smaller. SUMMARY
The values of the acid strength of twenty-three monosubstituted benzoic acids relative to benzoic acid have been determined in the solvent n-propyl alcohol by a potentiometric method. These results indicate that, for pure solvents having dielectric constants less than 24.2, log K A , B o is not linear in the reciprocal of the dielectric constant of the solvent. The author wishes to express his thanks to Professor Martin Kilpatrick, under whose direction this work was done, and to the Graduate School of the University of Pennsylvania for permission to use their facilities under a Ph.D. courtesy. REFERESCES ~ K E R L B FG, . : J . Am. Chem. SOC.64,4125 (1932). B R ~ N S T EJ. DK.: , Trans. Faraday SOC.28,430 (1927).
BRUNELL, R . F., CRENLHAW, J . L., AND TOBIN,E . : J . Am. Chem. Sor. 43, 561 (1921). ELLIOTT, J. H., AND KILPATRICK, M.:J . Phys. Chem. 46, 454 (1911). ELLIOTT, J. H.,AND KILPATRICK, hf.: J . Phys. Chem. 46, 466 (1941). ELLIOTT, J. H . , AND KILPATRICK, M . : J. Phys. Chem. 46, 472 (1941). ELLIOTT, J. H.,AND KILPATRICK, M.:J . Phys. Chem. 46,485 (1941). HOVORKA, F., . ~ N DSIMMS, J . C.: J. Am. Chem. SOC.69, 92 (1937). KILPATRICK, M., AND MEARB, W . H . : J. Am. Chem. SOC.82,3047 (1940).
MOLECULAR FORMULA O F CESIUM TETRAIODIDE
(10) (11) (12) (13) (14)
227
KILPATRICK, &I., AND MEARS,W . H.: J. Am. Chem. SOC.62, 3051 (1940). M.: J. Am. Chem. SOC.69,572 (1937). MASON,R . B., AND KILPATRICK, MINNICK,L. J., A N D KILPATRICK, M.: J. Phys. Chem. 43, 259 (1939). WOOTEN, L.A , , AKD HAMMETT, L. P.: J. .4m. Chem. SOC. 67, 2289 (1935). WYXNE-JONES, W.F. K . : Proc. Roy. Sot. (London) A140, 440 (1933).
POLYIODIDES OF CESIUM. I V
X KOTEON
THE
MOLECL-LAR FORMULA OF CESICMTETRAIODIDE 8. S HTJBhRD‘
Depai tment
0.f
C h e m i s t r y , Cornell L‘nztersziy, Ithaca, Neui York Recezved J u l y Z8,1941
Although the existence of cesium tetraiodide has been definitely established (3), there has been considerable speculation about its molecular formula and structure, and the same can be said of “even” polyhalides in general. This paper presents some pertinent data on cesium tetraiodide; the conclusions can perhaps be applied also to cesium tetrabromide (8) and to the sodium tetraiodide recently reported (2, 4). The empirical formula of cesium tetraiodide is Cs14,but it is extremely doubtful that this is the true molecular formula, as will now be shown. If the tetraiodide group were actually I4-, the total number of valence electrons would be 29, the sum of the 28 (4 X 7) due to iodine plus the one captured from cesium. Thus an unpaired electron would be present. On the other hand, if the true formula were some even multiple of Cs14, there would be no unpaired electrons. The simplest and most probable of these formulas is CszIg, corresponding to 58 (2 X 29) valence electrons in the polyiodide group. Therefore, a measurement of the magnetic susceptibility of the compound seemed advisable and was obtained by Dr. h.hl. Saum, to whom we are very grateful. The tetraiodide proved to be diamagnetic, with a susceptibility of -220.5 X C.G.S. units per mole, which agrees well with the theoretical value of -227.8 X C.G.S units calculated on the basis of the additivity principle.? A formula weight of 640 (corresponding to CsIJ was assumed in obtaining both valuesa Present address: Tusculum College, Greeneville, Tennessee. This value was calculated from data for cesium iodide and iodine given by Klemm (6) and by Bhatnagar and Mathur (1). Since the available data are not entirely concordant, an average value is reported. The lowest and highest values calculated were, respectively, -219.7 X 10-0 and -236 X 10-ec G.S. units. If the true formula is CsJs, the observed and theoretical values become, respectively, -441.0 X lo+ and -455.6 X IO4 C.G s. units. 2
’
